ELSEVIER
Energy and Buildings
28 ( 1998)
15-23
1Jpper limits of air humidity for preventing warm respiratory discomfort
J@m Toftum *, Anette S. Jgrgensen, P.O. Fanger
of Indoor
Laboratory
Environment
and Energy,
Department
Lyngby
Received
of Energy
DK-2800,
3 March
Engineering,
Denmark
1997; accepted 9 March
Technical
University
of Denmurk,
Building
402,
1997
Abstract
‘The effect of humidity and temperature of inhaled air on perceived acceptability of the air was studied. Thirty-eight subjects evaluated air
at 14 combinations of temperature and humidity to verify that insufficient evaporative and convective cooling of the mucous membranes in
tha upper respiratory tract is a cause of local warm discomfort and of a perception of poor air quality. Unpolluted,conditioned air was led
from a climate chamber to a box where the subjects one by one evaluated the air. The inhaled air was rated warmer, more stuffy and less
acceptable with increasing air humidity and temperature. A model was developed that predicts the percentage of persons dissatisfied due to
insufficient respiratory cooling as a function of the actual evaporative and convective cooling of the respiratory tract. Togetherwith a previously
proposed mode1 for predicting discomfort due to high skin humidity, the respiratory model may be used to specify upper limits for humidity
0 1998ElsevierScienceS.A. All rightsreserved.
in the indoor environment.
Ke words:
-
Respiratory
cooling;
Humidity
limit;
Local discomfort;
Air quality;
1. Introduction
Humidity may be a causeof discomfort due to inhalation
of humid and warm air or to an uncomfortably high level of
sk,!nhumidity. In the present study, human subject experiments were conducted to investigate the effect of humidity
an’dtemperatureof inhaled air and of skin humidity on percelved discomfort. This paper presentsthe results of the
experiments on humidity and temperatureof inhaled air on
discomfort, whereasdiscomfort causedby skin humidity will
be discussedin a separatepaper [ 81.
In humans,the respiratory tract actsasan air-conditioning
system that regulatesthe humidity content and temperature
of the inspired air on its way to the lungs. The site of the
conditioning processis determinedby the pulmonary ventilation rate and the temperature and water content of the
inhaled air [ 11.With air at temperaturesandhumiditiescommonly found indoorsand at low pulmonary ventilation rates,
thr, main part of the conditioning takes place in the upper
respiratory tract. In most indoor environments, inhalation of
air will causea cooling of the mucous membranesin the
upper respiratory tract, which contributes both to the perception of the thermal environment and to the perceived air
quality. Cooling takesplace through a combination of con-* lorresponding
e-nlail: [email protected]
author.
0378.7788/98/$19.00
PIrSO378-7788(97)00018-2
0
Tel.:
f45
1998 Elsevier
45 254028;
fax:
+45
45 932366;
Science S.A. All rights reserved.
Inhaled
air
vective and evaporative heat loss. A low temperatureof the
inhaled air results in greater convective cooling and low
humidity causesgreater evaporative cooling. At high air temperatures and humidities, the respiratory cooling will
decreasewith the result that the air may be perceivedasstuffy
and uncomfortable. Berglund and Cain [2] consideredthe
possibility of supplying cool and dry air to a spaceto alleviate
the perception of poor air quality, without even removing
contaminants in the air. In their study, Berglund and Cain
showed that air freshness and acceptability perceptions
decreasedwith increasingtemperatureand humidity. In particular, relative humidities above 65% were associatedwith
unacceptableair quality judgments.Thesefindings were confirmed by Fang et al. [ 31, who showedthat air polluted by
different building materials and unpolluted background air
were found lessacceptablewhen temperatureand humidity
were increased.In addition, the impact of temperature and
humidity on perceived air quality dominatedthe influence of
the pollution sourcesat high enthalpiesof the inhaled air.
In standardsprescribing acceptablethermal indoor environments,upper limits for humidity have beenspecifiedthat
were unrelatedto thermal comfort or related solely to discomfort causedby too high levels of skin humidity [ 4-71. In a
separatepaper by the authors, a model was introduced that
predicts discomfort due to high skin humidity in the comfort
range of temperatures[ 81. This skin humidity model can be
16
J. Tqfium et al. /Energy and Buildings 28 (19Yll) 15-23
used to determine discomfort due to high skin humidity at
many combinations of environmental parametersand clothing characteristicsfor sedentarypersons.In the comfort zone,
the model allows high air humidities without causing substantial discomfort from humid skin. The upper limit for
indoor air humidity may be determinedrather by discomfort
due to inhalation of humid or warm air.
The purposeof the presentstudy was to verify that insufficient respiratory cooling is a causeof local thermal discomfort and to develop a model predicting the percentage of
dissatisfiedpersonsasa function of respiratory cooling. Respiratory cooling is anticipatedto be a function of temperature
and humidity of the inhaled air.
2. Experimental method
The impact of temperatureand humidity on perception of
inhaled air was studiedat I4 combinationsof air temperature
andrelative humidity asshownin Table I. For eachcondition,
human subjectsassessed
the air with respectto thermal sensation,freshnessand acceptability.
2. I. Facilities
Air from a climate chamberdesignedfor air quality studies
[9] was presented to the subjects in a small box placed
outside the chamber. By controlling the temperature and
humidity of the air in the chamber,the desiredcondition in
the box could be provided. By meansof a glasstube system
containing a fan, the air wasled from the climate chamberto
the box, where it wasassessed.
The air moved upwardsin the
box and the air motion resembleda piston flow. To promote
a homogeneousair velocity profile, a perforated plate was
mounted acrossthe air inlet in the bottom of the box. The
assessments
were madein the center of the box where the air
velocity (without subject) was 0.25-0.30 m/s. The box was
made of stainlesssteel with dimensions0.95 X0.34X0.29
m3.
Air temperature and humidity in the box and inside the
climate chamber were monitored continuously by temperature andhumidity transmitters.The transmittermeasures
temperatureby meansof a Pt- 100 sensorin the range - 20°C to
80°C with an accuracy of +0.2”C and relative humidity in
the range 0% to 100% with an accuracy of rt 2% rh in the O90% rh range and + 3% rh in the 90-100% rh range. Air
temperature and humidity were carefully controlled before
and during eachseriesof assessments.
Betweenassessments,
the subjectsrestedin the sameroom
as that in which the box was located. Air temperature, air
velocity and humidity in the waiting room were monitored
continuously during the experimental period.
2.2. Subjects
Thirty-eight subjects, 19 females and 19 males, most of
them university students,were paid to take part in the exper-
Table 1
Combinations
the subjects
of temperatures
Air temperature
and relative
humidities
of the air assessed by
(“C)
Relative
humidity
(S)
Partial vapor
pressure
(Pa)
20
20
45
70
1053
36.7
20
23
23
90
50
1638
2106
1406
46.1
53.6
45.4
70
90
35
1968
2530
1177
54.5
63.6
44.8
55
70
90
25
1850
2355
3027
55.6
63.9
75.0
1002
40
55
70
1603
45.0
54.7
64.5
74.4
23
26
26
26
26
29
29
29
29
2204
2805
Enthalpy
(kJ kg-‘)
iments.All subjectswere volunteerswho gave their informed
consentprior to the experiments.Anthropometric data for the
subjectsare shown in Table 2.
To maintain a low skin humidity and a thermal sensation
near neutral, the subjectswere dressedin a thin, one-layer
experimentalclothing ensemblemadeof cotton. The clothing
insulation value of the ensemblewas not measured,but was
estimatedto approximately 0.5 clo. The cotton fabric had a
very low resistanceto diffusion of water vapor ( N 2 mm still
air equivalent). In addition, the subjects wore socks and
highly moisturepermeablecotton shoes.
2.3. Experimental
procedure
The experimentswereconductedin four repeatedsessions,
in the mornings and afternoons on two consecutive days.
Between eight and twelve subjectsparticipated in each session.On arrival, the subjectsput on the experimentalclothing
ensembleand were then seatedin the waiting room outside
the climate chamber.Before the first assessment
of the air in
the box, the subjectsevaluatedtheir generalthermalsensation
and perception of skin humidity, and the air in the waiting
room wasjudged asregardsthermal sensation,freshnessand
acceptability. The subjectsthen in random order approached
the box, positionedthe headinside the box and evaluated the
inhaled air after three to four inhalations according to the
Table 2
Anthropometric
data for the subjects
Sex
Age
(Y)
Height
Cm)
Weight
(kg)
Dubois
(m-3
Females
Males
Females and
males
23.0f3.1
22.s*
1.9
1.70*0.05
1.82*0.07
1.76kO.08
67.6f7.7
77.5 *9.9
72.6+
10.1
1.78+0.10
22.8 f 2.6
1.97f0.14
1.881tO.16
area
J. To&m
et al. /Energy
and Buildings
questionnaire shown in Fig. 1. Fig. 2 shows a subject prior to
ard during evaluation of the air in the box. After a subject
had finished an assessment and completed a questionnaire,
he.or she was seated and another subject began an assessment.
When all subjects had evaluated air in one condition, the air
temperature and humidity in the climate chamber were
changed to new levels. After 10 to 1.5 min, when a stable
condition inside the box had been reached, the assessment
procedure was repeated. Due to limitations in the climate
chamber air-handling system, the conditions in the box could
not be presented to the subjects in random order without
unacceptably long waiting times. Instead, the order of conditions shown from top to bottom in Table 1 was used. The
subjects made five to six assessments of the air in the box per
hour and were asked to evaluate the conditions in the waiting
room every half hour during the experimental period.
2.4. Calculations
Heat and moisture transport in the upper respiratory tract
are complex processes.The cooling of the mucous membranes during inhalation is proportional to the temperature
and water vapor gradientsfrom the surfaceof the respiratory
tract to the inhaled air. On its way to the lungs, the inspired
air passesthe nostrils, the nasalpassageandthe nasopharynx
3
How do you thermally rate
me air m the box?
WarIll
1
Sightly
NeUfral
Slightly cwl
-2
COOI
-3
Cold
Fig. 1, Questionnaire
At the mucosalsurface,the condition for water vapor concentration is assumedto be saturation, which is a function of
warm
0
Neither
concerning
Fig. 2. A subject prior and during
inhalation
thermal
lresh nor stuffy?
HOW do you rate the mermal
acceptabilAy
of the air in the box:
I
I
sensation, freshness and acceptability
of conditioned
17
C,,,=K,(30-t,)
Fresh?
-1
15-23
located at the back of the nasalpassage.The cooling of the
mucous membranesat the surface of the nasal passageis
presumedto be decisive for the thermal perception of the
inhaled air. The surface temperature of the nasalpassageis
therefore important for an empirical correlation of respiratory
cooling and subjective thermal perception of the air. Air
entering the alveoli has a temperature near that of the body
(37°C) ] lo]. With nasalbreathing, experimentshave shown
that the temperature of inspired air at 23’C increased to
32.4”C in the nasopharynx [ 111.With forced breathing (high
activity), the temperature of ambient air at 27°C rose from
28.3”C within the nostrils to 32°C in the nasopharynx. Thus,
for use in the presentstudy, an average surface temperature
of the nasal passageof 30°C was anticipated for sedentary
subjectsin comfortable environments. Inside the nasalpassage, the temperature of the inhaled air changesalong the
path to the nasopharynx, simultaneouslyaffecting the heat
transfer between the inner surface of the nose and the air.
Nevertheless,for the purposeof the presentstudy this temperature change was disregarded, and the dry respiratory
cooling thus approximated by
UC you lee1 the a,, KI the box to ix
Hot
2
28 (1998)
Acceptable
Just acceptaMe
Just unacceptable
Unacceptable
of the inhaled air.
air in the assessment box outside the climate
chamber.
18
J. Tojtum et ul. /Energy
and Buildings
the surface temperature [ IO]. The mucosal lining consists of
95% water and 5% glycoprotein [ II]. The saturated vapor
pressure at the surface of the mucosal lining therefore deviates
slightly from saturated vapor pressure of water in air at 3O”C,
which is 4250 Pa. This difference is presumed to be of only
little importance, and therefore the latent respiratory cooling
was evaluated by
E,,,=K,(42.5-O.Olp,)
The constants K, and K2 are functions of averageconvective and evaporative heat transfer coefficients, respectively,
and specific properties of air and water vapor. t, is the temperature and pa the water vapor pressureof the surrounding
air. Adding C,,, and E,,, provides an expressionfor the total
respiratory cooling. Basedon theseassumptions,the thermal
perception of the inhaled air may be linked to the respiratory
cooling by the expression
Y_f[K,(30-r,)+K,(42.5-O.Olp,)]
in which Y in this study designatessubjective thermal sensation, freshnessor acceptability of the inhaled air (Fig. 1).
2.5. Data processing
Multiple linear regressionanalysis was used to correlate
the driving potentials for convective and evaporative heat
losses with subjective perception of the inhaled air. As
dependent variables, the average for all subjects’ (N= 38)
votes of thermal sensation,freshnessand acceptability were
used at each assessedcombination of air temperature and
humidity.
The acceptability scale shown in Fig. 1 may be used to
record both a pseudo-continuousacceptability responseand
a binary dissatisfactionresponse.A mark put on the upper
half of the scaledesignatessatisfactionand on the lower half
dissatisfaction. From assessments
recorded with this scale,
logistic regressionanalysiswasusedto correlate the driving
potentialsfor convective andevaporative heatlosseswith the
percentage of dissatisfied due to decreased respiratory
cooling.
3. Results
At all exposures,the temperatureand the relative humidity
in the box deviated lessthan 0.2”C and 3% rh, respectively,
from the planned levels shown in Table 1. The discrepancy
in experimentalexposuresbetweensubjectsand sessions
was
thus negligible. The experimental condition at 29”C, 25% rh
was evaluated by 21 subjectsonly, as the climate chamber
wasunableto condition the air to the desiredlevel of humidity
in two of the four sessions.
Multiple linear regressionwith the driving potential for
convective ( 30 - t,) and evaporative (42.5 - 0.0 II),,) heat
transfer asexplanatory variableswasconductedwith thermal
28 (19YXj IS-23
meanvote, freshnessvote andacceptability vote asdependent
variables. The applied regressionmodel was
V=a+h(30-t,)+c(42.5-O.Olp,)
The results of the regressionanalysis are shown in Table
3. The coefficients describing the sensibleand latent heat
transfer between the surface of the nasal passageand the
inhaled air (K, and K2) are containedin the regressioncoefficients b and c. The regressionequationslink the convective
and evaporative heat transfer processesto subjective thermal
perceptionof the inhaled air. For fully suppressed
respiratory
cooling (t;, = 30°C andpa= 4250 Pa) the empirical equations
predict votes very near the end points of the applied scales
(warm, stuffy, unacceptable).
Fig. 3 showsthe meanthermal sensationvote asa function
of the potential for evaporative cooling at the four applied
levels of convective cooling. At increasing convective and
evaporative cooling, the subjectssensedthe air as cooler. In
addition, the inhaled air wasperceived increasingly stuffy at
decreasedevaporative and convective cooling (Fig. 4).
Accordingly, the acceptability of the air washigher the cooler
and more fresh the air was rated, i.e. at low air temperatures
and humidities (Fig. 5). In Table 4, the percentageof dissatisfied among the 38 subjectsdue to decreasedrespiratory
cooling is shown at the air temperaturesand water vapor
pressuresto which they were exposedin the box.
During all experimental sessions,the air temperaturein the
waiting room rose from approximately 23.5-24°C at the
beginning of a sessionto 25”-25.5”C at the end (average for
all sessions).The relative humidity varied in a rather narrow
interval between 45% rh and 50% rh. It was attempted to
maintain a dry skin and an overall thermal sensationfor the
body around neutral. Fig. 6 shows thermal sensationand
sensationof skin humidity, averagedfor all 38 subjects,asa
function of time. Thermal sensationvaried between neutral
and slightly warm, whereasthe sensationof skin humidity
was very near normal throughout the experimental period.
When the subjects evaluated the air in the box, they may,
consciously or unconsciously, have compared this with the
air in the waiting room. Therefore, somebias from the conditions in the waiting room on the assessments
of the air in
Table 3
Regression coefficients
determined by multiple
IIIdl
mean vote, freshness vote and acceptability
Y
Thermal mean vote
Freshness mean
vote
Acceptability
mean
vote
n
2.95
28.2
- I .06
linear regression with thervote as dependent variables
b
‘
R2
-0.171
-0.60
- 0.074
-0.46
0.98
0.96
0.038
0.99
0.046
Thermal vote: - 3 = very cold, 3 = hot.
Freshness vote: 0 = fresh, 26 = stuffy.
Acceptability
vote: I = acceptable, - 1 =unacceptable.
J. Toftum
et al. /Energy
and
Buildings
28 (199X)
15-23
SI. warm
3>
s
t
Neutral
0
3
!i
E
Cool
-,
!
SI. cool
-2 1
10
25
20
42.5-O.Olp.
15
30
35
Fig. 3. Mean thermal sensation of inhaled air (N= 38) as a function of the vapor pressure difference (42.5 - O.Olp,)
be:ween the surface of the nasal passage and the air. The dotted lines show predictions made by the regression equation
15
20
25
30
42.5-O.Olp.
Fi;;. 4. Mean freshness vote of inhaled air (N= 38) as a function of the vapor pressure difference (42.5 -O.Olp,)
the surface of the nasal passage and the air. The dotted lines show predictions made by the regression equation
Acceptable
and temperature difference
given in Table 3.
1
30-k
(30 - r,) between
("C)
0 10
q7
‘.
+4
b
&
...
0.0 ,:.
0
d
.. . . .
t
.-b
.,
Al
+
!
,. .-a
:’
.‘.
..’
-0.5 -c
Unacceptable
(30 - t,)
35
0.5
fi
0.
8
9
and temperature difference
given in Table 3.
+
A’
-1
10
15
20
25
42.5-0.01~.
30
35
Fig. 5. Mean acceptability
vote of inhaled air (N= 38) as a function of the vapor pressure difference (42.5 - O.Olp,)
be! ween the surface of the nasal passage and the air. The dotted lines show predictions made by the regression equation
the box was expected, although in what way was unknown.
Fig. 7 shows thermal sensation, freshness and acceptability
of the air in the waiting room as time progressed. Initially,
and temperature
difference
given in Table 3.
(30 -
t,)
the thermal sensation of the air was very near neutral, but
increased to between neutral and slightly warm. During the
course of experiments, the air in the waiting room was per-
J. Toftum et al. /Energy
20
Table 4
Air temperature,
water vapor pressure and the percentage of subjects
satisfied with the thermal perception of the inhaled air at this condition
Air
temperature
(“C)
Water vapor
pressure
(@a)
Percentage
dissatisfied
(%I
20
20
20
23
23
23
26
26
26
26
29
29
29
29
1053
1638
2106
1406
1968
2530
1177
1850
2355
3027
1002
1603
2204
2806
11
11
21
26
18
50
18
45
55
82
19
42
68
76
and Buildings
4. Discussion
of
Both temperature and humidity of the inhaled air had an
impact on the human perception of respiratory thermal sensation, freshness and acceptability. The subjects perceived
the inhaled air as cooler, less stuffy and more acceptable at
low temperatures and humidities. The results of the present
study substantiated the hypothesis that the sensation of air
humidity is linked to the cooling of the respiratory tractduring
inhalation. Overall thermal neutrality may be attained at
numerous combinations of the thermal environmental parameters, even at high air humidities. But local thermal discomfort may still be caused by for example cold or warm floors
or by too high vertical temperature gradients. Requirements
have therefore been specified for the surface temperature of
floors and for maximum acceptable vertical temperature gradients in order to avoid these types of local thermal discomfort
[4,6]. Likewise,
requirements for the temperature and
humidity of the air need to be specified in order to avoid local
thermal discomfort due to insufficient cooling of the mucous
membranes in the upper respiratory tract. Thus, the experimental data presented in Table 4 were used to correlate the
driving potentials for convective and evaporative heat trans-
-_-__.--
1.0
-~
5 SI. humid
-*Skin
Neutral
3 SI. dry
30
sensation of body and perceived
0
Fig. 7. Mean thermal
sensation,
humidity
0.0
0
Fig. 6. Thermal
15-23
dis-
ceived as neither stuffy nor fresh. Also, the air was rated
rather close to acceptable (0X3-0.7). Altogether, the subjects
assessed the air in the waiting room as being satisfactory.
SI. warm
28 (1998)
freshness and acceptability
60
Time (min)
humidity
30
90
120
of skin. Votes cast every
60
Time (min)
of the air in the waiting
half-hour
so
room, Votes cast every
during
the experimental
sessions.
120
half-hour
during the experimental
sessions.
J. Toftum et al. /Energy
and Buildings
100
l+exp[-3.58+0.18(30-r,)+O.l4(42.5-O.OlpJ]
21
15-23
The model was developed on the assumptions that the
average surface temperature of the nasal passage was 30°C
and that the condition at the surface was saturation. The
temperature of the nose depends to some extent on the environmental temperature and on the degree of vasoconstriction,
which for persons feeling cold is typically high on the face.
Nevertheless, the paranasal sinuses (mucosal-lined air spaces
surrounding the nasal passages) may serve as insulators helping to maintain a relatively constant temperature within the
nose [ lo]. The proposed model is valid for sedentary, thermally neutral persons having a low skin humidity and for first
impressions of air acceptability. On respiration, the mucous
membranesin the upper respiratory tract alternately are
cooled by inhalation of the surroundingair and heatedduring
exhalation by the warm and humid air coming from the alveoli. The period of the respiratory cycle is aroundfour seconds
(sedentary activity) implying that a steady-statecondition at
the mucosalsurface will never occur. Adaptation to insuffi-
fe.r with the percentage of dissatisfied due to decreased respiratory cooling (PD) using logistic regression analysis. The
analysis resulted in a model for predicting the percentage of
d; ssatisfied due to decreased respiratory cooling:
PD=
28 (1998)
%
Observed and predicted percentages of dissatisfied are
compared in Fig. 8. The sample correlation coefficient
between observed and predicted percentages of dissatisfied
was 0.97. The model is based on assessments of air at temperatures in the range 20-29°C and water vapor pressures
ranging from 1000 Pa to 3000 Pa. Fig. 9 shows combinations
OFair temperature and humidity which cause lo%, 20% and
30% dissatisfied due to decreased respiratory cooling as pred,cted by the model. The figure specifies rather stringent
requirements for the temperature and humidity content of the
inhaled air, even for unpolluted air.
%
40
60
100 %
80
Dissetisfled (observed)
Fig. 8. Comparison
sample correlation
of observed
and predicted percentages of dissatisfied due to decreased
between observed
and predicted percentage of dissatisfied was 0.97.
19
20
21
22
23
Air temperature
Fi ;. 9. Combinations
of air temperature
the logistic regression model.
and humidity
corresponding
respiratory
24
cooling
25
under all experimental
26
conditions
studied. The
27
(“C)
to IO%, 20% and 30% dissatisfied
due to decreased
respiratory
cooling
calculated
from
.I. Toftum el ~1. /Energy
22
and Buildings
28 (1998)
15-23
50
60
0.8
0.6
0.4
g
0.2
33
0
:: -0.2
Q
-0.4
-0.6
-0.8
-1
0
10
20
30
Enthalpyofair
Fig. 10. Comparison
of relationships
between
acceptability
40
of inhaled air and enthalpy
cient respiratory cooling is therefore unlikely to occur. Also,
no change in respiratory cooling over time is expected insofar
as the nose can maintain the mucosal lining and the temperature of its inner surface.
A 1°C change in air temperature had the same effect on
acceptability and freshness as 12 1 and 130 Pa change in vapor
pressure, respectively. For thermal sensation, a 1°C change
in air temperature corresponded to 23 1 Pa change in vapor
pressure, i.e. the relative influence of air temperature on thermal sensation was almost twice that on freshness and acceptability. Consequently, the relative influence of humidity on
freshness and acceptability was higher than on thermal sensation. Dry ambient air at a high temperature may therefore
be perceived as warm without being stuffy and unacceptable.
Humid ambient air at high temperature, on the other hand,
will be perceived as being warm, stuffy and unacceptable.
Berglund and Cain [ 21 found that, on average, a 1“F change
in air temperature had the same effect on staleness (freshness) as a 6°F change in dew point temperature. In the present
study, a 1°C change in air temperature on average had the
same effect on freshness as 1.1“C change in dew point temperature. In this study, the observed relative weight of humidity on the freshness vote was thus markedly higher than that
found by Berglund and Cain. It should be borne in mind,
however, that the subjects in this study made an assessment
of the air after 10 to 1.5s of exposure, whereas Berglund and
Cain in their analysis used steady-state values after 60
minutes of exposure. Thus, the effect of humidity on perceived freshness seems to be most pronounced in first impressions of the inhaled air.
The air supplied to the assessment box had a low level of
background pollution due to the construction of the climate
chamber (non-polluting stainless steel), and the materials
used in the glass duct system. In addition, no significant
sensory emission sources were present in the climate chamber. Therefore the votes of stuffiness should be caused by the
temperature and humidity of the inhaled air itself. This is also
reflected in the air being perceived as highly fresh at low
70
80
(kJ/kg)
observed
in the present study and by Fang et al. 131.
temperatures and humidities. Fang et al. [ 31 showed that the
acceptability of air containing pollutants emitted from different building materials was influenced significantly by temperature and humidity. At high temperature and humidity
levels, the acceptability of the air was mainly determined by
these thermal factors, whereas the pollution level dominated
at low temperatures and humidities. Fig. 10 compares the
acceptability votes concerning the inhaled air cast in the present study and obtained by Fang et al. [ 31 as a function of the
enthalpy of the air. In the present study, the subjects assessed
the air in a box as shown in Fig. 2, whereas the subjects in
[ 31 were presented to the air in cones covering only the nose.
In spite of the difference in the methods used, the relationships between acceptability and enthalpy in the two studies
are almost the same.
In the present study, assessments of the air were made with
nose breathing. With mouth breathing, a large part of the air
by-passes the nasal passages, and therefore the perception of
the inhaled air originates from cooling of the mucous membranes in the mouth. Most adults prefer breathing through the
nose, but nasal obstruction may result in mouth breathing
[ 121. Also, with increased ventilation demands, e.g. on physical activity, mouth breathing will constitute a larger part of
the pulmonary ventilation. Berglund and Cain [ 21 found only
a slight effect of activity on perceived freshness-stuffiness
and acceptability of the inhaled air at activity levels ranging
from 1 to 3 met. This indicates that at activity levels typical
for indoor environments (not sports arenas), the acceptability
of the inhaled air will depend only slightly on the pulmonary
ventilation rate. Nevertheless, Berglund and Cain found that
the air was perceived as more humid with increasing pulmonary ventilation, but this did not affect the acceptability.
The present study confirms that both humidity and temperature of the air in indoor environments have an important
impact on the discomfort perceived by humans. At high air
humidity or air temperature, the cooling of the mucous membranes in the upper respiratory tract on inhalation is
decreased, causing the air to be perceived as stuffy and unac-
J. Toftum et al. /Energy
ccptable. Together with a previously proposed model for
predicting discomfort due to high skin humidity [ 81, the
present results add requirements to the air humidity in buildings, based on thermal comfort for the occupants. For a maximum accepted percentage of dissatisfied, the two models can
specify upper limits far humidity for the idaarenviranment.
A comparison of the two models showed that the respiratory
model prescribes the most restrictive limits, excluding arange
oFenvironments accepted by the skin humidity model. Combined, the two models provide an extension of the existing
d&p &i&a 6.~ tire &err& irrtienvironrnenc.
Erruirunmerits where people feel thermally neutral may be established
al many combinations of activity, clothing, air temperature,
mean radiant temperature, velocity and air humidity. The two
models proposed may exclude some of these conditions
n*hich cause an uncomfortable skin humidity or an unaccq?aibk idTztkd wk.
5. Conclusions
Air humidity and temperature have a significant impact on
PL?ii%iEd fL+AR!Iv; w.Tf~?.fci??~, *?ii%&lR”?*> -ad -ib-IX!?%biii‘rL
-&
inhaled air.
The effect is assumed to be related to warm discomfort in
the respiratory tract caused by an insufficient evaporative and
c anvective cooling of the mucous membranes.
A model has been elaborated that predicts the percentage
ot-persons dissatisfied due to insufficjent respiratory cooling
at a Function of temperature and humidity of the inhaled air.
Ii applies for thermally neutral persons at sedentary activity
levels. The respiratory mode1 provides more stringent requirements for air humidity than a previously developed skin
humidity model.
and Buildings
28 (1998)
15-23
23
Acknowledgements
Financial support for this study from Vera and Carl Johan
Michaelsens Fund is gratefully acknowledged.
References
III E.R. McFadden
et al., Thermal mapping of the airways in humans, J.
Appl. Phys., 58 ( 1985) 564.
121L.G. Berglund and W. Cain, Perceived air qualityand the thermal
envirnnmeti,
Proc.. LQ ‘119, ,Sanllieg~~,&
19R19 .w. 9X-99.
131L. Fang, G. Clausen and P.O. Fanger, The impact of temperature and
humidity on perception and emission of indoor air pollutants, hoc.
Indoor Air ‘96, Vol. 4, 1996, Nagoya, Japan, pp. 349-353.
55-1992, Thermal environmental
conditions
for human
141 ASHRAE
occupancy, American Society of Heating, Refrigerating
and Air Conditioning Engineers, 1791 Tullie Circle, Atlanta, USA, 1992.
ASHRAE 55a-1995, Addendum to Thermal environmental
conditions
for human occupancy, American Society of Heating, Refrigerating
and Air Conditioning
Engineers, Atlanta, GA, 199.5.
of
[61 IS0 7730-1994, Moderate thermal environments-determination
the PMV and PPD indices and specification
of the conditions
for
thermal comfort,
International
Organization
for Standardization,
Geneva. Switzerland,
1994.
for buildings, Design criteria for the
[71 prENV 1752-1996, Ventilation
indoor environment,
Draft CEN (European Committe for Stdndardization) TC 156 (pre-standard),
1996.
[81 3. Toftum, A.S. Jorgensen and P.O. Faqter. U,quer limits for indoor
air humidity to avoid uncomfortably
humid skin, Energy Build., 28
(1998) l-13.
0. Albrechtsen,
Twin climatic chambers to study sick and healthy
buildings, Proc. Healthy Buildings ‘88, Vol. 3, 1988, pp. 537-541.
D.F. Proctor and D.L. Swift, Temperature and water vapor adjustment,
in J.D. Brain, D.F. Proctor and L.M. Reid (eds.), Respiratory Defense
Mechanisms,
Part I, Marcel Dekker, New York, 1977.
J. .4nderson, G. .!mxJq&
D.F. Pxmu,
cited by D.E Pmxur, md
D.L. Swift, Temperature
and water vapor adjustment, in J.D. Brain,
D.F. Proctor, L.M. Reid (eds.), Respiratory defense mechanisms, Part
I, Marcel Dekker, New York, 1977.
D.L. Swift and D.F. Proctor, Access of air to the respiratory
tract, in
J.D. Brain,
D.F. Proctor, L.M. Reid (eds.), Respiratory
Defense
Mechanisms, Part I, Marcel Dekker, New York, 1977.