Health and Safety
Executive
The use of infra-red (tympanic) temperature
as a guide to signs of heat stress in industry
Prepared by the Institute for Occupational Medicine
for the Health and Safety Executive 2013
RR989
Research Report
Health and Safety
Executive
The use of infra-red (tympanic) temperature
as a guide to signs of heat stress in industry
Richard Graveling, Laura MacCalman and Hilary Cowie
Institute for Occupational Medicine
Research Avenue North
Riccarton
Edinburgh EH14 4AP
Previous IOM research, showed that the use of a simple infra-red (IR) ear thermometer did not provide a reliable
prediction of core body temperature for use in industrial situations (Graveling et al, 2009). The aim of this
research was to explore the use of an IR ear thermometer further, and to determine whether the consistency and
accuracy of the measurements obtained could be improved sufficiently to provide a reliable indication of the risk
of an individual suffering from heat strain.
Published studies where IR temperature has been compared with core temperature benchmarks such as rectal
temperature to provide further detail on likely sources of variation in measured tympanic temperature were
re-examined. Based upon the factors identified, a number of experimental studies were carried out to explore
the influence of these factors, together with revised measurement methods aimed at reducing their influence.
The results showed that the technique devised as a result of these studies did give more reliable results than
those found previously although the predictive relationship determined showed that IR tympanic temperature
could still not be used to predict actual core body temperature with a sufficient degree of accuracy for it to be
used in industry. However it is suggested that this provides a possible basis for the use of IR thermometry as a
screening tool in monitoring hot workplaces for possible risks of thermal strain.
This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents,
including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily
reflect HSE policy.
HSE Books
© Crown copyright 2013
First published 2013
You may reuse this information (not including logos) free
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ii
Contents
Summary1
1 Introduction
2
2 References to measurement technique in the literature
3
3 Further laboratory studies
7
4 Discussion
26
5 Conclusions
32
References33
Appendix 1
35
Summary
Previous IOM research, showed that the use of a simple infra-red (IR) ear
thermometer did not provide a reliable prediction of core body temperature for use in
industrial situations (Graveling et al, 2009). There are conflicting reports in the
scientific literature with some echoing that view whilst others suggest that such
thermometers can reliably be used.
The research reported here was carried out to explore the use of an IR ear
thermometer further, and to determine whether the consistency and accuracy of the
measurements obtained could be improved sufficiently to provide a reliable indication
of the risk of an individual suffering from heat strain.
The research re-examined the published studies where IR temperature has been
compared with core temperature benchmarks such as rectal temperature to provide
further detail on likely sources of variation in measured tympanic temperature.
Based upon the factors identified, a number of experimental studies were carried out
to explore the influence of these factors, together with revised measurement methods
aimed at reducing their influence.
The results showed that the technique devised as a result of these studies did give
more reliable results than those found previously although the predictive relationship
determined showed that IR tympanic temperature could still not be used to predict
actual core body temperature with a sufficient degree of accuracy for it to be used in
industry. However, at an IR temperature of 37.5 °C approximately 85% of people
would have an IG (core) temperature of between 38 °C and 39 °C with approximately
10% having a lower temperature and 5% a higher temperature. It is suggested that
this provides a possible basis for the use of IR thermometry as a screening tool in
monitoring hot workplaces for possible risks of thermal strain.
However, these results were obtained from a limited number for subjects, in a
restricted range of thermal environments and were obtained by a single researcher.
There are indications that the technique (and therefore the results obtained) is
dependent on individual technique, to the extent that the outcome depended on
which ear the temperature was obtained from. The role of the make and model of
thermometer also requires further investigation as the outcome varied between
different instruments. It remains to be seen whether the technique can reliably be
applied by others, taking measurements from more people in a wider range of
climates with a variety of makes and models of thermometer.
1
1
Introduction
Previous IOM research, showed that the use of a simple infra-red (IR) ear
thermometer did not provide a reliable prediction of core body temperature for use in
industrial situations (Graveling et al, 2009). Whilst this was in agreement with some
published studies (e.g. Roth et al, 1996), others, such as Stavem et al (1997), have
suggested that the technique could be used with a reasonable degree of confidence.
There are many methodological differences which could possibly account for these
differences in findings. For example, the study by Roth and co-workers was of
marathon runners with suspected hyperthermia who had been taken into an acute
medical treatment area at the event. The authors do not document whether any
active cooling or other treatment had been commenced but, if it had (which would
seem likely) this could result in local cooling effects depressing the ear canal
temperature. In contrast, Stavem and co-workers, working in a hospital setting, used
the mean of readings obtained from both ears which would have reduced the
influence of random measurement errors by diluting their effect.
The research reported here was carried out to explore the use of an IR ear
thermometer further, and to determine whether the stability and accuracy of the
measurements obtained could be improved to the extent that it could be used in
industry to provide a reliable indication of the risk of an individual suffering from heat
strain.
The research commenced with a re-examination of the published studies where IR
temperature has been compared with core temperature benchmarks such as rectal
temperature. This was to provide further detail on likely sources of variation in
measured tympanic temperature. It was anticipated that these could include operator
error in inserting the temperature probe (e.g. not pushing it fully ‘home’); anatomical
variation restricting the ‘view’ of the ear drum; and a relatively narrow ear canal.
There are also suggestions (e.g. Betta et al, 1997) that different measuring
instruments, employing differing technologies, can vary in their accuracy.
Based upon the factors identified from this re-appraisal of the literature, a number of
experimental studies were carried out to explore the influence of these factors,
together with revised measurement methods aimed at reducing their influence.
2
2
References to measurement technique in the
literature
.1
Introduction
In order to inform attempts to improve the reliability of the measurement of tympanic
temperature using an IR thermometer the literature on this measurement
methodology was explored. The appraisal was limited primarily to those studies
identified during the previous research (Graveling et al, 2009) comparing this
technique with measurements obtained from other body sites.
These were
supplemented by other papers identified through a search of recently published
literature. The rationale for focussing on these papers was that, as the main
emphasis of such papers was on the technique itself, these would be more likely to
explore details of the measurement method used than, for example, a research study
utilising this technique as its measure of core temperature. Review papers were
excluded as they gave little detail of relevance.
The findings from this overview were used to develop experimental approaches
aimed at exploring the influences on the reliable measurement of tympanic
temperature using an IR thermometer.
.2
Descriptions of technique from the literature
In many instances, the papers reviewed gave little insight into the details of the
technique used. Reference was commonly simply made to the provision of prior
training, usually in-house (e.g. Jakobsson et al, 1992; Roth et al, 1996; Moran et al,
2007). In one case (Valle et al, 1999) an instructional video was used. In many
papers, descriptions of the actual technique were limited to the fact that this was in
accordance with the manufacturers’ instructions (e.g. Modell et al, 1998; Casa et al,
2007).
A subset of papers described the use of prior examination of the ear canal using an
otoscope. In some instances this was to exclude those with damaged tympanic
membranes or whose membrane could not be visualised (e.g. Giuliano et al, 1999).
In other cases, otoscopic examination was used to estimate the coverage of the
tympanic membrane by cerumen (wax) although where this was the case (e.g.
Stavem et al, 1997; Modell et al, 1998) subsequent statistical analyses suggested
that cerumen did not significantly contribute to the predictive relationship between
tympanic and rectal temperatures.
Research opinion appears to be divided on this issue. Thus, whilst Robinson et al
(1998) echo these findings, in citing work suggesting that cerumen presence is of
“minimal clinical importance”, Fulbrook (1997) cites studies promoting the opposite
view with one suggesting systematic differences in tympanic temperature of up to
0.30°C where cerumen is present. However, this finding should be interpreted with
caution as it stems from a study in which a cerumen plug was inserted, rather than
relying on naturally occurring cerumen. It is likely that this plug was not in good
contact with the tympanic membrane, which would have been likely to have impaired
conductive heat transfer and accounted for at least some of the reduction in
measured temperature.
Similarly, Fulbrook (1997) (which reported smaller differences), did not involve
closely consecutive readings but rather compared inter-ear differences before and 3-
3
4 days after irrigation to remove cerumen. One paper (Giuliano et al, 1999) did
exclude those patients where it was not possible to visualise the tympanic
membrane, suggesting a particularly convoluted ear canal. However, only four out of
106 were excluded in this manner suggesting it is not a major contributory factor.
A number of authors (e.g. Childs et al, 1999) made specific reference to leaving
subjects in a stable thermal environment before taking a reading (10 minutes).
Although others did not specifically identify this factor it is likely that the nature of the
studies (trials with clinical in-patients) means that this condition was met in any case.
Related to this was the possible contribution of other local thermal factors. Thus,
Fulbrook (1997) observed that obtaining readings from an ear shortly after a patient
had been lying on that side appeared to contribute to a difference in readings as did
the concurrent use of a cooling fan as part of their treatment.
The possible influence of the measurement procedure itself was also acknowledged
by some authors. For example, Giuliano et al (1999) indicated that the ear canal was
allowed to re-warm for two minutes after the use of the otoscope and Valle et al
(1999) referred to earlier work as demonstrating that the time from insertion of the
thermometer itself into the ear canal, until the temperature was taken, was an
important factor, presumably because of the local cooling induced by the probe.
A common procedure referred to by a number of authors and seen as being of
importance is the use of an ear tug, apparently to straighten the line of the ear canal.
Bricknell (1997); Childs et al (1999); and Moran et al (2007) are amongst the authors
emphasising this element. However, it is interesting to note the work of Weiss et al
(1998) who, in a comparative study of three different devices, only used an ear tug
for the one device where it was specially included in the manufacture’s instruction.
Of course it is possible that there were many other factors influencing the outcome
but it is interesting to note that, although the three instruments each displayed a
different offset from the comparator temperature (oral) the particular instrument in
question did not ‘stand out’ in terms of yielding less variable results.
These latter authors (Weiss et al, 1998) list a number of potential factors through
which operator technique could contribute to erroneous readings. As well as failing
to administer an ear tug these included variation in the angle of placement of the
instrument; an occlusive seal created by the device in the ear canal (it is not clear if
this is presented as a positive or negative feature) and movement of the device as
the measurement is obtained. The authors also refer to the influence of the patient
previously lying on that side of the body (limiting ambient air exposure) with higher
temperatures being obtained as a result. However, it is not clear which temperatures
would be regarded as more correct.
One possible source of apparent error in variable climates is the time lag in response
between rectal temperature and other sites, including the IR-tympanic. For example,
Newsham et al (2002) reported that IR temperature rose more rapidly than rectal in a
group of subjects exercising in hot, humid conditions but that rectal temperature
continued to rise after exercise cessation, such that peak temperatures from the two
measurement systems were not significantly different.
A detailed description of the technique used is provided by Childs et al (1999).
According to these authors:
“The pinna was gently pulled backwards and the ear thermometer inserted
into the external auditory meatus, turned and directed towards the eye.”
4
This differs from some others (e.g. Moran et al, 2007) who refer to pulling the pinna
upwards and backwards. In addition, it is difficult to interpret the reference to
‘turning’ the thermometer as no plane of rotation is indicated.
Terndrup and Rajk (1992) provided an even more detailed description in their study,
which included a formal comparison of the use (or non-use) of an ear tug. On the ear
tug itself they describe this as pulling:
“posteriorly and superiorly on the external ear at the mid-point between the
apex of the helix and inferior border of the lobule.”
Further detail, relating to actually inserting the thermometer probe, is also given:
“the probe was placed in the auditory canal with a gentle back and forth
motion of the probe while directing the tip at the midpoint between the
eyebrow and sideburn on the patient’s left side.” (i.e. pointing slightly
upwards towards the left hand temple).
Although the impact was not statistically significant the authors report that an ear tug
decreased the mean differences between rectal and IR-tympanic temperature
readings from 0.35 ± 0.44°C to 0.26 ± 0.30°C.
As an addendum to this paper, it should perhaps be noted that the authors record
that:
“some auditory canal discomfort during probe insertion was verbalised by
several participants.”
Possibly one of the most thorough examinations of technique has been reported by
Rohrberg et al (1997). The authors found the most accurate readings to be obtained
with the thermometer in what they described as the ‘telephone handle’ position,
because of the manner in which it mirrored the position of a conventional telephone
handset (in line with the ramus mandibulae - the more vertical portion of the lower
jaw). The reading should be taken immediately because a delay of five seconds will
result in significantly lower temperatures and, if repeat measurements are obtained,
at least 90 seconds should be left between readings or the second reading will be
lower because of the cooling effect of the sensor probe.
One issue which does not seem to have been reported in any of the papers
examined is that of the pressure used in inserting the temperature probe. Intuitively,
the force which is used to press the probe into the ear canal will influence the
distance of the sensor from the ear drum, and might also have an effect on the
quality of the ‘line of sight’ achieved between the sensor and the drum.
Finally, in addition to issues relating to the measurement procedure, it is apparent
from the literature that differences can be identified between the different measuring
instruments used, not only in the absolute temperatures obtained but also in the
variation of those temperatures. Pusnik et al (2004) cite a European Standard (CEN
2003) as requiring IR thermometers to be accurate within ±0.2 °C. It is apparent from
the various published studies that, in practical use, this is seldom achieved.
A number of papers have been published which have compared different
instruments.
In addition, papers using a single instrument to compare IR
temperature measurement against a benchmark such as rectal temperature usually
5
specify the type used although, in these cases, it is difficult to apportion errors and
inaccuracies to the instrument rather than other aspects of the studies.
Pusnik et al (2004) and Pusnik and Drnovsek (2005) express some concerns about
the fundamental accuracy and stability of IR thermometers, including the difficulty of
calibrating or recalibrating them. In the latter paper they indicate:
“For accurate measurements with IRETs they have to be calibrated regularly
with an appropriate and traceable calibration system. Such systems are
neither widely available nor are there many competent (accredited) laboratories
which can provide traceability for IRETs.”
The authors suggest that the ‘Genius First Temp’ is the best instrument in this regard
as it was the only one to have a calibration mode. In a comparison of four different
instruments the authors concluded that none achieved the required accuracy of ±0.2
°C although, if an offset was applied to the indicated temperature, some models did
display the required accuracy, around that offset.
Betta et al (1997) also carried out laboratory-based (simulation) studies in which they
constructed an artificial ear canal to test the instruments in what they considered to
be realistic conditions. The authors concluded that the First Temp gave better
repeatability than accuracy, favouring the Thermoscan HM-1 over the First Temp and
another device. Arguably however, a stable ‘inaccuracy’, manifested as an offset,
can be compensated for by adjusting the reading whilst an unstable/unrepeatable
reading presents more difficulties.
Weiss et al (1998) also favoured the Genius First Temp over a Thermoscan device
(Pro-1) although, more recently, Smitz et al (2009) favoured another Thermoscan
model (the Pro 3000) over the Genius First Temp, although the 95% confidence
intervals of ±0.62 °C and ±0.76 °C respectively in predicting rectal temperature were
not that dissimilar.
It would seem therefore that a number of potential influences on the accurate and
consistent measurement of body temperature using an IR tympanic thermometer can
be identified.
Most papers where cerumen presence has been recorded and treated as a variable
have not found it to make a significant contribution to the variability in measurement.
However, it would seem to be important to ensure that the ear canal is straightened
with an ear tug, carefully administered; the probe is inserted firmly and correctly
orientated; the temperature is taken soon after insertion; and adequate time is
allowed for re-warming before any repeat measurements are obtained. It remains to
be seen whether these factors can be used to establish a sufficiently reliable
measurement procedure.
In addition, consideration should be given to the
observation that certain IR thermometers appear to be more accurate than others.
6
3
3.1
Further laboratory studies
Introduction to studies
As a result of this overview of the published literature, the following series of
experimental studies were identified:
1.
The use of an otoscope for preliminary visualisation of the ear drum
and to determine the optimum insertion angle and ear pull to straighten the ear
canal and maximise the proportion of the ear drum ‘visible’ to an IR
thermometer.
2.
The effect of handset pressure against the head on IR temperature
readings
3.
Examination of the effect on the IR temperature obtained of delaying
the temperature reading or of taking multiple readings in quick succession (less
than 60 seconds).
4.
Examination of the possibly increased accuracy and/or reliability of
measurement of alternative IR thermometers to that already available (the
Braun Thermoscan 6022).
5.
The variability in measured IR temperature in stable thermally neutral
conditions (office environment).
6.
Comparison between the instrument selected on the basis of the study
in (4) and method for IR thermometry with a ‘gold standard’ in stable thermally
neutral conditions (office environment).
These were considered to present the main issues identified and it was anticipated
that the cumulative findings from the first four studies would be used to devise a
definitive measurement technique which would be used firstly in study five, to
establish the stability of the selected technique, and then in study six in which this
technique would be compared against rectal temperature as a ‘gold standard’.
For operational reasons it was decided to merge the last three studies, using
measuring instruments from two manufacturers to obtain readings of IR tympanic
temperature across a working day and compare the readings to temperatures
obtained using intra-gastric (IG) temperature pills. The use of these pills represented
a departure from the original plan, which was to use rectal temperature. However,
not enough subjects were willing to volunteer for this aspect of the series of studies
to provide a statistically valid sample and the pills were adopted as a more
acceptable alternative.
The IG temperature pills and their use as an alternative ‘gold standard’ has been
described previously (Graveling et al, 2009). Although some variation in the
temperature measured by the IG system would be expected across a day, as the pills
themselves pass down the digestive tract, it was considered that, in essentially
sedentary subjects, this variation would be kept to a minimum. In addition, it was
recognised that the intake of food or drink at various temperatures, previously
7
identified as a potential source of considerable error, could be easily controlled in an
office environment.
Ethical approval for this work was obtained through the University of Aberdeen where
the primary author is an Honorary Senior Lecturer in the Faculty of Medicine and
Dentistry.
3.2
Subjects
The subjects were drawn on a voluntary basis from amongst the staff of the IOM.
Details of the proposed studies were circulated electronically to all staff and, in
accordance with the ethics committee approval, no inducements to participate were
used. Volunteers provided their informed consent and retained the right to withdraw
from the study at any time without offering any reason for doing so. In order to
maximise recruitment for the majority of the studies all subjects were given the
opportunity to opt-out of the final study. As stated above, because of the number of
subjects exercising this right, changes were made to the final study, utilising IG rather
than rectal temperature measurement.
Ten subjects were required for the series of studies and the same ten were retained
throughout (with a reserve in place in the event of non-availability). Volunteers were
accepted into the study on a first-come basis. One was excluded following the initial
visual examination because of marked ear canal inflammation and replaced by a
reserve. Of the ten subjects eight were female and the group had an average age of
36.5 years (range 24-60).
3.3
Study 1: Determination of the optimum insertion angle and ear pull
3.2.1 Aim
As stated above, the purpose of this study was to identify the optimum ear tug and
insertion angle to be used to maximise the proportion of the ear drum which can be
seen and to establish the impact of this on the tympanic temperatures recorded.
3.2.2 Method
Ten subjects were used, with both ears used from each subject. A standard
otoscope was used to view the ear canal. Initially it was inserted without any ‘ear tug’
and the proportion of the ear drum visible recorded by sketching the field of view
through the otoscope. The process was then repeated using a variety of ear tugs,
including the normally recommended ‘standard tug’ (pulling horizontally backwards
on the external ear) and at angles of approximately 45° above and below this. Other
directions were used, based upon the visualisation of ear drum, to determine what
influence if any each tug had on the extent of the ear drum which could be seen.
Once the optimal ear tug angle had been established for a subject, the otoscope
was replaced by an IR tympanic thermometer (Braun ThermoScan 6022) and
temperatures obtained using no tug and the optimal tug as previously determined.
Six pairs of measurements (tug/no tug) were obtained from each ear, with the order
of testing balanced with subjects being randomly assigned to the treatment sequence
determined according to a latin square. The values obtained were recorded. At least
1 minute was left between consecutive readings in one ear to minimise any cooling
effects of inserting the plastic thermometer probe.
8
A mixed model regression analysis was carried out on the results. This analysis used
as a response variable the measured temperatures, and fitted terms for differences
between individuals (as a random effect) and between the different methods or time
intervals (as fixed effects). The results of the regression model provided an estimate
of the statistical significance of any differences between methods.
3.2.4 Results and Discussion
Visual observations of ear canals revealed considerable detailed anatomical
differences between subjects and, to a lesser extent, between the two ears of the
same subject. The external opening for the ear canal was not always visible without
some form of ear tug. However, although a tug could be helpful in locating the tip of
the otoscope (and subsequently the thermometer tip) into the ear canal, it was not
necessary to obtain a reading as, once it was inserted, the rigid projection of the
otoscope would maintain the necessary pathway.
With the otoscope superficially inserted it was noted that, in most instances, various
forms of ear tug would cause some movement in the outer portion of the canal.
However, once the otoscope was inserted sufficiently to visualise the ear drum, this
moved the tip past the mobile area.
Beyond the mobile section of the ear canal, most subjects were noted not to have a
completely straight canal, with part of the canal wall tending to intrude into the full
view of the drum. Tugging on the external ear did not appear to have any appreciable
effect in moving this and it was often therefore not possible to completely straighten
the ear canal by tugging. Nevertheless in all cases some visualisation of the ear
drum was possible. Cerumen was visible to some extent in most ears although in no
case did it prevent sight of the ear drum.
Table 3.1 presents the findings from the temperature readings obtained with or
without an ear pull being applied to the pinna (Pull and No pull). Results of
measurements taken from each of the subject’s ears are presented separately (Left
and Right ear). Values are presented as the mean and standard deviation (s.d.).
Table 3.1: Mean (s.d.) temperature (°C) by subject, Left and Right ear, and Pull or
No pull
Subject
1
2
3
4
5
6
7
8
9
10
All
Left ear
No pull
37.0
36.3
36.8
36.5
36.8
36.5
36.1
36.6
36.5
36.6
36.6
(0.17)
(0.05)
(0.21)
(0.16)
(0.05)
(0.08)
(0.17)
(0.12)
(0.08)
(0.08)
(0.28)
Pull
37.0
36.4
36.9
36.6
36.8
36.7
36.1
36.6
36.7
36.7
36.7
(0.04)
(0.19)
(0.29)
(0.40)
(0.08)
(0.26)
(0.04)
(0.32)
(0.08)
(0.15)
(0.32)
Right ear
No pull
36.9 (0.14)
36.3 (0.08)
37.0 (0.12)
36.8 (0.00)
36.8 (0.18)
36.4 (0.05)
36.1 (0.08)
36.8 (0.10)
36.5 (0.04)
36.6 (0.16)
36.6 (0.29)
Pull
36.9
36.3
37.0
36.9
36.7
36.5
36.0
36.9
36.6
36.6
36.7
(0.12)
(0.19)
(0.06)
(0.15)
(0.09)
(0.05)
(0.12)
(0.14)
(0.12)
(0.06)
(0.31)
On average, the temperatures were slightly higher for the Pull condition compared to
No pull and this difference was greatest, and close to statistical significance (p=0.06)
9
for measurements from the left ear. Differences for mean measurements from the
right ear were not statistically significant.
Figures 3.1 and 3.2 display these results graphically, with boxplots representing the
mean value, and the middle two quartile distributions (25th and 75th percentiles). The
results are shown for the left and right ear measurements respectively.
A visual analysis of these plots reveals that there did not appear to be any systematic
differences between Pull or No pull in terms of the consistency of the readings
obtained. In some subjects using a pull gave a smaller spread of readings whilst, in
others, not using a pull did so.
Temperature Left Ear (∞C)
37.2
37.0
36.8
36.6
36.4
36.2
NP
P
NP
P
NP
P
NP
P
NP
P
NP
P
NP
P
NP
P
NP
P
35.8
NP
P
36.0
1
2
3
4
5
6
7
8
9
10
No Pull (NP) and Pull (P) (subjects 1-10)
Figure 3.1: Boxplot comparison of Pull v No pull – left ear temperature by subject
37.0
36.8
36.6
36.4
36.2
NP
P
NP
P
NP
P
NP
P
NP
P
NP
P
NP
P
NP
P
35.8
NP
P
36.0
NP
P
TemperatureRight Ear (∞C)
37.2
1
2
3
4
5
6
7
8
9
10
10
No Pull (NP) and Pull (P) (subjects 1-10)
Figure 3.22: Boxplot comparison of Pull v No pull – right ear temperature by subject
3.4
3.4.1
Study 2: The effect of handset pressure
Aim
The purpose of this study was to examine the influence of the pressure used to hold
the IR tympanic thermometer in place in the ear when obtaining a temperature
reading.
3.4.2
Method
The same ten subjects as for Study 1 were used, with the temperature in both ears
measured from each subject. Using the optimum ear tug established for each
individual subject in the previous study the IR tympanic thermometer was used to
measure the temperature in each ear using two different levels of force. The original
intention was to utilise three different force conditions:
• Holding the probe loosely in the ear canal;
• Pushing the probe into the ear canal so that it was just sealing against
the surrounding skin;
• Pushing the probe into the ear canal so that it deformed the surrounding
skin. The maximum firm force acceptable to the subject was to be used.
However, experience from the first study indicated that, especially when the opening
was not visually apparent, a reasonable level of force was required in opening the
orifice of the ear canal to insert the sensor probe and it was not considered possible
to obtain viable readings using the minimal force implied in the first planned
condition. Consequently, the experimental plan was revised to just two conditions,
defined as follows:
• Pushing the probe into the ear canal using the standard procedure and
force deemed necessary to firmly position the probe as determined from
Study 1;
• Pushing the probe into the ear canal using the standard procedure but
then requesting the subject to press against the firmly supported probe
with the side of their head, to exert the maximum level of force they
regarded as acceptable.
The latter approach was considered to be an appropriate manner of achieving an
acceptable maximum force without the experimenter inadvertently causing pain.
Six sets of each measurement in each ear were obtained, with the order of testing
balanced according to a latin square allocation within subjects, as before. The
values obtained were recorded. At least 1 minute was left between consecutive
readings in one ear to minimise any cooling effects of inserting the plastic probe.
The results were analysed for any systematic differences using a mixed model
regression analysis. This used as a response variable the measured temperatures,
and fitted terms for differences between individuals (as a random effect) and between
the different methods or time intervals (as fixed effects).
11
3.4.3
Results and Discussion
Results were available for 10 subjects with 6 replications per method for each
subject. The results are summarised in table 3.2.
Table 3.2: Mean (s.d.) temperature (°C) by subject, by Forced v Normal pressure
Left ear
Forced press
36.7 (0.12)
36.7 (0.27)
36.8 (0.08)
36.6 (0.23)
36.8 (0.21)
36.9 (0.26)
36.1 (0.40)
37.0 (0.14)
36.9 (0.10)
36.8 (0.26)
36.7 (0.33)
Subject
1
2
3
4
5
6
7
8
9
10
All
Normal press
36.9 (0.17)
36.6 (0.08)
36.7 (0.19)
36.6 (0.40)
36.7 (0.10)
36.8 (0.26)
35.9 (0.05)
36.9 (0.18)
36.9 (0.21)
36.9 (0.27)
36.7 (0.34)
Right ear
No pull
36.8 (0.11)
37.0 (0.15)
36.8 (0.22)
36.9 (0.08)
36.6 (0.10)
36.6 (0.21)
35.9 (0.12)
37.1 (0.08)
36.5 (0.14)
36.6 (0.28)
36.7 (0.36)
Pull
36.8
37.0
36.8
36.9
36.6
36.6
35.8
36.9
36.7
36.6
36.7
(0.04)
(0.16)
(0.19)
(0.15)
(0.18)
(0.05)
(0.08)
(0.08)
(0.27)
(0.10)
(0.35)
On average, the temperatures were very slightly higher for Forced compared to
Normal pressure but these small differences were not statistically significant for
measurements from either ear.
Again, a visual analysis of these plots. shown in Figures 3.3 and 3.4 for the left and
right ears respectively, does not suggest any systematic difference in terms of the
consistency of readings, with a wider spread of values with Normal pressure for
some subjects and a wider spread with Forced pressure for others.
37.0
36.8
36.6
36.4
36.2
FP
NP
FP
NP
FP
NP
FP
NP
FP
NP
FP
NP
FP
NP
FP
NP
35.8
FP
NP
36.0
FP
NP
Temperature Left Ear (∞C)
37.2
1
2
3
4
5
6
7
8
9
10
Forced (FP) and Normal (NP) Pressure (subjects 1-10)
12
Figure 3.3: Boxplot comparison of Forced v Normal pressure – left ear temperature
(°C) by subject
Temperature Right Ear (∞C)
37.2
37.0
36.8
36.6
36.4
36.2
36.0
FP
NP
FP
NP
FP
NP
FP
NP
FP
NP
FP
NP
FP
NP
FP
NP
FP
NP
35.6
FP
NP
35.8
1
2
3
4
5
6
7
8
9
10
Forced (FP) and Normal (NP) Pressure (subjects 1-10)
Figure 3.4: Boxplot comparison of Forced v Normal pressure – right ear temperature
(°C) by subject
3.5
Study 3: The effect of the length of the time interval between readings
3.5.1 Aim
The purpose of this study was to examine the effect on the temperature obtained of
either taking a series of temperatures in rapid succession or of allowing intervals of
different lengths between successive readings from the same ear.
3.5.2
Method
Using the technique and normal pressure identified from the first two studies,
successive IR tympanic temperatures were obtained. Four different patterns of
measurement were utilised:
• Six readings obtained as quickly as possible (given the limitations of the
measuring instrument – these are described as ‘zero seconds’
measurements in the results below), without removing the probe from its
position in the ear canal (using the memory function on the instrument);
• Six readings obtained with each of the following time intervals between
removing the sensor (having obtained a reading) and re-inserting the
sensor: 10 seconds; 1 minute; 2 minutes. Longer intervals were not
considered viable given the long-term aim of using the technique to
obtain temperatures from workers in industry.
As before, the order of use of the time intervals was balanced between ten subjects
using a latin square design and the values obtained were recorded.
13
The results were again analysed for any systematic differences and to establish the
size of those differences.
3.5.3
Results and Discussion
Results were available for 10 subjects with 6 replications for each time interval per
subject. These are summarised in tables 3.3 and 3.4 for left and right ear
respectively.
Table 3.3: Mean (s.d.) temperature (°C) by subject by time interval – left ear
Subject
1
2
3
4
5
6
7
8
9
10
All
Left ear temperature
0 seconds
10 seconds
36.9 (0.12)
36.8 (0.09)
36.4 (0.18)
36.2 (0.00)
36.9 (0.10)
36.7 (0.21)
37.2 (0.05)
36.9 (0.20)
37.1 (0.06)
36.9 (0.31)
37.2 (0.10)
37.2 (0.53)
36.5 (0.26)
36.1 (0.12)
37.0 (0.10)
36.8 (0.05)
36.8 (0.20)
36.7 (0.05)
37.0 (0.16)
37.0 (0.08)
36.9 (0.30)
36.7 (0.39)
60 seconds
37.0 (0.10)
36.3 (0.08)
36.7 (0.05)
37.1 (0.09)
37.2 (0.10)
36.9 (0.25)
36.1 (0.06)
36.8 (0.05)
36.7 (0.06)
37.0 (0.08)
36.8 (0.34)
120 seconds
36.9 (0.12)
36.5 (0.24)
36.7 (0.10)
37.1 (0.06)
37.3 (0.10)
36.8 (0.19)
36.1 (0.08)
36.9 (0.19)
36.8 (0.28)
36.9 (0.18)
36.8 (0.35)
Table 3.4: Mean (s.d.) temperature (°C) by subject by time interval – right ear
Subject
1
2
3
4
5
6
7
8
9
10
All
Right ear temperature
0 seconds
10 seconds
37.1 (0.04)
36.8 (0.00)
36.7 (0.14)
36.7 (0.00)
36.9 (0.05)
36.8 (0.04)
37.1 (0.04)
37.0 (0.16)
37.2 (0.05)
37.1 (0.09)
37.0 (0.08)
36.4 (0.18)
36.4 (0.04)
36.0 (0.32)
37.3 (0.08)
37.2 (0.00)
36.7 (0.04)
36.5 (0.16)
36.9 (0.04)
36.8 (0.08)
36.9 (0.26)
36.7 (0.38)
60 seconds
36.8 (0.11)
36.6 (0.05)
36.8 (0.00)
37.4 (0.40)
37.2 (0.08)
36.5 (0.08)
36.0 (0.15)
37.1 (0.05)
36.6 (0.11)
36.7 (0.10)
36.8 (0.40)
120 seconds
36.9 (0.27)
36.7 (0.19)
36.7 (0.10)
37.1 (0.15)
37.3 (0.14)
36.8 (0.12)
36.0 (0.14)
37.1 (0.08)
36.5 (0.10)
36.9 (0.19)
36.8 (0.38)
The statistical analysis was undertaken for all four time intervals and also with the 0
second time interval excluded (as it was felt that this might not be achievable in
practical situations). In both analyses the differences between time intervals, though
small, were significant statistically.
Looking at the pairwise comparison between time points, for the first analysis,
temperatures at 0 seconds were significantly higher than at each of the other 3 time
points; and temperatures at 10 seconds were significantly lower than at 120 seconds.
In the analysis excluding the 0 second time point, the temperatures at 10 seconds
were again significantly lower than those at 120 seconds. However, as stated above,
the mean differences in temperature recorded with different time intervals between
measurements were small, being no more than 0.2 °C.
14
The results are shown graphically in Figures 3.5 and 3.6 for the left and right ear
measurements respectively.
37.6
Temperature Left Ear (∞C)
37.4
37.2
37.0
36.8
36.6
36.4
36.2
1
2
3
4
5
6
7
8
9
0
10
60
120
0
10
60
120
0
10
60
120
0
10
60
120
0
10
60
120
0
10
60
120
0
10
60
120
0
10
60
120
0
10
60
120
35.8
0
10
60
120
36.0
10
Inter-reading intervals of 0, 10, 60 and 120 seconds (subjects 1-10)
Figure 3.5: Boxplot comparison of time intervals between readings – left ear
temperature (°C) by subject
37.0
36.5
1
2
3
4
5
6
7
8
9
0
10
60
120
0
10
60
120
0
10
60
120
0
10
60
120
0
10
60
120
0
10
60
120
0
10
60
120
0
10
60
120
35.5
0
10
60
120
36.0
0
10
60
120
Temperature Right Ear (∞C)
37.5
10
Inter-reading intervals of 0, 10, 60 and 120 seconds (subjects 1-10)
Figure 3.6: Boxplot comparison of time intervals between readings – right ear
temperature (°C) by subject
15
The results from the statistical analyses showed that, once the 0 seconds readings
were omitted, temperature readings which were more spaced out (120 second
intervals) tended to result in higher mean readings. This would seem to imply that
more frequent readings did indeed result in a cooling of the ear canal by the
temperature probe as has been suggested in the literature. If this was the case then,
given the relatively large thermal mass of the temperature probe, it would be
expected that the zero second temperatures, taken without withdrawing the sensor,
might demonstrate a cooling effect across the series of six readings as the probe
cooled the tissues in the ear.
Tables 3.5 and 3.6 show the relevant individual temperature readings for the left and
right ears respectively. From this it can be seen that there is no consistent trend
apparent. Although in some series the first reading is the highest obtained this is
matched by a similar number of instances where the readings are higher after the
first. In addition, in many of the series any variation is limited to 0.1°C (which is at
the limit of resolution of the instrument).
Table 3.5: Set of six temperature readings from left ear, no break between readings.
Subject
No
Temperature readings (°C) Left ear
1
2
3
4
5
6
1
36.7
36.9
37.0
37.0
37.0
37.0
2
36.7
36.2
36.3
36.3
36.4
36.5
3
36.7
36.9
36.9
36.9
36.9
37.0
4
37.3
37.3
37.2
37.3
37.2
37.2
5
37.2
37.0
37.1
37.1
37.1
37.1
6
37.4
37.3
37.1
37.2
37.2
37.2
7
36.0
36.5
36.6
36.6
36.7
36.7
8
37.2
36.9
37.0
37.0
37.1
37.1
9
36.4
36.8
36.9
36.9
36.9
36.9
10
36.7
37.0
37.1
37.1
37.1
37.1
As shown by the (generally) smaller spreads of data in the graphical presentations,
these temperatures obtained with no time interval (not withdrawing the thermometer
probe) were generally more consistent. This is probably because, in taking these
readings, the sensor head was kept still and therefore pointed at the same element in
the ear canal throughout.
A visual examination of the raw data and of the pairs of graphs (left ear/right ear)
from the three studies suggested that readings might be generally more consistently
obtained from the right ear than the left. A statistical appraisal of the variability of
these data indicated that, while the standard deviations for the left ear were on
average almost invariably greater than those for the right ear, the differences were
not statistically significant for any of the treatments.
16
Table 3.6: Set of six temperature readings from right ear, no break between readings.
Subject
No
Temperature readings (°C) Right ear
1
2
3
4
5
6
1
37.0
37.1
37.1
37.1
37.1
37.1
2
36.5
36.6
36.7
36.7
36.8
36.9
3
36.9
36.9
37.0
37.0
37.0
36.9
4
37.2
37.1
37.1
37.1
37.1
37.1
5
37.2
37.2
37.2
37.1
37.1
37.1
6
37.2
37.0
37.0
37.0
37.0
37.0
7
36.3
36.4
36.4
36.4
36.4
36.4
8
37.1
37.3
37.3
37.3
37.3
37.3
9
36.7
36.7
36.7
36.7
36.7
36.6
10
36.8
36.9
36.9
36.9
36.9
36.9
3.6
Study 4: Comparison of IR with IG temperature, using two different IR
thermometers
3.6.1
Aim
The aim of this study was to compare the temperatures measured using two different
IR thermometers against IG temperature, measured using an ingestible temperaturesensitive pill.
3.6.2
Technique selection
It was apparent from the experience gained during the series of three studies that it
was possible, at least on occasions, to obtain a consistent temperature reading from
the ear canal although clearly, at this stage, it was not possible to establish whether
these consistent values differed from any actual core temperature value; the
magnitude of any difference; or the consistency of any such differences between
individuals.
Readings were most consistent for the continuous series of measurements, obtained
without moving or withdrawing the probe. This at least demonstrated that there was
little fundamental ‘noise’ in the sensing circuitry, introducing a random error.
However, it was also apparent that seemingly subtle differences in probe placement
could result in sizeable differences in the values obtained. This was most apparent in
the rapid series of measurements (10 second intervals) where there was subjectively
a strong carry-over memory of probe placement. Particularly with some individuals,
consistent readings could nevertheless be achieved with longer time intervals
between measurements. Reinsertion in a highly consistent manner with any one
individual usually resulted in consistent readings. Subjectively, this appeared to be
particularly the case with readings from the right ear, possibly indicating a degree of
handedness in the (right-handed) experimenter in obtaining readings.
17
It appeared that there were marked individual differences between individuals in the
extent to which this was possible, with some having seemingly ‘difficult’ ears. Again
subjectively, this appeared to be related to some extent to the size and/or immediate
accessibility of the opening to the ear canal, as much as the possible degree of
visualisation of their ear drums.
A measurement system was therefore devised to attempt to maximise the
repeatability of the readings obtained. This was based upon the approach of initially
visualising the ear drum using an otoscope. This recorded orientation was then used
to guide the placement of the temperature probe in obtaining readings.
3.6.3
IR temperature measurement method selected
A recording sheet was devised to permit the recording of the orientation of the
otoscope (relative to the side of the head) that gave the best view of the ear drum
(see Appendix 1). In essence, an otoscope was inserted into the ear canal and its
orientation manipulated until the best view achievable of the ear drum was
established. The angle at which the otoscope was held, relative to the side of the
head; and the direction in which it was pointed (referenced to a clock face with the
top of the ear deemed 12.00); were then noted on the record sheet as an arrow
(direction) and the estimated angle. It was noted that, in a number of ears, it was
necessary to deform the side of the ear canal by pressing with the side of the
otoscope viewing piece. The need for such pressure and the direction of pressing
was therefore also recorded by placing a ‘#’ mark on the circumference of the clock
face to indicate the direction to press. This procedure was then repeated for the
second ear. No attempt was made to clean wax from any ear and no subject was
excluded because of such wax or any other difficulty in achieving a good view of the
ear drum.
3.6.4 Procedure
Eight of the volunteers from the earlier studies participated in this final study.
Because of the change in measurement technique from that originally planned,
further informed consent (and ethical approval) was obtained from the volunteers and
two exercised their right to withdraw, as they did not wish to take the temperature pill.
In a preliminary session, prior to taking the pill, an otoscope was used to examine the
ear canals of each subject to determine the optimum viewing angle as described
above. The volunteers were then each given a sealed bag containing a single-use
intra-gastric temperature pill (IG) (CorTemp) together with instructions to activate the
pill by removing the switching magnet and swallowing the pill the evening before the
pre-arranged temperature measurement day.
After the start of work the following day, an IG temperature logger (CorTemp) was
used to verify the operation of the IG temperature pill. Two IR thermometers (Braun
Thermoscan Type 2022 and Brannan IR Ear Thermometer) were then each used to
obtain IR temperatures in triplicate from each ear, using the optimum orientation and
measurement procedure as previously determined. The highest temperature of each
set of three was used in subsequent statistical analyses rather than the mean on the
basis that it was thought to be unlikely that there would be any surface inside the ear
canal which was hotter than the ear drum and that this would therefore offer the most
accurate indication of core temperature. An IG temperature was measured for each
set (using the logging device) as a comparator. This procedure was repeated a
further seven times during the day, at approximately hourly intervals, giving a total of
eight sets of readings per subject. At other times during the day, the volunteers
carried on with their normal (desk-based) work activities.
18
The pill was swallowed the evening before the measurements were to be obtained in
order to minimise any risk of false readings due to the consumption of hot or cold
food or fluids. As a further safeguard, volunteers were asked to refrain from
consuming large volumes of food or drink in the 20 minutes or so preceding a
reading. During the afternoon of the measurement day, the IG temperature signal
was lost from one of the eight subjects. Enquiries indicated that the pill had probably
been voided. To avoid any missing data, this subject was asked to consume a
replacement IG pill. No readings were taken until the IG temperature readings from
the replacement pill were comparable to the last value obtained before the loss of
signal. This volunteer then refrained from further drink or food consumption for the
remainder of the afternoon (until all required readings had been obtained).
3.6.5 Results
As stated above, eight sets of temperatures were obtained from each volunteer
across the course of a day, consisting of maximum IR tympanic temperatures from
each ear using two IR thermometers together with a corresponding IG temperature.
The individual sets of data are shown as scatter plots in Figure 3.7. This shows the
results for each of the IR thermometers for each ear separately, plotted against the
equivalent IG temperature. It also shows the maximum IR temperature obtained for
each set of measurements from each subject from either ear. The means and
standard deviations from the data for each subject are shown in Table 3.7.
Table 3.7: The mean and standard deviation of each measurement device/ear
combination.
Subject
Brannan
(Left)
Braun
(Left)
Brannan
(Right)
Braun
(Right)
IG
Mean
SD
Mean
SD
Mean
Mean
Mean
SD
Mean
SD
1
38.4
0.54
38.1
0.37
37.4
0.15
37.2
0.20
37.6
0.20
2
37.5
0.57
37.3
0.19
37.2
0.06
36.9
0.14
37.4
0.11
3
36.9
0.48
36.9
0.36
36.7
0.29
36.9
0.13
37.6
0.13
4
36.9
0.32
36.8
0.26
37.4
0.54
37.0
0.29
37.5
0.18
5
37.3
0.46
37.2
0.26
37.1
0.19
37.0
0.21
37.6
0.28
6
36.2
0.27
36.5
0.15
36.6
0.15
36.6
0.14
37.2
0.24
7
37.6
0.31
36.9
0.29
37.2
0.17
36.7
0.19
37.5
0.18
8
36.7
0.38
36.6
0.47
36.9
0.19
36.5
0.27
37.2
0.26
All
37.2
0.73
37.0
0.56
37.1
0.38
36.9
0.29
37.5
0.26
From the scatter plots it is clear that the IR measurements obtained using the Braun
thermometer are closer to the IG temperatures, although they do tend to measure
lower, i.e. the majority of the points are above the line of equality shown on the plots.
The Brannan thermometer has more variation in the measurements obtained and
appears to be somewhat independent of the IG temperature (as confirmed by the
poor correlations – see below). For both IR thermometers, the temperature measured
in the right ear tends to be closer to the IG temperature. Looking at the data in Table
3.7, the overall average of the temperatures using the Brannan thermometer is
closest to the overall average of the IG temperature; however the relationship looks
better when comparing the temperatures obtained using the Braun with the IG
temperatures.
19
Braun thermometer, left ear
Brannan thermometer, left ear
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
Subject 6
Subject 7
Subject 8
38.5
39
O
IG Temperature C
O
IG Temperature C
38.0
37.5
37.0
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
Subject 6
Subject 7
Subject 8
36.5
36.5
37.0
37.5
38.0
37
36
36.0
36.0
38
36
38.5
37
Braun thermometer, right ear
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
Subject 6
Subject 7
Subject 8
39
O
IG Temperature C
O
IG Temperature C
38.0
37.5
37.0
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
Subject 6
Subject 7
Subject 8
36.5
36.0
36.0
36.5
37.0
37.5
38.0
38
37
36
36
38.5
37
Braun thermometer, max, either ear
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
Subject 6
Subject 7
Subject 8
39
O
IG Temperature C
O
IG Temperature C
38.0
37.5
37.0
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
Subject 6
Subject 7
Subject 8
36.5
36.0
37.0
37.5
39
Brannan thermometer, max, either ear
38.5
36.5
38
IR Temperature OC (Brannan right)
IR Temperature OC (Braun right)
36.0
39
Brannan thermometer, right ear
38.5
35.5
38
IR Temperature OC (Brannan left)
IR Temperature OC (Braun left)
38.0
38
37
36
36
38.5
37
38
39
IR Temperature OC (Brannan max)
IR Temperature OC (Braun max)
Figure 3.7: Scatter plots of IR temperature against the IG temperature. The points
are shown for each subject. The plots are shown individually for the two models of
thermometer (Braun and Brannan) and for each ear (Left and Right) as well as the
maximum obtained for either ear (bottom row).
In the earlier investigation (Graveling et al, 2009) ‘Bland-Altman plots’ (Bland and
Altman, 1986) were used for comparing temperature measurements made using
different instruments. Figure 3.8 shows a series of such plots for the collected pairs
of data. Each plot includes a line representing the mean difference between an IR
reading and its equivalent IG reading, together with two further lines indicating the
20
limits around this mean, determined by adding and subtracting two standard
deviations as recommended by Bland and Altman (op cit).
Braun thermometer, left ear
Brannan thermometer, left ear
2.0
Difference between IG & IR (Brannan left)
Difference between IG & IR (Braun left)
2
1
0
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
Subject 6
Subject 7
Subject 8
-1
-2
1.5
1.0
0.5
0.0
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
Subject 6
Subject 7
Subject 8
-0.5
-1.0
-1.5
-2.0
36.6
36.8
37.0
37.2
37.4
37.6
37.8
38.0
36.0
Average IG and IR Temperature (Braun left)
37.5
38.0
38.5
39.0
Brannan thermometer, right ear
2
Difference between IG & IR (Brannan right)
2
Difference between IG & IR (Braun right)
37.0
Average IG and IR Temperature (Brannan left)
Braun thermometer, right ear
1
0
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
Subject 6
Subject 7
Subject 8
-1
-2
1
0
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
Subject 6
Subject 7
Subject 8
-1
-2
36.6
36.8
37.0
37.2
37.4
37.6
37.8
38.0
36.0
Average IG and IR Temperature (Braun right)
36.5
37.0
37.5
38.0
38.5
39.0
Average IG and IR Temperature (Brannan right)
Braun thermometer, max, either ear
Brannan thermometer, max, either ear
2
2
Difference between IG & IR (Brannan max)
Difference between IG & IR (Braun max)
36.5
1
0
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
Subject 6
Subject 7
Subject 8
-1
-2
1
0
Subject 1
Subject 2
Subject 3
Subject 4
Subject 5
Subject 6
Subject 7
Subject 8
-1
-2
36.0
36.5
37.0
37.5
38.0
36.0
Average IG and IR Temperature (Braun max)
36.5
37.0
37.5
38.0
38.5
39.0
Average IG and IR Temperature (Brannan max)
Figure 3.8: Bland-Altman plots showing the average of each pair of measurements
against the difference.
In comparing IR and IG temperature readings, the previous study reported a BlandAltman range of approximately 2 °C, which was considered to represent too wide a
range to allow the IR tympanic temperature readings to be used as a direct surrogate
for IG temperature as a measure of core temperature. In contrast, as can be seen
21
from Figure 3.8, in the best case from the present series of measurements (Braun,
right ear), the range was approximately half of this figure.
As with the previous investigation, further statistical analyses were performed in
order to investigate whether IR temperature readings could be used to provide an
indication of exceeding the limiting criterion of a core temperature of 39 °C. This was
on the basis that it is not necessary to know precisely the actual core temperature but
to know (with reasonable confidence) that the core temperature is unlikely to have
exceeded a safe limit.
Examination of the relationship between the different temperatures obtained using IR
thermometers and the core temperature was done using generalised linear statistical
models. Table 3.8 shows the resulting relationships, all of which were, individually,
significant. As before, the table shows the analysis based on the temperatures
obtained from each ear separately and for the maximum value obtained in either ear
for the two instruments. The best fit was to the Braun IR temperature measured in
the right ear, which is in agreement with the Bland-Altman plots where the Braun
right had the least variation around the mean difference.
Table 3.8: Results of the regression modelling where core temperature is the
response, the tables show the estimated coefficient, its standard error (s.e.) and the
p-value.
Braun - Left ear
Estimate
s.e.
Brannan – Left ear
p
Estimate
s.e.
p
Constant
29.54
3.13
<.001
32.05
1.55
<.001
IR Temp
0.21
0.08
0.014
0.15
0.04
<.001
Braun – Right ear
estimate
s.e.
Brannan – Right ear
p
estimate
s.e.
p
Constant
16.09
3.17
<.001
30.21
2.01
<.001
IR Temp
0.58
0.09
<.001
0.20
0.05
<.001
Braun – Maximum either ear
Brannan – Maximum either ear
Estimate
estimate
s.e.
p
s.e.
p
Constant
24.83
3.35
<.001
31.45
1.67
<.001
IR Temp
0.34
0.09
<.001
0.16
0.04
<.001
As before, with eight pairs of measurements taken from each person, the pairs of
data are not independent from each other. This requires the use of some form of
repeated measures analysis which the Bland-Altman approach does not
accommodate. To attempt to adjust for repeated measurements therefore a mixed
effects linear regression model was fitted to the data, with the participant as a
random effect and the IG temperature as a fixed effect. However, unlike the previous
data set, this did not have any significant influence on the outcome and was not
pursued further.
22
Some authors have advocated the use of the mean of temperature readings from
both ears. Additional statistical analyses were conducted to determine whether this
improved the predictive power of the IR temperature readings. The results were
mixed. For the Braun thermometer there was no improvement over using the right
ear temperature alone. For the Brannan, the mean temperature provided a
marginally better predictive ability than the maximum (R2 = 0.18) but it remained
poor.
Focussing on the results from the right ear using the Braun thermometer, which
clearly gave the most consistent comparisons, figure 3.9 shows the scatter plot of IG
vs IR temperatures using these data. Using confidence intervals in the conventional
manner effectively restricts the prediction to the subjects on which the prediction
model was based. The confidence interval of a predicted IG temperature from a new
IR temperature, (known as the prediction interval or PI), is a lot wider as there is
more variance associated with predicting a future value than with fitting an existing
value. Figure 3.9 also therefore presents the best line through these data, based on
the fitted regression and both the corresponding 95% confidence and prediction
intervals around this line for the inverse regression. The inverse regression is used
because the normal regression provides a prediction of IR temperature from IG
temperature whereas the reverse (predicting IG temperature from IR temperature) is
what is required. Further explanation of this can be found in Graveling et al, (op cit).
Based on these prediction intervals, Table 3.9 shows what the predicted IG
temperature would be for a series of given IR temperatures, together with the 90%
prediction intervals around these values. Thus, it would be predicted that, for a
measured IR temperature using the Braun of 38 °C, the predicted IG temperature
would be 39.05 °C with a prediction range of 38.42 - 39.75 °C. This can be
compared to that derived from the previous report (Graveling et al, op cit) where,
using the same model of thermometer, an IR temperature reading of 38 °C would
predict an IG temperature of 39.25 °C with a much wider prediction range around this
of 37.91 - 40.84 °C.
23
38.0
37.5
O
IR Temperature C (Braun right)
38.5
37.0
36.5
36.0
35.0
35.5
36.0
36.5
37.0
37.5
38.0
38.5
IG Temperature OC
Figure 3.9: Scatter plot of the IG temperature vs. the IR temperature. The solid fitted
line of the inverse regression is shown, along with the confidence interval (dashed
[black] line) and prediction interval (outer [red] lines).
Table 3.9: Predicted IG temperatures with confidence and prediction intervals for
these, given a specific IR temperature.
IR
temperature
O
( C)
Predicted IG
temperature
O
( C)
36
36.3
35.8
36.5
35.5
36.8
37
37.7
37.5
37.7
37.1
38.2
38
39.0
38.7
39.5
38.4
39.7
39
40.4
39.8
41.3
39.6
41.5
IG 90% CI
IG 90% PI
Using this same predictive relationship it can be calculated that, to be confident that
there was only a 5% probability of any individual having a core body temperature in
excess of 39 °C, an IR limit of 37.5 °C would have to be adopted based on these
findings. This can be compared to the previous study which, using the same
criterion, concluded that an IR limit of 37 °C would be necessary. This is clearly an
improvement over the relationship determined from the previous study.
The question of a safe limit for core temperature is the subject of some debate.
Although a limit of 39.0 °C has been used in the analysis described above some
authorities recommend a lower exposure limit.
For example, the WHO
recommendation is for an increase of core temperature of no more than 1 °C. This is
usually taken to indicate a limit of 38.0 °C based on an assumed ‘normal’ body
temperature of 37 °C. This limit of 38.0 °C is also believed to provide the basis for
24
the usually recommended exposure limits for the WBGT thermal index1, used in
assessing the risk of heat strain in an environment. According to the predictive
model therefore, a measured IR temperature of 37.5 °C would indicate an average
IG temperature of 38.4 °C with a lower (5%) limit of 37.8 °C and an upper (95%) limit
of 39.0 °C. Thus, at this indicated IR temperature, slightly more than 5% would have
a core (IG) temperature less than the WHO recommendation. Although clearly not
ideal, this does suggest the possible use of the Braun thermometer to measure IR
temperature as a preliminary screening tool.
As a final part of the investigation, a visual examination of the relationships between
IR and IG temperatures was carried out on an individual subject by subject basis.
This was to determine whether there was any indication that IR temperatures could
be used more reliably with a selected subgroup of people, or with individual analyses.
There was little indication that this was the case and no statistical explorations were
carried out.
1
BS EN 27243. Hot environments - Estimation of the heat stress on working man, based on
the WBGT-index (wet bulb globe temperature). London: BSI.
25
4
Discussion
The overall study aim was to explore the influence of factors affecting the accuracy
and consistency of infra-red (IR) tympanic temperature measurement as a measure
of body (core) temperature and to devise a measurement method aimed at reducing
their influence. This was prompted by the findings of a previous study (Graveling et
al, op cit) that had concluded that the uncertainty of measurement meant that IR
tympanic temperature measurement was not sufficiently reliable to be used to
indicate high body temperatures and therefore risk of thermal strain.
This earlier study had concluded that, to ensure that no more than 5% of individuals
had an IG temperature of over 39 °C, it would be necessary to adopt a maximum
permissible IR tympanic temperature of 37.04 °C. It was concluded that this was
unnecessarily restrictive as, if such a limit was applied to the data obtained in that
study, more than 50% of the employees studied would have to be studied in more
detail (perhaps with more accurate temperature measurement such as the IG pill)
placing a considerable burden on employers (and other burdens on employees)
despite the fact that there was no evidence from IG temperatures amongst those
taking part in the study of any significant thermal strain.
The present study explored the contribution of a number of potential variables on the
acquisition of a reasonably reliable IR tympanic temperature. The initial emphasis
was on consistency rather than accuracy; as a consistent offset in temperature
reading can readily be accounted for.
Pinna pull
Pulling on the pinna, ostensibly to straighten the ear canal, was investigated first.
This did show a slight trend towards the ear pull yielding higher temperatures
although this only approached statistical significance in temperatures obtained from
one ear (the left). Perhaps more importantly, no systematic difference could be
observed in the spread of repeated measurements. On some occasions, with some
subjects, using an ear pull seemed beneficial in that more consistent readings were
obtained. However, on other occasions, sometimes with the same subject, the
reverse applied.
Based upon visual observation of the ear canal and drum using an otoscope it
appeared that pulling on the pinna mainly affected the outermost section of the ear
canal and that insertion of the rigid otoscope cover (and presumably also the IR
temperature probe) had the effect of straightening this section. Any further (deeper)
convolutions of the ear canal could be seen to be unaffected by any acceptable level
of pulling.
Although pulling on the pinna was therefore of little apparent value in obtaining a
more reliable temperature measurement it was noted that it was usually of some help
in inserting the probe into the canal opening, especially where this was hidden by the
tragus of the pinna itself (the fleshy flap to the front of the ear canal opening). It was
therefore considered worthwhile to retain this action as part of the standard
procedure.
Although this perhaps appears contradictory to the ‘received wisdom‘ relating to the
insertion of ear plugs for protection against noise, it should be recognised that, in the
latter case, the plug is inherently flexible and therefore any attempt otherwise to just
26
‘push it home’ is likely to be ineffective. However, contrast this with the use of a rigid,
cone-shaped IR temperature sensor and the different behaviour of the probe can
perhaps be better understood.
Insertion pressure
The next issue to be explored was that of the force used to press the IR temperature
probe into the ear canal. Intuitively, pressing the probe more firmly ‘home’ would, if
nothing else, reduce the distance from the ear drum to the sensor tip and, through
this, possibly result in higher temperatures being recorded. However, this has to be
balanced against any discomfort experienced by those whose temperature is being
measured and consequent reluctance to allow any measurements to be obtained.
As a way of resolving this, a psychophysical approach was adopted in which the
subjects were instructed to exert the level of pressure they considered to be the
maximum acceptable by pressing their head against the firmly held probe once it had
been inserted into the ear canal. Care had to be taken to ensure that the probe
angle remained the same whichever approach was being used.
It was noted that, in adopting this approach, some subjects applied little discernable
extra force, suggesting that the standard force applied was already close to what
they considered to be the maximum acceptable. In contrast others would press very
much more firmly, requiring considerable resistance to stop the thermometer from
moving backwards.
As with the ear pull, there was a slight tendency for the higher pressure to result in
higher temperature readings being obtained on average. However, the effect was not
consistent and did not therefore result in any statistically significant comparisons.
Similarly there was again no common pattern to the consistency of the readings
obtained with or without the additional pressure, even within a subject on different
occasions. Although not explored statistically, on examination of the data there was
no discernable pattern relating the degree of additional force exerted by the subject
to the consistency of the readings obtained.
It appears likely that this apparent ‘failure’ can be related to the degree of force used
in the standard procedure. In taking a measurement it is clearly necessary to ensure
that the sensor is correctly inserted into the ear canal. With most ear canals
observed not being straight, this requires a reasonable degree of initial pressure to
straighten the canal thus allowing the probe tip to be fully inserted. In taking a
reading, the probe is inserted ‘fully’, to the extent that it forms physical contact with
the outer ear around its whole circumference. These two measures themselves
therefore ensure that a reasonable degree of force is applied, even with the standard
technique. Examination of the outer ear shows that, once inserted to this point, the
probe rests against the relatively incompressible cartilage/bone surrounding the
opening to the ear canal. At least in the ears observed in this study there would not
usually be a significant thickness of skin or subcutaneous tissue in this area which
would be more compressible. Pressing the probe more firmly against the head does
not therefore result in appreciable inward movement of the sensor so that the
sensor/ear drum distance is little different. Furthermore, with such little movement,
the scope for any further straightening of the ear canal is restricted, making it unlikely
that the higher force will result in any appreciable improvement of the ‘visualisation’
of the ear drum by the sensor.
It would seem therefore that, as long as the sensor probe is firmly inserted, sufficient
to result in a visual seal against the skin of the outer ear around the circumference of
27
the probe, there is no benefit in applying additional force in inserting it and taking a
reading.
Interval between insertions
There is some suggestion that inserting the head of the thermometer into the ear
canal has a potential cooling effect (assuming that the environmental temperature is
lower than body temperature, resulting in erroneous readings. Thus, according to
one manufacturer, the probe at room temperature has a cooling effect and ‘can
result’ in an inaccurate temperature reading being obtained. It is not known whether
or not this is based on any clinical measurements or a marketing ploy to justify their
apparently unique selling point of a heated tip to the sensor probe. The model of
thermometer used for the present studies (Braun Thermoscan 6022) features this
technology.
The results from leaving different time intervals between any two readings suggest
that, for this device at least, any cooling effect did not introduce any meaningful
variation. Although the overall mean IR temperature obtained with 10 second
intervals between readings was statistically significantly lower than those obtained
with 2 minute (120 secs) intervals the difference was less than 0.1 °C, which is the
limit of resolution of the display. The values obtained without withdrawing the probe
between readings (designated as 0 secs between readings) was statistically
significantly higher than those with any other time interval although again the
absolute differences were relatively small, with a difference of 0.1 °C between the
mean for 0 secs and that for the next highest (120 secs).
Theoretically, any impact of inserting a relatively cold instrument into the ear canal is
likely to be highest when that sensor remains in place for longer. It would therefore
be hypothesised that the first temperature reading obtained would be higher than
those taken subsequently as any cooling effect has its influence. As an additional
check for this, the temperatures obtained through the sequences of six readings
obtained from each subject without withdrawing the probe were analysed. This
analysis identified no statistically significant variations, with the first reading obtained
not being significantly different from those taken subsequently. It is not known
whether this is attributable to the benefits of the heated tip technology, or simply that
cooling of the ear canal by the measurement probe is not a significant issue. It would
appear therefore that there is little advantage of introducing an artificial delay in the
time intervals before taking any repeat measurements.
Measurement protocol selected
Drawing together each of these elements, a standard measurement protocol was
devised which was then used to compare IR temperature readings with IG
temperatures as a gold standard. As the IR sensor relies on a ‘line of sight’ with the
ear drum, the ability to visualise the ear drum was clearly vital and the preliminary
use of an otoscope was adopted as part of the standard procedure. It was apparent
from these sightings that the optimum orientation for the sensor could vary to a
considerable extent between individuals.
An ear pull was usually helpful in initially inserting the sensor probe although not
apparently having any impact on the temperature reading subsequently obtained. A
pull was therefore adopted as part of the protocol.
28
There was apparently little value in attempting to apply additional pressure with the
IR thermometer beyond that necessary to ensure an adequate seal of the probe
within the ear canal and no additional force requirements were included.
There also appeared to be no value in seeking to pause between readings or
otherwise control the timing between a sequence of readings.
Additional observations
Although it was usually possible to gain some sight of the ear drum with an otoscope
there were marked differences between subjects in the proportion of the drum which
could be visualised at all and in the ease with which this visualisation could be
achieved. In some instances an individual’s ear canal seemed to be relatively
straight and a good sight of a large area of the drum was readily achievable. With
others, an unobstructed view could not be achieved at all and there was part of the
wall of the ear canal, or wax, within the optimum otoscope field of view. It is not
known how large a ‘view’ the IR thermometer takes or requires in obtaining a
reading. Clearly the sensor will respond to the IR radiation it senses, wherever that
comes from and, if the field of view of the sensor includes waxy deposits or at least
part of the wall of the ear canal, then that will be integrated into the temperature
reading obtained.
No attempt was made to exclude subjects for whom a full (or reasonably full) view of
the ear drum could not be obtained, on the basis that excluding such people would
not be an option in an occupational setting. However, although it was not possible to
examine this formally it did not seem, in obtaining IR temperatures, that those with
less visible ear drums were giving systematically lower values. Certainly this was the
impression gained from carrying out a visual examination of the data plots for each
separate individual.
Previous studies have suggested a possible direct influence of the environmental
temperature on the IR readings obtained. For example, when used in a clinical
setting, the advice given is to avoid taking measurements from an ear that the patient
has recently been lying on. Although not formally studied in the present series a
slight tendency was noted for a greater mismatch between IR and IG when an
individual had recently arrived at work, having cycled to work in cold conditions. It
can be hypothesised that core temperature would have been slightly elevated by the
physical activity and the tissues of the face would have been cooled by the cold
conditions, thus maximising any difference between IR and IG temperatures.
However, this phenomenon could not be not systematically examined and should
only be regarded as anecdotal evidence.
Differences between thermometers
Two different thermometers were used, from two different manufacturers. The
statistical analysis identified the Braun thermometer as providing temperature
readings with a more consistent relationship with IG temperature than those obtained
using the Brannan thermometer. In the earlier study (Graveling et al, 2009) one of
the problems identified was that the relationship between the IR and IG temperatures
was temperature dependant, with the regression predicting that, below temperatures
of around 38 °C, the Braun would give a lower temperature than IG whilst, at higher
temperatures, it would give a higher temperature than IG. This phenomenon was
much less apparent with the Braun thermometer in the current series of
measurements (using the same model of thermometer), although the Brannan
thermometer appeared to be less consistent. One reason for this was the occasional
29
tendency for the Brannan thermometer to give what were clearly erroneous readings
(a maximum of 39.4 °C was recorded on one occasion). Any explanation for this
would be purely speculative. It is known, for example, that some such devices
incorporate circuitry which requires a minimum level of stability in the IR levels
received before displaying a result. In the absence of any such provision it is
possible that movement of the probe in the ear canal created some form of artefact.
Care should be taken in placing too much emphasis on these findings, based on a
sample of one example of each instrument. Nevertheless, the observations that
different makes and models of IR thermometer give different tympanic temperature
readings and that some are more consistent than others in those readings are not
new. Clearly, it is a matter of some concern that the instrument used to obtain a
measurement can play a potential role in determining the reliance that can be placed
on the values obtained.
‘Handedness’ of results
Throughout the series of studies, differences were often noted in the temperatures
obtained from the two ears of each subjects. This has been noted previously in the
medical literature. For example, Stavem et al, (1997) reported that, in taking IR
tympanic temperature measurements, the accuracy compared to the pulmonary
artery benchmark was improved by using the average measurement from each ear.
It was also found that, using the maximum of the two ear measurements obtained
also improved the accuracy of prediction, albeit to a slightly lesser extent.
In that particular study, measurements from the right ear seemed to be slightly more
variable. In contrast, in the present study, when contrasted with IG temperature,
measurements obtained from the right ear seemed more consistent and therefore
provided a potentially more accurate estimate of IG temperature. There is some
precedent for this. Weiss et al, (1998), did report an influence of handedness on the
temperatures obtained, although the relationships were complex. Using the mean
temperature or highest value from both ears did not improve the predictive
relationship. No physiological explanation has been found for any systematic
difference in ear drum temperatures or ear canal shape. The most plausible
explanation therefore is that of some form of interaction between the handedness of
the researcher and the ear being measured.
Overall outcome
The overall outcome of the refinement of the measurement technique was a marked
improvement in predictive ability over previous attempts to devise a stable
relationship between IR and IG temperatures. The results indicated a reasonably
stable mean offset which could readily be factored in to any measurement regime.
Although the potential error around the mean prediction was much reduced it
remains relatively high and certainly IR temperature cannot be used to predict actual
core (IG) temperature with any degree of clinically acceptable accuracy.
However, the potential value of the technique lies in its use as an early warning, to
flag a need for more detailed investigations. Based on the predictive relationship
determined in this study, if the usual convention of protecting 95% of the workforce is
adopted, an IR tympanic temperature greater than 37.5 °C would have to be used.
On the basis of the results from the present study, this would be above the
temperature to be expected from sedentary subjects in thermally neutral
environments.
30
Using the predictive relationship determined in this study, at an IR temperature of
37.5 °C, the vast majority of the workforce would have a core body (IG) temperature
between 38 °C and 39 °C although a minority (around 10%) would have lower
temperatures and 5% higher. Given that a core body temperature of 38 °C
represents the conservative limit currently adopted by some authorities it would seem
that this would represent a viable, workable, limit. However, the dependency of the
relationship on factors such as the type of instrument used, the handedness of the
measurer, and the precise measurement technique adopted means that care should
be taken over recommending its widespread use at present. These factors need to
be explored further, as does the consistency of the predictive relationship at differing
environmental temperatures.
31
5
Conclusions
Investigations were carried out to systematically examine different aspects of the
technique used to obtain measurements of tympanic temperature using proprietary
infra-red (IR) thermometers. This was prompted by earlier work which suggested
that measurement of ear canal temperature in this way was not sufficiently reliable to
be used as a surrogate for more invasive and complex methods in industrial
situations.
A series of studies were carried out to refine the technique used, followed by a
comparative study in which IR temperatures were compared to intra-gastric (IG)
values (obtained using an ingested temperature sensitive pill).
The results showed that the technique devised as a result of these studies did give
more reliable results than those found previously. The predictive relationship
determined showed that IR tympanic temperature could not be used to predict core
body temperature with a sufficient degree of accuracy for it to be used to detect
excessive heat strain in industry. However, at an IR temperature of 37.5 °C
approximately 85% of people would have an IG (core) temperature of between 38 °C
and 39 °C with approximately 10% having a lower temperature and 5% a higher
temperature. It is suggested that this provides a possible basis for the use of IR
thermometry as a screening tool in monitoring hot workplaces for possible risks of
thermal strain.
These results were obtained from a limited number for subjects, in a restricted range
of thermal environments and were obtained by a single researcher. It remains to be
seen whether the technique can reliably be applied by others, taking measurements
from more people in a wider range of climates. The role of the make and model of
thermometer also requires further investigation.
32
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34
Appendix 1: Record sheet devised for recording IR
thermometer placement based on visualisation of the
ear drum using an otoscope.
Ear canal temperature – orientation of thermometer
Subject Name………………………………
Date of observation……………………
Left
Right
Using clock face, indicate direction tip of sensor points towards. “
→”
Estimated angle of inclination (relative to perpendicular from side of head):
LEFT: ……………..
Ear tug:
LEFT: YES
RIGHT:………………..
NO
RIGHT:
YES
NO
(delete as applicable)
Using clock face, indicate direction of any lateral movement of barrel of sensor. “#”
Check temperatures:
1
2
3
35
Published by the Health and Safety Executive
10/13
Health and Safety
Executive
The use of infra-red (tympanic) temperature
as a guide to signs of heat stress in industry
Previous IOM research, showed that the use of a simple infrared (IR) ear thermometer did not provide a reliable prediction
of core body temperature for use in industrial situations
(Graveling et al, 2009). The aim of this research was to explore
the use of an IR ear thermometer further, and to determine
whether the consistency and accuracy of the measurements
obtained could be improved sufficiently to provide a reliable
indication of the risk of an individual suffering from heat strain.
Published studies where IR temperature has been compared
with core temperature benchmarks such as rectal temperature
to provide further detail on likely sources of variation in
measured tympanic temperature were re-examined. Based
upon the factors identified, a number of experimental studies
were carried out to explore the influence of these factors,
together with revised measurement methods aimed at
reducing their influence.
The results showed that the technique devised as a result of
these studies did give more reliable results than those found
previously although the predictive relationship determined
showed that IR tympanic temperature could still not be used
to predict actual core body temperature with a sufficient
degree of accuracy for it to be used in industry. However it is
suggested that this provides a possible basis for the use of IR
thermometry as a screening tool in monitoring hot workplaces
for possible risks of thermal strain.
This report and the work it describes were funded by the
Health and Safety Executive (HSE). Its contents, including
any opinions and/or conclusions expressed, are those of the
authors alone and do not necessarily reflect HSE policy.
RR989
www.hse.gov.uk