The effect of high air temperature and high CO2 concentration on
physiological responses and work performance
Weiwei Liu1,2, *, Pawel Wargocki2
1
2
*
Central South University, Changsha, Hunan, China
Technical University of Denmark, Kongens Lyngby, Denmark
Corresponding email: [email protected]
SUMMARY
The effects were examined of exposures to 35°C at 380 ppm and 3,000 ppm of carbon dioxide
(CO2) compared with exposure to 26°C and CO2 at 380 ppm. Pure CO2 was added to increase
its concentration. Physiological responses, subjective ratings and work performance were
measured. Twelve subjects (six men and six women) were exposed for 3 hours, two persons at
a time. The order of exposure was balanced. Their eardrum temperature, skin temperature,
heart rate and body weight loss increased significantly at this exposure, while arterial oxygen
saturation decreased at 35°C, compared with 26°C. Performance of addition and subtraction
decreased significantly, as well. No significant changes in responses were observed when CO2
was increased to 3,000 ppm at 35 °C suggesting that CO2 does not interact with temperature
and may be considered harmless at the examined concentration.
PRACTICAL IMPLICATIONS
The observed negative effects of elevated temperature have important implication for people
working outdoors, where they are difficult to mitigate.
KEYWORDS
High air temperature; High CO2 concentration; Physiological responses; Work performance
1 INTRODUCTION
Outdoor temperature of 35°C is defined as the threshold of high temperature yellow warning
for hot weather in China (www.cma.gov.cn). Temperatures higher than or close to 35°C result
in changes of important physiological parameters indicating that thermal stress is perceived.
Significant rise in core temperature, heart rate and sweat rate was observed, when subjects
stayed in high temperatures ranging from 35 to 50°C for 80 ~120 min (Epstein et al., 1980;
Hocking et al., 2001; Mohr et al., 2012; Shi et al., 2013; Tamm et al., 2013). In the series of
experiments, both systolic and diastolic blood pressures decreased, when temperature
increased from 20°C to 34 ~ 42°C (Lu and Zhu, 2007) during exposures of up to 4 hours.
Human mental performance was also observed to be affected during exposures to
temperatures close to and higher than 35°C. For example, Wilkinson et al. (1964) observed
significant decrease in performance of two-digit addition and increase in performance of
vigilance test (subjects listened to a series of tones) when body temperature was maintained at
38.5°C with ambient temperature at 37°C for 2 hours. Epstein et al. (1980) found that
percentage of errors in a shooting TV game rose gradually with the rise in the air temperature
from 24°C to 37°C and to 50°C during 2 h exposure to each temperature. This effect was an
indication of deterioration in psychomotor functions caused by the heat load. Hocking et al.
(2001) found that increased thermal stress at 35°C induced deficits in working memory,
information retention and information processing, and there was a marked difference in the
electrical responses of the brain when subjects were thermally strained.
Exposure to carbon dioxide (CO2) at levels much higher than these typically occurring
indoors (>10,000 ppm) can have negative effect on health and can impair mental performance
(e.g. Zhang et al., 2016a). At typical indoor levels no harmful effects are generally expected.
Only few studies were performed examining the effects of CO2 at levels < 5,000 ppm, which
are typically occurring indoors. No negative effects on health were seen at these levels (Zhang
et al., 2016a, 2016b, 2016c) and one experiment suggested that exposures to CO2 at 3,000
ppm increased fatigue and reduced wellbeing (Kajtar and Herczeg, 2012). In case of the
effects of CO2 on performance at the levels typically occurring indoors that reported
experimental data is inconsistent. On one hand studies of Satish et al. (2012) and Allen et al.
(2015) suggest that the exposures to CO2 at levels as low as 1,000 ppm result in reduced
performance of the tests examining the ability and functions needed to take decisions, while
studies of Zhang et al. (2016a, 2016b, 2016c) show no effects of CO2 on performance of
neurobehavioral tests measuring different cognitive skills and tasks examining the skills
needed to perform office work at levels of CO2 up to 4,900 ppm. Kajtar and Herczeg (2012)
showed however that performance of proof-reading one of the tasks used by Zhang et al.
(2016a) was reduced when CO2 increased above 4,000 ppm. In the studies mentioned above
the exposure duration was from 2,5 to 8 hours.
One objective of the present study was to examine whether elevated air temperature affects
physiological responses and impairs work performance. Another objective was to examine the
combined effect of high temperature and high CO2 level i.e. whether increasing CO2 level at
elevated temperature will result in changes of the responses mentioned above. The authors
were not able to identify the study in the published literature, which actually examined the
combined effect of temperature and CO2.
2 METHODS
Approach
Twelve subjects, two individuals at a time, were exposed for 3 hours in the chamber to three
conditions: temperature of 26°C and 35°C at CO2 level of 380 ppm and temperature of 35°C
at CO2 level of 3,000 ppm. During exposure they performed different cognitive tests, rated
their acute health symptoms and assessed the environmental conditions. Many physiological
reactions were monitored.
Subjects
6 male and 6 female college students (mean ± SD of age: 24.8 ± 2.6 years, height: 172.4 ± 8.5
cm, weight: 68.8 ± 18.3 kg) were recruited for the experiment. All subjects were Chinese
living in Denmark. They were healthy non-smokers who were not taking prescription
medication and had no history of cardiovascular disease. All protocols were approved by the
university’s ethics committee and conformed to the guidelines contained within the
Declaration of Helsinki. Verbal and written informed consent was obtained from each subject
prior to the participation in the protocol. Subjects were asked to avoid caffeine, alcohol, and
intense physical activity at least 12 h prior to each experimental session and keep a good
mental status. The female subjects were not in their menstrual period when attending the
experiment.
Protocol
There were 3 experimental conditions as listed in Table 1. 12 subjects were divided into 6
groups, with 1 male and 1female in each group. Two subjects in one group participated in the
experiment at the same time.
Table 1. The experimental conditions.
Condition
T26
T35
T35C3000
Air temperature (°C)
26
35
35
CO2 concentration (ppm)
380
380
3000
A fully balanced design was applied for the order of the experiment conditions (see Table 2).
For each group, all the experiments for the three conditions were done in one week. The
subjects got familiar with questionnaire and experimental protocol, and practiced cognitive
tests in a 1.5 h long session prior to the day when actual exposures took place.
Table 2. The order of the experiment conditions.
Group
1st day
2nd day
3rd day
4th day
1
T26
T35
T35C3000
2
T26
T35C3000
T35
Pratice
3
T35
T26
T35C3000
&
4
T35
T35C3000
T26
Instruction
5
T35C3000
T26
T35
6
T35C3000
T35
T26
During the experiment, subjective (psychological) and objective (physiological) responses and
work performance (neurobehavioral test) were investigated and measured. In one day, the
experiment under one condition was conducted for one group from 13:30 to 17:00. The
procedure was shown in Fig.1.
During the exposure, the subjects performed different cognitive tests. They were part of the
battery with neurobehavioral tests, which have been used successfully to evaluate the effect of
temperature on performance (Lan et al., 2009). Subjects performed also the d2 test used to
examine attention and concentration. During the test, the subjects had to find and cancel out
all target d2 characters ( “d” character with a total of two dashes placed above and/or below
the character) placed among the nontarget characters (“d” characters with more or less than
two dashes, and “p” characters with any number of dashes). Subjects performed also TsaiPartington test. This is a cue-utilization test providing indication of arousal. The test was
presented to subjects on a paper. Twenty random numbers were randomly distributed on the
paper. The task was to connect numbers in ascending sequence, beginning from the “start”.
The time for completion of the test was set to 60 s. It was not possible to complete the task in
the indicated time. The nuumber of correct links and errors were used as a measure of
performance.
In case the gradual improvement of performance was observed independently of the exposure
conditions (so called learning) the following adjustment was made to the performance metric
(Lan and Lian, 2009),
P
(1)
Pn',i = 1 × Pn ,i
Pn
where Pn',i is the performance of the ith participant at the nth presentation after correction, n is
the sequence of presentation, Pn refers to averaged performance at the nth presentation, and
Pn ,i means the performance of the ith participant at the nth presentation before correction.
Figure 1. The procedure of the experiment. SPO2 is arterial oxygen saturation and ETCO2 is
end-tidal partial CO2 pressure. TC, PAQ, AHS, SLP and SEP are respectively the ratings of
thermal comfort, air quality, acute health symptoms, sleepiness and self-estimated
performance.
The experiment was done in a chamber at Technical University of Denmark during May and
June in 2014. The subject was asked to sit quietly at the square outside the chamber when he
arrived. As shown in Fig.1, after 30 minutes he can enter the chamber and begin the
experiment. In the experiment, the subjects wore their own clothing. The clothing included
short sleeve, trousers and sports shoes (Total clo about 0.57), which can keep the subjects
feeling neutral at 26 °C. The same clothing was compulsory for each condition.
During the experiment, the air relative humidity was kept at a low level (34% in 26 °C and
24% in 35 °C). All the subjects were blind to the air temperature and CO2 concentration.
Statistical analysis
Friedman ANOVA test for related samples was used to detect differences in different
outcomes between the three exposure conditions studied. The significance level was set at
p=0.05. If the difference was significant (p < 0.05), post-hoc analysis was done. Pair-wise
comparisons were additionally made using Wilcoxon Signed Ranks Test for paired samples.
Two comparisons were made: the outcomes at T26 and T35 (both at CO2 of 380 ppm) were
compared to reveal whether there were any statistically significant differences as a result of
increased temperature, and the outcomes at T35 and T35C3000 to examine whether adding
CO2 at elevated air temperature caused change in the outcomes. Assuming two independent
hypotheses were tested for above two comparisons: elevated temperature affects some
outcomes and elevated CO2 affects some outcomes, the significance level was set at
p=0.05/2=0.025 (2tail) according to a Bonferroni correction. The statistical analysis was
performed with SPSS 19.0 software (SPSS Inc., Chicago, USA).
3 RESULTS
The eardrum temperature and loss of body weight were higher at 35 °C than at 26 °C (Table
3). SPO2 was lower at 35 °C and the difference reached significance near the end of the
exposure. No significant differences in blood pressure were observed between 26°C and 35°C,
however the diastolic blood pressure was systematically lower at 35 °C compared with 26 °C.
Adding CO2 at 35°C did not significantly change the measured eardrum temperature, body
weight loss, SPO2 and blood pressure at this temperature and low CO2 of 380 ppm.
Table 3 Mean ( ± SD) of eardrum temperature, arterial blood oxygen saturation (SPO2), blood
pressure and body weight loss at each experimental condition
Time during exposure (min)
Eardrum temperature (°C)
T26
T35
T35C3000
p
SPO2 (%)
T26
T35
T35C3000
p
Systolic pressure (mmHg)
T26
T35
T35C3000
p
Diastolic pressure (mmHg)
T26
T35
T35C3000
p
Body weight loss (g/min)
T26
T35
T35C3000
p
20 ~ 30
70 ~ 80
100 ~110
150-160
36.3 ± 0.2
36.9 ± 0.3##
36.9 ± 0.2
<0.01**
36.3 ± 0.2
37.0 ± 0.3##
37.0 ± 0.2
<0.01**
36.3 ± 0.2
37.0 ± 0.3##
37.0 ± 0.2
<0.01**
36.3 ± 0.3
37.0 ± 0.3##
37.1 ± 0.2
<0.01**
96.6 ± 0.8
96.7 ± 0.9
96.8 ± 0.9
0.63
96.9 ± 1.0
96.5 ± 0.7
96.7 ± 0.9
0.36
97.1 ± 1.0
96.7 ± 0.6
96.6 ± 0.8
0.18
97.6 ± 1.2
96.5 ± 0.7##
96.6 ± 0.5
<0.01**
102.3 ± 11.9
102.1 ± 9.5
101.3 ± 11.8
0.56
103.0 ± 10.5
102.3 ± 9.3
106.0 ± 14.3
0.32
102.3 ± 9.1
101.8 ± 8.8
101.7 ± 9.5
0.93
103.9 ± 10.2
106.9 ± 12.5
102.8 ± 11.3
0.73
70.2 ± 9.0
68.6 ± 10.2
70.3 ± 9.1
0.84
72.9 ± 11.0
68.3 ± 10.6
70.5 ± 9.3
0.1
73.5 ± 8.8
69.5 ± 8.1
71.2 ± 8.8
0.04*
71.1 ± 6.7
70.1 ± 9.9
69.3 ± 8.7
0.93
0.1 ± 0.1
0.1 ± 0.1
0.1 ± 0.1
1.0
0.3 ± 0.2
0.9 ± 0.2##
1.0 ± 0.4
<0.01**
0.5 ± 0.4
1.4 ± 0.4##
1.6 ± 0.5
<0.01**
0.7 ± 0.3
2.1 ± 0.5##
2.1 ± 0.6
<0.01**
*
Significant difference (p<0.05) among T26, T35 and T35C3000 based on Friedman ANOVA test.
Significant difference (p<0.01) among T26, T35 and T35C3000 based on Friedman ANOVA test.
#
Significant difference (p<0.025) with respect to T26 based on Wilcoxon Signed Ranks test.
##
Significant difference (p<0.01) with respect to T26 based on Wilcoxon Signed Ranks test.
**
Fig. 2 shows the measurements of heart rate and mean skin temperature. The heart rate was
significantly higher at 35°C than at 26 °C, especially after 90 min of exposure. Adding CO2
concentration at 35°C slightly reduced heart rate, however the change was not significant. The
mean skin temperature at 35°C was significantly higher compared with 26°C, while adding
CO2 at 35 °C to reach 3,000 ppm did not impact the mean skin temperature.
Table 4 shows the performance of the neurobehavioral tests, d2 test and Tsai-Partington test.
The speed was corrected according to equation (1). The corrected results were used for further
analysis. The average performance at a condition was calculated and analyzed (Table 4).
Among the neurobehavioral tests, accuracy of the “Addition & Subtraction” test significantly
decreased at 35°C compared with 26°C; adding CO2 concentration to 3,000 ppm at 35°C did
not significantly change the performance of this test. No significant difference in the
performance of the other neurobehavioral tests, d2 test and Tsai-Partington test was found
between all experiment conditions.
(a) Heart rate
(b) Mean skin temperature
Figure 2. Average heart rate and mean skin temperature for five periods during exposure to
different conditions.
Table 4 Mean ( ± SD) performance of the cognitive tests at different experimental conditions
No.
1
2
3
4
5
6
7
8
9
10
11
12
Experiment condition
Mental Redirection
Accuracy (%)
Speed (Units/min)
Grammatical Reasoning
Accuracy (%)
Speed (Units/min)
Stroop
Accuracy (%)
Speed (Units/min)
Addition & Subtract
Accuracy (%)
Speed (Units/min)
Multiplication
Accuracy (%)
Speed (Units/min)
Visual Reaction Time
Accuracy (%)
Speed (Units/min)
Visual Learning Memory
Accuracy (%)
Response time (s)
Digit Span Memory
Span
Stroop with feedback
Speed (Units/min)
Calculation with feedback
Speed (Units/min)
d2 test
Total processed (Units)
Accuracy (%)
Tsai-Partington
Correct links (Units/min)
Error links (Units/min)
T26
T35
T35C3000
p
97.2 ± 2.4
48.5 ± 11.5
96.8 ± 3.5
49.1 ± 12.6
97.2 ± 2.8
50.0 ± 10.6
0.690
0.856
94.1 ± 4.8
8.3 ± 2.2
91.8 ± 10.6
9.1 ± 1.9
91.9 ± 9.2
8.2 ± 0.8
0.833
0.383
98.1 ± 2.0
24.0 ± 4.7
97.8 ± 4.0
23.3 ± 5.6
97.9 ± 2.4
23.1 ± 4.2
0.446
0.751
98.9 ± 0.9
21.1 ± 2.6
97.3 ± 1.7##
20.2 ± 3.0
97.8 ± 2.0
19.7 ± 3.3
0.014*
0.191
93.3 ± 6.5
5.7 ± 2.1
92.5 ± 5.4
6.1 ± 1.9
90.8 ± 6.7
5.7 ± 1.8
0.740
0.751
97.9 ± 1.6
92.9 ± 15.7
97.6 ± 1.8
94.5 ± 15.7
98.1 ± 1.5
93.4 ± 17.5
0.536
0.434
89.3 ± 10.2
90.5 ± 14.4
89.2 ± 10.0
90.2 ± 14.0
87.0 ± 11.6
91.2 ± 14.9
0.811
0.751
9.90 ± 2.2
9.92 ± 2.1
9.90 ± 2.1
0.798
27.6 ± 5.0
27.9 ± 3.8
27.0 ± 5.0
0.844
15.3 ± 2.2
14.8 ± 2.3
15.0 ± 2.1
0.978
644 ± 18
97.1 ± 1.6
634 ± 31
97.0 ± 1.3
643 ± 22
97.2 ± 1.1
0.249
0.5
13.6 ± 3.7
1.7 ± 1.9
13.8 ± 4.0
1.3 ± 1.5
13.9 ± 3.9
1.6 ± 1.9
0.518
0.533
4 DISCUSSION
The difference between conditions T26 and T35 were as expected and reflected the effect of
high air temperature on human physiological and subjective responses. The observed results
are consistent with these reported by previous studies (e.g. Shi et al., 2013). The occurrence of
the above physiological responses was the result of thermoregulation of human body under
high air temperature. The thermoregulation maintained the body temperature in normal range
at 35°C, as reflected by the slight increase in the eardrum temperature compared to 26°C. In
order to enhance heat dissipation of body at high temperature, skin blood vessels dilate and
more blood is pumped from internal organs to the skin, which leads to increase of heart rate
and skin temperature. At the same time, more water evaporates through the skin by diffusion
to increase heat loss from core, causing bigger loss of body weight. Regulatory sweating did
not occur, as no significant decrease in mean skin temperature was observed at 35°C.
The accuracy of the task “Addition & Subtraction” declined significantly at 35°C compared
with 26°C, which indicated that high temperature can impair human ability to perform
cognitive tasks. This confirms the previous findings (e.g. Epstein et al., 1980; Hocking et al.
2001; Wilkinson et al., 1964). During the experiments, the subjects had few adaptive
possibilities at 35°C. They could not open windows, use local cooling and adjust clothing and
activity level. Therefore, they experienced significant heat stress as indicated by increased
core temperature and heart rate, which can reduce their cognitive performance. Although the
high temperature did not change the performance of the other neurobehavioral tests in this
study, the subjects had to exert more effort to maintain the performance, at the cost of
increasing mental fatigue and general neurobehavioral symptoms.
The comparison between conditions T35 and T35C3000 revealed that no further change of the
physiological and subjective responses and the performance of all tasks during the whole
experiment period were caused by adding CO2 to achieve moderately high concentration of
3,000 ppm at 35°C. This suggests no effect of elevated CO2 concentration at high temperature.
None of the above results were different from those found in the relative studies carried out at
normal air temperatures (Zhang et al., 2016a; Zhang et al., 2016b; Zhang et al., 2016c). In
these studies, no significant effects of increased CO2 (< 5,000 ppm) on human responses and
work performance were observed when temperature was not elevated. This may additionally
suggest no interaction between high temperature and elevated CO2 concentration.
5 CONCLUSIONS
(1) When raising the air temperature from 26°C to 35°C at CO2 of 380 ppm, the eardrum
temperature, skin temperature, heart rate, respiratory ventilation rate and body weight loss
increased, while SPO2 decreased. The accuracy of the task “Addition & Subtraction”
decreased.
(2) The moderate increase in CO2 concentration to 3,000 ppm at 35°C did not cause changes
in responses of subjects. This may suggest no interaction and no effect of CO2 at high
temperature, and generally shows no effect of CO2 on these outcomes at concentration
below 3,000 ppm.
ACKNOWLEDGEMENT
The project was financially supported by Bjarne Saxhof Foundation in Denmark (2014) and
National Natural Science Foundation of China (No. 51478471).
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