UOEH #479077, VOL 7, ISS 7
Evaluation of a Carbon Dioxide Personal Cooling Device for
Workers in Hot Environments
Yang Zhang, Phillip A. Bishop, James Matthew Green, Mark T.
Richardson, and Randall E. Schumacker
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Evaluation of a Carbon Dioxide Personal Cooling Device for Workers in Hot Environments
Yang Zhang, Phillip A. Bishop, James Matthew Green, Mark T. Richardson, and Randall E. Schumacker
Journal of Occupational and Environmental Hygiene, 7: 1–8
ISSN: 1545-9624 print / 1545-9632 online
c 2010 JOEH, LLC
Copyright DOI: 10.1080/15459621003785554
Evaluation of a Carbon Dioxide Personal Cooling Device for
Workers in Hot Environments
Yang Zhang,1 Phillip A. Bishop,1 James Matthew Green,2 Mark T.
Richardson,1 and Randall E. Schumacker3
5
1
Human Performance Laboratory, The University of Alabama, Tuscaloosa, Alabama
Department of Health, Physical Education and Recreation, University of North Alabama,
Florence, Alabama
3
Department of Educational Studies in Psychology, Research Methodology, and Counseling,
The University of Alabama, Tuscaloosa, Alabama
2
10
15
20
25
30
This study tested the effectiveness of a carbon dioxide
cooling device in reducing heat strain for workers in a hot
and humid environment. Ten participants completed two trials
in an environment of 30◦ C WBGT (75% relative humidity) with
a novel liquid carbon dioxide cooling shirt (CC) or no cooling
(NC) in a randomized order. Mean time-weighted workload
for each individual equaled 465 W (400 Kcals·h−1 ). In the CC
condition, the work time was significantly increased by 32%
(97 ± 36 min) compared with NC (74 ± 26 min) (p < 0.05).
There was no significant difference in mean skin temperature
over the trials. Rectal temperature (Tre ) was significantly
different after 50 min (p < 0.05). Mean heart rate, the delta Tre
increase rate, and heat storage at 55 min (last point with n = 8)
were significantly lower in CC (p < 0.05). Overall heat storage
was 54 ± 41 W and 72 ± 40 W for CC and NC, respectively
(p < 0.05). Participants also indicated favorable subjective
responses for CC vs. NC (p < 0.05). These findings suggest
that this novel cooling device would effectively attenuate heat
strain and increase work productivity for personnel working
in a hot and humid environment. Practical aspects of use, such
as cost, convenience, weight, cooling duration, and rise in
ambient CO2 concentration in confined spaces must also be
considered.
Keywords
35
heat stress, heat tolerance, personal cooling, thermoregulation
Address correspondence to: Yang Zhang, The University of
Alabama—Kinesiology, 102 Moore Hall, Tuscaloosa, AL; e-mail:
[email protected].
INTRODUCTION
S
ignificant numbers of workers on occasion endure long
periods of work in conditions of elevated heat stress.
When moderate to high workloads are required, as can often
40 be the case in emergency response, industrial, and military
tasks, metabolic heat production increases accordingly, and
if such work must be accomplished in hot environments, the
elevated metabolic heat production, along with the external
environmental heat gain, can be a considerable danger. It has
been well documented that heat stress can greatly diminish
work capabilities(1–5) and put workers at increased risk for
heat injuries.(6)
Because heat stress can threaten the life, health, and productivity of workers, mitigating heat stress with microclimate
personal cooling is important. Many efforts have focused
on different types of personal cooling systems, including
(1) ice vests,(7,8) (2) cooled air vests,(9–11) and (3) liquid
circulated garments.(12,13) Each of these methods is effective
in reducing heat stress, attenuating physiological strain, and
increasing work time. However, lack of mobility, limited
cooling capacity, logistics, cost, and other factors limit their
practical application.
Recently, an alternative approach to cooling has been
developed. A nonpowered cooling garment based on the
endothermic vaporization of liquefied carbon dioxide (CO2 )
(Porticool Personal Cooling System, Porticool, Inc., Morrisville, N.C.) is now available. The cooling consists of two
phases. In Phase 1, liquid CO2 dispensed through a control
valve travels through flexible tubing, undergoing a continuous
pressure drop toward atmospheric pressure. As this happens,
the liquid flashes to vapor absorbing energy equal to the heatof-vaporization for CO2 .
In Phase 2, the newly vaporized cool and dry CO2 is
vented over a thin textile layer (100% cotton blended fabric)
in direct contact with the skin, providing further cooling
to the extent that it enhances ambient sweat evaporation.
Heat and moisture are then expelled from the system with
the flow of the gaseous CO2. Through these processes, this
personal cooling system provides heat removal during work
in temperate and hot environments. In operation, the system
has no moving parts, and is portable, relatively lightweight,
and relatively inexpensive to operate due to the low cost of
CO2 .
Journal of Occupational and Environmental Hygiene
July 2010
1
45
50
55
60
65
70
75
The purpose of this study was to evaluate this novel personal
cooling device for workers in a hot and humid environment.
Effective control of heat stress in industrial workers, first
responders, and military personnel working in the heat can
improve worker safety and increase productivity. In the case
of emergency response personnel, these improvements may
85 reduce mortality and morbidity of the workers and those being
rescued.
80
METHODS
90
95
100
105
110
115
120
125
130
Participants
Ten physically active males volunteered for this study.
Females were not included because this was a preliminary
study specifically targeted at industrial labor populations. The
normally large variability in male thermoregulatory responses
would be further expanded by the addition of the gender
difference in thermoregulation. The physical characteristics
of age, height, weight, estimated body fat percentage, and
VO2max were (mean ± standard deviation): 26 ± 3 yr, 177.0
± 7.8 cm, 75.9 ± 17.1 kg, 13 ± 6% body fat and 55 ± 9
ml·kg−1 ·min−1 , respectively. In general, this was a diverse
group of young males varying in fitness level. None of the
participants were acclimated to the heat, though two had a
high level of aerobic fitness (VO2max > 60 ml·kg−1 ·min−1 ). All
participants were informed of the nature of the study, cleared
by a physician for participation, and signed a written informed
consent before participation. This study was approved by
the University of Alabama’s Institutional Review Board for
Protection of Human Subjects.
Determination of Exercise Regimen
The metabolic rate for the experimental trial for each
participant was determined prior to heat exposure by collecting
and analyzing ventilatory expirations (Vacu-med Vista MX;
Vacumetrics Inc., Ventura, Calif.) to establish the desired
metabolic rate. The metabolic system was calibrated before
each trial with a gas of known composition. A 7-L syringe
(Hans Rudolph, Kansas City, Mo.) was used to calibrate the
measurement of ventilation. The cooling shirt including the
control system was worn during this initial measurement of
metabolic rate. Once determined, that individualized exercise
regimen was fixed for all successive trials.
R
According to the classification by the ACGIH , a moderate
work rate of 465 W was chosen. The exercise regimen
at a rate of 465 W (400 Kcals·h−1 ) (oxygen uptake 1.35
L·min−1 ) consisted of 12 min (80% of total work cycle time)
of walking on a motor-driven treadmill (Q55xt, Series 90;
Quinton Instrument Co., Seattle, Wash.) at 1.33 m·s−1 and at
a grade to elicit a metabolic rate of 523 W (450 Kcals·h−1 )
(oxygen uptake of 1.5 L·min−1 ) followed by 3 min (20% of
total work cycle time) of arm curls at a curl rate that elicited
a metabolic rate of 209 W (180 Kcals·h−1 ) (oxygen uptake
of 0.6 L·min−1 ). Arm curls were done with a bar weighing
13.9 kg. This work rate is similar to work rates used many
times by our laboratory.
2
Experimental Design
This study was designed to evaluate the physiological
responses to a novel cooling shirt worn in a hot and humid
environment. The participants were asked to refrain from
heavy exercise and to drink adequate water to ensure normal
hydration the day before the experimental trials. The CO2
cooling shirt (CC) trial and the control trial with no cooling
(NC) were assigned in a random order with a minimum of 72
hr between trials.
On arriving at the lab, the participant’s nude weight (with
shorts only) was measured before each trial on a calibrated
scale (Detecto Scales Inc., Brooklyn, N.Y.). Then participants self-inserted a rectal thermocouple (Physitemp, Clifton,
N.J.) 8 cm beyond their anal sphincter. Skin thermocouples
(Physitemp) were placed on the right forearm, upper-right
chest (approximately 2–3 cm above the last cooling tube of the
cooling shirt), and the right calf. Participant’s thermocouples
were monitored with a portable system (Thermalert Thermometer TH-8; Physitemp). Environmental thermocouples
were monitored with a computerized system (Iso-Thermex
Model 256, Columbus Instruments, Columbus, Ohio).
All thermocouples were calibrated prior to use and a reference thermocouple in a calibrated water bath was monitored
during trials. A heart rate monitor (Polar Electro Inc., Lake
Success, N.Y.) was worn throughout the test. During the trials,
participants wore their own cotton t-shirt, shorts, jeans, socks,
and athletic shoes. For the CC trial, the cooling shirt was worn
over the t-shirt. A full-clothed weight was recorded.
On dressing, participants entered the test chamber and
exercised at a temperature of 30◦ C wet bulb globe temperature
(WBGT: 33◦ C dry bulb, 29◦ C wet bulb, 33◦ C globe; relative
humidity of 75%). This environmental setting represents
typical temperature in the southeastern United States during
the summer time. Also, the cooling system would be less likely
to be used in milder environments. Participants did the leg and
arm work at their predetermined treadmill inclination and arm
curl pace. Water was freely available during the whole trial
period, and the total volume consumed was recorded.
Any of the following criteria were used to terminate
the trial: (1) rectal temperature ≥ 38.7◦ C; (2) heart rate ≥
95% of age-predicted maximum over a 3-min period; (3)
participant’s volition; (4) symptoms of onset of heat injury
(e.g., lightheadedness, dizziness, exhaustion, heat cramps);
or (5) work cycle time equaled a maximum of 4 hr. After
finishing the trial, participants’ clothed body weight and nude
body weight were immediately recorded again.
135
140
145
150
155
160
165
170
175
Instrumental and Sensory Measurements
Rectal temperature (Tre ), body skin temperature (chest,
forearm, and calf), heart rate (HR), and thermal rating 180
were recorded every 5 min during the test. Weighted skin
temperature at the chest, forearm, and calf was computed
as mean skin temperature (Tsk ).(14) Heat storage (S) was
calculated as: S = human body specific heat (3.47 kJ·kg−1 ·
◦ −1
C )× (0.8 ×Tre + 0.2 ×Tsk )÷ TT in Watts, where TT 185
stands for tolerance time.(15) Change in rectal temperature
Journal of Occupational and Environmental Hygiene
July 2010
190
195
200
205
210
215
over the whole experimental period (Tre rate) was also
calculated as the difference of post-exercise and pre-exercise
rectal temperature divided by the heat tolerance time.
Pre-exercise, post-exercise nude, and clothed body weights
and fluid intake were recorded. Sweat production rate was
calculated from the difference of pre-exercise and postexercise nude body weight adjusted by total fluid intake,
divided by trial duration. Sweat evaporation rate was calculated
from the difference of pre-exercise and post-exercise clothed
body weight adjusted by total fluid intake, divided by the trial
duration. Evaporative efficiency was defined as the percentage
of sweat evaporation relative to sweat production. Because it
was not logistically possible to determine the sweat volume
dripped, this represents an error in calculating the sweat
evaporation rate and evaporative efficiency. Water intake rate
was calculated from the total water consumed divided by trial
duration. Hypohydration was calculated as change in nude
body weight adjusted for fluid intake divided by initial nude
body weight times 100.
During the experimental trials, participants reported their
thermal rating every 5 min. The thermal rating scale(16) ranged
from 0.0 to 7.0 in increments of 0.5 with 0.0 representing
“unbearably cold” and 7.0 was labeled “unbearably hot.” At
baseline and immediately on test termination, participants
rated their perceptions of overall comfort. Using a pencil, they
marked their ratings on a 100-mm visual analog scale (from
“0” representing “Very Uncomfortable” to “100” representing
“Very Comfortable”), skin wetness (from “0” representing
“Not Sticky” to “100” representing “Very Sticky”), clothing
stickiness (from “0” representing “Very Dry” to “100” representing “Very Wet”), and perceived temperature (from “0”
representing “Very Cool” to “100” representing “Very Hot”).
Data Analysis
Data were reported as mean values and standard deviations. 220
A two-factor (time and condition) repeated measures analysis
of variance (ANOVA) was used to determine differences
among treatments for Tre , Tsk , HR, heat storage, and thermal
rating over the trial period. A post hoc Tukey’s test was applied,
when appropriate, with attention on the main effect for the 225
CC vs. NC, particularly at 40 min (last point with n = 10),
heat storage at 55 min (last time point with n = 8). Student’s
t-test was used to compare the overall heat storage, sweat
production rate, sweat evaporation rate, evaporative efficiency,
water intake rate, and hypohydration between the two trials. 230
Type I error level was set at p < 0.05.
RESULTS
O
f the total 20 trials, 17 were stopped due to reaching
predetermined rectal temperature limit of 38.7◦ C. One
trial (CC) was stopped for equipment failure, whereas one 235
participant each was removed for voluntary fatigue (CC) or
muscle cramping (CC).
Rectal Temperature and Mean Skin Temperature
Figure 1a illustrates the changes in rectal temperature
(Tre ) across time. Tre rose steadily across time under both 240
FIGURE 1. (a) Mean (± SD) rectal temperature, (b) mean skin temperature, (c) mean heart rate, and (d) thermal rating, across time for each
of two conditions: with cooling (CC) and without cooling (NC) under a WBGT of 30◦ C and at a work rate of 465 W (sample size is indicated at
each time point, with statistical power of 0.8 for n = 8). *Indicates p < 0.05 CC vs. NC. Key time points are marked.
Journal of Occupational and Environmental Hygiene
July 2010
3
TABLE I. Mean Tolerance Time, Change in Rectal
Temperature (T re ) Rate, Heat Storage Rate at 40 min
(HS40 ) (n = 10), Heat Storage Rate at 55 min (HS55 ) (n
= 8), and Overall Heat Storage Rate for Each of Two
Conditions: With Cooling (CC) and without Cooling
(NC) under a WBGT of 30◦ C and at a Work Rate of
465 W
Variables
Tolerance time (min)
Tre rate (◦ C·h−1 )
HS40 (watts)
HS55 (watts)
Overall HS rate (watts)
A Significantly
CC
NC
97 ± 36A
0.7 ± 0.4A
84 ± 52
54 ± 14A
54 ± 41A
74 ± 26
1.0 ± 0.4
85 ± 42
74 ± 27
72 ± 40
different from NC (p < 0.05).
conditions, with and without cooling device, from the start
of exercise. Significantly lower Tre was found at 55 min for
CC compared with NC (CC = 38.2 ± 0.3◦ C, NC = 38.4
± 0.3◦ C; n = 8, p < 0.05). A similar trend was found in
245 the mean skin temperature (Tsk ) across time (Figure 1b).
The rectal temperature and mean skin temperature response
across time was consistent with a reduced body heat storage at
55 min under CC (Table I).
Cardiovascular Strain
Changes in heart rate in the two conditions are shown in
Figure 1c. No differences in average HR were found between
the CC and NC during the first 20 min. After this time point,
the differences in HR became evident. Significant differences
(p < 0.05) were observed at 25 min (CC: 128 ± 16 vs. NC:
255 136 ± 15 beats·min−1 ), 40 min (CC: 141 ± 20 vs. NC: 146
± 17 beats·min−1 ), 45 min (CC: 115 ± 16 vs. NC: 124 ±
13 beats·min−1 ), 55 min (CC: 142 ± 17 vs. NC: 147 ±
17 beats·min−1 ), 65 min (CC: 140 ± 19 vs. NC: 147 ± 17
beats·min−1 ), and 70 min (CC: 143 ± 18 vs. NC: 152 ± 16
260 beats·min−1 ).
250
Heat Storage and Tolerance Time
Cooling condition significantly increased (p < 0.05) work
time by 32% over NC condition (Table I). Mean heat tolerance
times were 97 ± 36 min for CC and 74 ± 26 min for NC, while
265 participants’ improvement varied from 9% to 64% (Table II).
Heat storage values under the two conditions are presented
in Table I. There was no difference in heat storage between
cooling conditions at 40 min (n = 10); however, at 55 min
(n = 8), heat storage was significantly greater (p < 0.05) for
270 NC. In addition, Tre rate and overall heat storage were also
significantly greater for the NC vs. CC (p < 0.05). Based on
the change in weight of the cooling bottle, and the latent heat of
vaporization of CO2 , we calculated the mean cooling capacity
of the CC to be ∼136 W.
4
TABLE II. Individual Overall Heat Tolerance Time
for Each of Two Conditions: With Cooling (CC) and
without Cooling (NC) under a WBGT of 30◦ C and at
a Work Rate of 465 W
Subject
1
2
3
4
5
6
7
8
9
10
Mean ± SD
A Significantly
CC
(min)
NC
(min)
Improvement
(%)
120
60
90
55
100
85
115
115
55
170
97 ± 36A
100
55
75
37
70
70
70
105
40
115
74 ± 26
20
9
20
49
43
21
64
10
38
48
32 ± 19
different from NC (p < 0.05).
Sweat Rate and Dehydration
275
Sweat production rate, sweat evaporation rate, and the
evaporative efficiency are presented in Table III for both trials.
Although sweat production rate (CC = 987 ± 262 g·h−1 , NC
= 1051 ± 292 g·h−1 ) and sweat evaporation rate (CC = 576
± 115 g·h−1 , NC = 540 ± 182 g·h−1 ) were similar (p > 280
0.05), the evaporative efficiency was significantly higher in
CC compared with NC (59.8 ± 10.2% vs. 51.4 ± 10.2%, for
CC and NC, respectively; p = 0.05). There was no significant
difference in the water intake rate; however, the hypohydration
was significantly higher (p < 0.05) in the CC (2.1%) condition 285
compared with NC (1.8%) (Table III), as the duration was
longer in the CC.
Sensory Measurements
Thermal rating across time for the two conditions is
graphically presented in Figure 1d. After 5 min of exercise- 290
heat stress and till the end (except at 25 min when loss of
cooling due to the CO2 bottle emptying), the thermal rating
TABLE III. Mean Water Intake Rate, Hypohydration,
Sweat Production Rate, Sweat Evaporation Rate,
Evaporative Efficiency for Each of Two Conditions:
With Cooling (CC) and without Cooling (NC) under a
WBGT of 30◦ C and at a Work Rate of 465 W (n = 10)
Variables
CC
NC
Water intake rate (ml·h−1 )
730 ± 320
779 ± 392
1.8 ± 0.9
Hypohydration (%)
2.1 ± 0.9A
987 ± 262 1051 ± 292
Sweat production rate (g·h−1 )
540 ± 182
Sweat evaporation rate (g·h−1 ) 576 ± 115
Evaporative efficiency (%)
59.8 ± 10.2A 51.4 ± 10.2
A Significantly
different from NC (p < 0.05).
Journal of Occupational and Environmental Hygiene
July 2010
FIGURE 2. Mean (± SD) scores from subjective 100-mm scale for each of two conditions: cooling (CC) and without cooling (NC) under
a WBGT of 30◦ C and at a work rate of 465 W (n = 10). (a) The higher score indicates more comfortable; (b) The higher score indicates a
perception of increased stickiness of the clothes; (c) The higher score indicates a perception of greater wetness of the skin; (d) The higher
score indicates a greater perception of heat in the body. *Indicates p < 0.05 CC vs. NC.
was consistently cooler (p < 0.05) for CC in comparison
with NC condition. Figures 2a–d show the changes in the
295 100-mm subjective scales under both conditions. There were
no significant differences between pre-trial scores, but posttrial scores were significantly different for clothing stickiness
and skin wetness (p < 0.05). These findings suggested an
overall greater subjective comfort under the CC condition.
300
DISCUSSION
T
his study evaluated a novel, personal cooling system
under conditions often encountered during the summer
months in the United States. The work rate and clothing were
selected to maximize both ecological and external validity. The
305 combination of environmental and exercise stress in the current
study created considerable heat stress as evidenced during
the NC condition. Because mean heat tolerance time in CC
significantly increased by 23 min (32% advantage) over NC,
work productivity significantly increased. Our findings suggest
310 that this portable, lightweight cooling system is effective in
attenuating heat strain of personnel working in a hot and
humid environment. Our previous experience suggests that
cooling systems are most effective under hot conditions and
high metabolic rates. Thus, the conditions of this study may
315 be close to optimal for effectiveness of this system while
preserving ecological validity.
When the cooling shirt was used, heat stress was significantly reduced. Though the increases of mean Tre and Tsk
over time were not statistically different between the two
conditions, the time per unit of Tre increase (Tre rate) was
slower (p < 0.05) with CC. The rates of overall body heat
storage and total heat storage at 55 min (last time point with
eight participants) of the sessions were both lower with CC.
This could have occurred because of improved heat transfer
between the skin and the surrounding gas phase, provided
by the presence of cool, dry gas. The CC also reduced
cardiovascular strain as demonstrated by lower HR after
20 min of the exercise-heat exposure for the CC compared with
NC responses. This finding is similar to our previous studies
employing cooling during exercise.(8,12) This lower HR in
CC likely resulted from a downward adjustment of cutaneous
circulation, which acts as a thermoregulation response.
During exercise-heat stress, redistribution of blood flow
away from muscle or inactive tissue to the skin can dissipate
body heat to the environment to slow heat storage. Cardiac
output requirements must be preserved for a given workload,
and consequently, an increased HR is observed. While
increased peripheral skin blood flow may delay the time to
reach a critically high body temperature, an increased HR
also places a limit on the capacity for maximal external work
performance. Therefore, a lower HR supports the notion of
better thermoregulation and greater reserve of work capacity
associated with the cooling method in this study. In line
with Tre , Tsk , and HR responses in CC, the thermal rating
was also significantly lower, indicating some psychological
relief with the cooling device while working in a thermally
stressful environment. These attenuated physiological and
sensory responses in CC ultimately led to a longer work time
Journal of Occupational and Environmental Hygiene
July 2010
5
320
325
330
335
340
345
350
355
360
365
370
375
380
under exercise-heat stress. From a practical standpoint, the
primary standard for determining the effectiveness of these
cooling devices in a work setting is the total time that can be
endured before a predetermined physiological cut-off point is
achieved, such as a body core temperature less than 38.7◦ C in
this study. The cooling shirt worked as expected.
Along with elevated heat storage resulting from working in
a thermally stressful environment, sweating and dehydration
also play an important role with regard to the onset of fatigue
and hyperthermia.(17) In this study, we found a significant
difference in the evaporative efficiency (60 ± 10% vs. 51 ±
10%, for CC and NC, respectively). Apparently, vaporized cool
and dry CO2 vented over the skin, absorbed moisture from the
skin, and permitted sweat evaporation, thus taking advantage
of the natural heat transfer mechanism by drawing heat from
the body equal to the heat-of-evaporation of sweat. This is
again evidenced by the subjective scores for clothing stickiness
and skin wetness, which were lower in CC relative to NC
(Figure 2). Although the higher sweat evaporative efficiency in
CC enhanced evaporative cooling power, the sweat production
rate was about 1.0 L·h−1 under both conditions (CC = 987 ±
262 g·h−1 , NC = 1051 ± 292 g·h−1 ). This is commonly seen
in many other commercially available microclimate cooling
systems.(8,9,13,18)
The impact of cooling on sweat production is important
because higher sweating rate increases the risk of dehydration
that may lead to decreased work performance and heat injury.
In contrast, we also saw a slightly greater hypohydration (with
statistical significance) during CC use (hypohydration level
for CC was 2.1 ± 0.9% total body weight loss, and for NC
was 1.8 ± 0.9%). This may represent a slight blunting in the
thirst drive due to external cooling and can be attributed in part
to the longer duration in the CC. Body weight loss over 2%
can result in decrements of exercise performance, with greater
impact under heat exposure conditions.(19,20) These findings
suggest that the current cooling shirt cannot provide enough
TABLE IV.
Authors
Summary of Studies on Different Types of Cooling Technology
Cooling Type
Bennett et al.(7)
Ice-cooled vest
Ice cooling
Bishop et al.(8)
Cadarette et al.(18) Ice cooling unit
Liquid cooling unit
Cadarette et al.(22) Liquid cooling garment
Cheuvront et al.(24) Liquid cooling garment
Muza et al.(9)
Air cooling vest
Shapiro et al.(15)
Water-cooled vest
Air-cooled vest
Vallerand et al.(10) Liquid-cooled vest
Air-cooled vest
Hand cooling cone
Zhang et al.(23)
Carbon dioxide cooling vest
Zhang et al.A
A Current
6
cooling power to substantially lower the sweat production
rate as a mean for body cooling. Furthermore, any benefit of
evaporative cooling under normal clothing conditions would
be reduced under protective clothing or other impermeable
clothing ensembles.
Muza et al.(9) studied the effect of a portable ambient
air microclimate cooling system in selected environmental
conditions. The cooling vest with battery packs weighed
5.45 kg, and the maximum theoretical cooling power varied
from 173–528 W based on different airflow rates. Muza and
colleagues found that tolerance time was extended (by 19%
and 42%, respectively, for different flow rates) compared
with a computer-predicted tolerance time of no cooling, and
concluded the air vest was effective in reducing heat stress.
However, they did not state the battery life, which is a main
concern for portable cooling systems.
A 5-kg ice-based personal ice cooling unit has also
been investigated.(18) When considering a cooling power of
150 W over a 30 min period, heat storage with the personal
ice cooling unit was 45 ± 14 W·m−2 . When compared with a
previous study in our lab,(8) using similar workloads (450 W)
and similar environmental condition (28◦ C WBGT), we found
that the current CO2 -based cooling shirt provided a 150%
advantage relative to a 2.2-kg ice-based cooling vest.
The issues of most microclimate cooling technologies for
practical application include cost-saving, ergonomics, and in
most cases, cooling capacity. In Table IV, a comparison of
system weight vs. cooling power is provided. Liquid cooling
technologies provide moderate to high cooling capacity (200–
500 W).(10,21,22) However, these systems usually require bulky
equipment or a tether to provide sustained cooling power.
Other cooling technologies such as air cooling,(10) phasechange cooling,(7) or portable hand cooling(23) may provide
cooling capacity similar to that found in this novel CO2 device
but at a cost of extra weight, which may restrict the freedom of
movement. Therefore, an ideal approach would be a balance of
Cooling Power
Power Supply
Six frozen gel thermostrips
Ice packs
150 W
Ice-based cooling unit
375 W
Vapor compression unit
305 W (constant flow) Water bath
6 W/body surface area Water bath
581 W
Rechargeable battery
91 W
80 W
459 W
Refrigeration unit
487 W
Air chiller setup
Rechargeable battery
136 W (medium flow) Carbon dioxide bottle
study.
Journal of Occupational and Environmental Hygiene
July 2010
Sustainable Time Weight
30 min
30 min
4.6 kg
2.2 kg
5 kg
10 kg
5.5 kg
25 min
2.7 kg
1.3 kg
385
390
395
400
405
410
415
420
425
430
435
440
445
450
455
460
465
470
475
acceptable cooling power, weight, and mobility. The cooling
shirt in this study uses a liquid CO2 bottle (454 g) as a heat
sink. The refillable bottle along with the control device (adjusts
flow rate) fit into a nylon pouch that was strapped to a belt
and allowed users to freely move around. The total weight
of the whole system is about 1.3 kg, and it provided 136 W
cooling power at the moderate setting (high and low settings
are available but were not tested) over about a 25-min period
under a hot and humid condition before the bottle had to be
replenished.
In general, the cooling systems with a high cooling capacity
also had a high weight. The present system is one of the lighter
systems and yet evidenced effective total cooling capacity.
The difference between heat storage at 55 min for CC and
NC was 18 W (Table I). The cooling power was calculated as
136 W (excluding evaporation of sweat). This meant that the
efficiency of delivery of the total cooling power was less than
13% (not including evaporative cooling). Whereas this sounds
low, there are inevitable losses to the atmosphere. Further,
maximizing heat loss in a worker involves physiological heat
transfer as well as mechanical heat transfer. Optimal efficiency
occurs only when the removal rate and the delivery rate are
exactly matched.
As might be expected, the thermal comfort rating was
consistently lower (p < 0.05) for the cooled condition for
most of the duration of the trials. Although this is a subjective
rating, it should not be discounted. Keeping workers more
comfortable should result in greater job satisfaction and could
lead to better morale, attention to task, and possibly better
productivity.
While we found refilling cooling bottles to be easy, under
some work conditions it may be much more trouble for a
worker to operate the system. Exhausting CO2 to a closed
environment such as a confined space may cause ambient CO2
concentrations to rise to hazardous levels. But, in our small,
closed lab area setting (volume of ∼ 29.5 m3 ), the highest CO2
level while a single cooling vest was in use was 0.4% monitored
very near the research participants. This is influenced by
numerous factors including air motion, ventilation, and other
vests in the vicinity.
Optimizing the cooling power for these cooling technologies would improve their practicality for field use. Recently,
Cheuvront et al. proposed that intermittent cooling(22,24)
is an effective way for reducing the power requirements
while maintaining an equivalent thermoregulation. These
authors(24) suggested that, if skin temperature falls below 32◦ C,
skin cooling could produce cutaneous vascular constriction
that decreases convective heat transfer from the body core to
the periphery, and it was observed that an intermittent cooling
strategy alleviated cardiovascular and thermoregulatory strain
comparable to a constant cooling method. This would be of
interest since a simple 4:4 min on-off cooling ratio could
double the cooling supply life.
However, in the current study, the Tsk was around 35◦ C,
well above the suggested critical Tsk of 32◦ C. Furthermore,
the liquid cooling technologies used(24) had a cooling capacity
higher than the current CO2 -based cooling system. It is unclear
whether the intermittent cooling strategy would still work at
a relatively high skin temperature and lower cooling power
conditions. The CO2 bottle in our study lasted around 25 min 480
before being replenished. Even using a 2:1 on-off perfusion
ratio would theoretically increase the cooling bottle life by
12.5 min.
Future research should examine this application. During
the first 20 min (Figure 1), the cooling shirt had little impact 485
on physiological change and added only comfort. One strategy
that should be tested would be to wait ∼ 25 min before donning
the PCS to save one bottle refill.
CONCLUSIONS
T
he carbon dioxide personal cooling system appeared to 490
be an effective method for reducing physiological stress
and increasing work productivity in the heat. It is clear from
these data that, under the conditions of this study, individuals
performing very short work bouts (e.g., less than 50 min)
would not likely benefit greatly on a physiological basis from 495
use of this cooling system. On the other hand, the subjective
responses were positive, and workers who feel better may work
longer and more efficiently. This CO2 -based cooling system is
light, portable, and apparently cost-effective for repeated use.
Furthermore, there is minimal risk in operating the cooling 500
system. The cooling system provides moderate cooling power,
and it offers a practical approach for reducing heat strain during
field operations.
ACKNOWLEDGMENT
T
his study was supported by a grant from the Kimberly- 505
Clark Corporation. The authors are in no way affiliated
with Porticool, Inc. or Kimberly-Clark. We thank all the
participants for their participation in this investigation.
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