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Journal of Occupational and Environmental Hygiene
ISSN: 1545-9624 (Print) 1545-9632 (Online) Journal homepage: http://www.tandfonline.com/loi/uoeh20
A method to reduce heat strain while clad in
encapsulating outerwear
Dennis Grahn, Megha Makam & H. Craig Heller
To cite this article: Dennis Grahn, Megha Makam & H. Craig Heller (2018) A method to reduce
heat strain while clad in encapsulating outerwear, Journal of Occupational and Environmental
Hygiene, 15:8, 573-579, DOI: 10.1080/15459624.2018.1470635
To link to this article: https://doi.org/10.1080/15459624.2018.1470635
Accepted author version posted online: 30
Apr 2018.
Published online: 30 Apr 2018.
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A method to reduce heat strain while clad in encapsulating outerwear
Dennis Grahn, Megha Makam, and H. Craig Heller
Department of Biology, Stanford University, Stanford, California
ABSTRACT
The use of personal protective equipment (PPE) increases the risk of heat related maladies.
A means to enhance heat dissipation capacity of individuals clad in PPE would be of benefit.
The glabrous skin regions of the hands, face, and feet are portals for direct heat transfer
between the body core and the external environment. The effects of PPE outerwear and
palmar glabrous skin cooling on heat storage were assessed. Subjects engaged in fixed load
treadmill exercise in a thermoneutral environment (T
a
¼20–24C) or rested in a hot environ-
ment (45 ± 0.5C). The use of PPE outerwear increased the rate of core temperature rise by
five-fold during vigorous exercise. Palm cooling using a stationary water circulation system
attenuated the rate of core temperature rise by 30–60% during rest and light, moderate,
and vigorous exercise while wearing PPE outerwear. However, the subjects were tethered to
the system. A wearable cooling system was devised that allowed free range of motion and
unrestricted mobility. The wearable system provided thermal benefits equivalent to the use
of the tethering cooling system. With optimization, this wearable cooling technique could
neutralize the negative thermoregulatory effects of wearing PPE while engaged in light
workload activities such as those encountered by healthcare professionals working in infec-
tious disease treatment centers. For individuals working at higher workloads, such as fire-
fighters, a wearable glabrous skin-based cooling system could extend work bout duration as
well as enhance heat loss during episodic recovery periods.
KEYWORDS
Core temperature; heat
illness; hyperthermia;
protective clothing;
temperature; work/
rest cycles
Introduction
Personal protective equipment (PPE) can greatly reduce
or eliminate the risk of exposure to hazardous chemi-
cals, physical hazards, and disease-causing organisms
such as the Ebola virus. However, wearing PPE increases
the risk of developing heat-related illnesses because the
encapsulating ensembles severely compromised convect-
ive, conductive, radiative, and evaporative heat dissipa-
tion modalities.
[1,2]
For example, PPE worn by Health
Care Personnel (HCP) working in the hot zones (patient
care units) of the Ebola Treatment Centers (ETC) in
tropical Western Africa during the 2014–2015 Ebola
outbreak consisted of overlapping impermeable material
coverings (a surgical cap and gown, impermeable
hooded coveralls, a face shield, a visor, a respirator, two
layers of gloves, a waterproof apron, and rubber
boots).
[3]
The use of PPE in the ETCs severely limited
HCP work times in the hot zones, potentially impaired
mental and physical performance, and increased the
risks of heat related maladies (including heat exhaustion,
heat cramps, heat stroke, and, in extreme cases, death).
The inability to dissipate heat while clad in PPE also
elevated the risk of self-contamination during the post-
work shift PPE removal process by: (1) limiting work
shift duration, thereby, increasing the frequency of PPE
doffing and (2) increasing the likelihood of a misstep
during the doffing process due to heat-related deterior-
ation of mental acuity. A means to facilitate heat dissi-
pation from PPE encapsulated individuals would
improve the safety of workers in situations that require
isolation from the local environment.
Unique radiator like vascular structures underlie the
glabrous skin regions of the body—the palms of the
hands, the soles of the feet, and regions of the face and
ears. These subcutaneous vascular structures (arterioven-
ous anastomoses and the associated retia venosa) provide
a direct thermal portal between the body core and the
external environment. The application of a heat source or
aheatsinktoglabrousskinsurfacesisaneffectivemeans
for transferring heat into or out of the body core.
[4–9]
The objective of this current effort was to replicate,
and expand on, previous studies that assessed the
effects of glabrous skin surface cooling on heat
stressed individuals. The hypothesis to be tested was
that a closed-loop water circulating system interfacing
CONTACT Dennis Grahn dagrahn@stanford.edu Department of Biology, 371 Serra Mall, Stanford University, Stanford, CA 94305.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/uoeh.
ß2018 JOEH, LLC
JOURNAL OF OCCUPATIONAL AND ENVIRONMENTAL HYGIENE
2018, VOL. 15, NO. 8, 573–579
https://doi.org/10.1080/15459624.2018.1470635
with the palms of the hands worn inside encapsulating
outerwear would reduce the rate of core temperature
rise during fixed load exercise ranging in intensity from
rest to vigorous. A first step was to determine the effect
of wearing PPE on heat accumulation during fixed load
exercise, followed by a series of trials on palm cooling
of subjects exercising at a variety of workloads. A stand-
alone and wearable cooling system that enabled a free
range of motion and unlimited mobility was developed.
The relative cooling efficacy of the wearable system was
compared with the use of commercially available sta-
tionary water circulation-based systems that tethered the
subjects to the heat sink and limited their mobility.
Materials and methods
The human subject research was divided into four dis-
crete protocols that assessed the effects on core tempera-
ture of: (1) the use of a PPE ensemble during vigorous
fixed load exercise, (2) palm cooling of PPE clad sub-
jects during light, moderate, and vigorous exercise under
normothermic ambient conditions, (3) palm cooling of
PPE clad subjects during rest under hot ambient condi-
tions, and (4) the use of a lightweight wearable cooling
system compared to the use of a stationary temperature-
controlled water circulation system that tethered the
subjects. The protocols were approved by the Stanford
University Institutional Review Board (IRB), and written
informed consent was obtained from each volunteer
prior to participation in the studies using an instrument
approved by the Stanford University IRB.
Subjects
Twenty-three volunteer subjects recruited from the
Stanford University community participated in these
studies (12 males and 11 females, age 23 ± 3 years,
mean ±SD). All subjects met the inclusion criteria of
being healthy (no illnesses or chronic debilitating condi-
tions),active(regularexerciseofaminimumof6hr/
week), and over 18 years of age. The subjects all partici-
pated in more than one protocol. Eleven subjects (six
male and five female) participated in protocol 1, 13 sub-
jects (six male and seven female) participated in protocol
2, 10 subjects (seven male and three female) participated
in protocol 3, and 12 subjects (six males and six
females) participated in protocol 4.
Clothing
In all trials, the subjects wore a lightweight exercise
clothing base layer (i.e., t-shirts, shorts, undergarments,
ankle height socks, and athletic shoes). An outer-layer
PPE ensemble that simulated the outerwear worn by
HCPs in the ETCs consisted of impermeable protective
coveralls with hood and foot coverings (TyCHEM QC,
Tyvek, DuPont, Wilmington, DE), a polypropylene bala-
clava worn under the PPE hood and covering the head
and face (Tullahoma Industries, Tullahoma, TN), and
plastic bags (Hi-density medical bags 7–10 gal, Elkay
Plastics Co., Los Angeles, CA) secured with surgical tape
(Transpore, 3M Corporation, St. Paul, MN) to the PPE
shell at the wrists to create a contiguous vapor barrier
around the hands.
Facilities and monitoring
The trials were conducted in either a thermoneutral
environment or a hot environment. The thermoneu-
tral environment consisted of an 8 m x 6 m x 3 m
(width, length, height) laboratory that housed four
treadmills (model SC7000, SciFit, Tulsa, OK). The
temperature and humidity of the room were main-
tained by the building’s central HVAC system
(T
a
¼20–24C, RH ¼15–35%). The hot environment
consisted of a 2.4 m x 3.3 m x 2.4 m (width, length,
height) temperature-controlled environmental cham-
ber (T
a
¼45 ± 0.5C, RH ¼20–35%).
Esophageal temperatures (T
es
)werecontinuously
monitored throughout all trials using general purpose
thermocouple probes (Mon-a-Therm #503-0028,
Mallinckrodt Medical Inc., St. Louis, MO) self-inserted
throughthenoseormouthtoadepthof38–39 cm and
held in place with a small piece of surgical tape
(Transpore, 3M Corporation, St. Paul, MN) attached to
the skin surrounding the anterior nares or the cheek.
The probe was connected to a laptop-based thermo-
couple transducer/data collection system (GEC instru-
ments, Gainesville, FL) that recorded temperature data
at 1-sec intervals. The T
es
data streams were uploaded
to a secure server from which they were subsequently
accessed for off-line analysis.
Cooling systems
The cooling systems consisted of water perfused pal-
mar surface heat exchange interfaces and a means for
delivering a continuous stream of temperature-con-
trolled water to the interfaces.
Heat exchange interfaces
The palmar heat exchange interfaces consisted of 13 x
13 cm urethane coated nylon cloth water perfusion
pads (American National Manufacturing, Corona,
574 D. GRAHN ET AL.
CA) held in close apposition to the palmar surfaces
with 4-way stretch fabric (Lycra) mitts sized for the
individual subjects (Figure 1(C)). The interfaces were
designed to provide minimal compromise of fine
motor activity by leaving the thumb and fingers
uncovered and unencumbered. The water perfusion
pads were connected via polyurethane tubing (1/4”ID
x 1/16”wall laboratory tubing, Tygon) to a tempera-
ture-controlled water circulating system.
Temperature-controlled water circulating systems
For protocols 2 and 3, the heat exchange interfaces were
connected to commercially available temperature-con-
trolled water circulating systems that circulated a 6.0 l/
min stream of 16 ± 0.5C water (either a Meditherm III,
Gaymar Industries, Orchard Park, NY or Blankroll III,
Cincinnati Sub-Zero, Cincinnati, OH).
For protocol 4, a small closed-loop temperature-
controlled circulating system was designed in house
(Figure 1). The system consisted of a pump (3.6 L/min
mini DC brushless submersible water pump, item #
EWP-DC30A1230, Light Objects, Sacramento, CA,
http://www.lightobject.com/), a wax piston-driven
thermostatic mixing valve (custom designed by Rostra
Veratherm, Bristol, CT), a bubble trap/expansion
chamber assembly, a heat sink water perfusion pad,
and the two hand interfaces, connected with plumbing
fittings and laboratory tubing and powered by four
AA (1.5 volt) batteries aligned in series. The path of
the water flow through the system was from pump
outlet to the inlet of a tee fitting. One outlet of the tee
fitting connected to a water perfusion pad (23 x
13 cm) that abutted the heat sink, which in turn was
connected to the cold inlet of the thermostatic mixing
valve. The second outlet of the tee connected directly
to the warm inlet port of the thermostatic mixing valve.
The outlet of the mixing valve fed into the palmar heat
exchange interfaces. The return from the interfaces
flowed through the bubble trap/expansion chamber
complex before returning to the inlet of the circulating
pump. The bubble trap/expansion chamber assembly
consisted of a 250 mL square wide mouth bottle
(Nalgene) plumbed to a bubble capturing manifold. A
hydration backpack (Model Lobo, Camelbak, Petaluma,
CA) was used to house the water circulating system and
heat sink. The heat sink was created by freezing 1 L of
water contained in the hydration pack bladder (the
bladder was lain flat during freezing). The wax in the
thermostatic mixing valve had a melting point of 18C.
This system delivered a 600 mL/min stream of
18C±2
C water to the hand interfaces. The entire sys-
tem weighed <2.25 kg (5 lb), was quiet (<10 decibels
at 10 cm), and fit under PPE outerwear.
Experimental protocols
Prior to participation in the experimental trials each
subject’s physical condition was assessed using a
Modified Bruce Test that determined VO
2max
.
[5]
Figure 1. A self-contained cooling system worn inside encapsulating outerwear (See “Methods”section for details). (A) Schematic
diagram of the components. Arrowheads indicate direction of water flow. Black outlined forms: core components connected with
rigid tubing (1. pump, 2. mixing valve, 5. bubble trap, and 6. expansion chamber). Shaded forms: peripheral components con-
nected via polyurethane tubing or electrical wire (4. palm interfaces, 3. heat sink interface, and 7. battery). Component 8 is the
external heat sink (e.g., frozen hydration bladder). (B) An individual clad in PPE wearing the cooling system. (C) The palm interface.
(D) The core of the system sitting atop the hydration pack in which it is housed.
JOURNAL OF OCCUPATIONAL AND ENVIRONMENTAL HYGIENE 575
All trials on individual subjects were conducted at
the same time of day and were separated by a min-
imum of 2 days. Prior to participation in a trial, sub-
jects measured their nude weight, self-inserted the
esophageal probe, attached the HR monitors, donned
the assigned attire (base clothing or base clothing and
PPE ensemble outerwear) and attached the palmar
heat transfer equipment. The orders of the treatments
were randomized. Upon completion of a trial the PPE
outerwear was removed and the subject rested quietly
in a thermoneutral environment for 30 minutes after
which the monitoring devices and heat transfer equip-
ment were removed and nude body weight was meas-
ured. No water was consumed during trials, but each
subject consumed a volume of fluid equivalent to his
or her weight loss prior to leaving the laboratory.
The slopes of the treadmills were adjusted for indi-
vidual subjects to achieve the desired exercise power
outputs based on the formula:
Power output Watts
ðÞ
¼0:11 þfractional grade
ðÞ
speed weight g;
where speed ¼treadmill speed (m/sec), fractional gra-
de ¼treadmill slope, weight ¼body weight (kg), and
g¼gravity (9.8 m/sec
2
).
[10]
The constant 0.11 accounts
for the power required to move the body mass over
level ground. The speeds of the treadmills were fixed
at 1.56 m/sec (3.5 mile/hr). The exercise was stopped
when one of the criteria was met: subjective fatigue,
T
es
¼39C, or an exercise duration of 40 min. The
subjects were encouraged to expectorate excess saliva
into a spittoon during the trials to minimize swallow-
ing artifacts in the T
es
data sets.
Protocol 1
Subjects engaged in vigorous (250 W) fixed load
treadmill exercise in a thermoneutral environment
while clad in the base layer only or in the base layer
and the PPE outerwear.
Protocol 2
Subjects clad in PPE outerwear exercised on the tread-
mill at power output levels simulating vigorous, mod-
erate and light intensity activity (250 W, 180 W,
and 100 W, respectively) in a thermoneutral environ-
ment with and without palm cooling.
Protocol 3
Subjects clad in PPE outerwear rested quietly (power
output ¼0 W) in a hot environment (T
a
¼45 ± 0.5C)
with and without palm cooling.
Protocol 4
Subjects clad in PPE outerwear engaged in light inten-
sity treadmill exercise with or without palm cooling
using the portable water circulating system or the
tethering stationary water-circulating device.
Data analysis
The raw T
es
vs.timedataweregraphedforeachtrial
using Microsoft Excel software and the graphs were
screened for irregularities. Artifactual data points were
expunged from the datasets. The data were then sorted
by 30-sec intervals and subjected to regression analysis
to determine rates of change of T
es
(DT
es
/min). Profiles of
T
es
vs. time during fixed-load exercise are typically bin-
ary—characterized by an initial rapid rise, followed by a
sustained shallower sloped linear rise (e.g., Fig. 2). The T
es
data from the final linear section of the plots (i.e., the last
20 min of the trials) were subjected to the regression ana-
lysis. The slopes from the regression analyses were tabu-
lated and sorted according to protocol, subject, and
treatment. Statistical analyses were performed using
Microsoft Office Excel for Windows data analysis tools.
The sorted data were summarized using descriptive statis-
tics (mean ± SD). The T
es
regression analysis data from
paired trials were analyzed using paired t-tests. Statistical
significance was established at a¼0.01.
Results
Protocol 1
The PPE ensemble compromised heat dissipation cap-
acity of individuals during vigorous fixed workload
treadmill exercise (power output ¼241 ± 53 W)
(Figure 2). The rate of core temperature rise (DT
es
/
Figure 2. The effect of PPE outerwear ensembles on T
es
dur-
ing vigorous fixed load treadmill exercise. Left: data from an
individual subject clad in PPE (gray) and without PPE (black).
Dotted lines represent the regression lines (regression line
equations, and R
2
coefficients are listed). Right: Rates of
changes in T
es
(C/min) during the last 20 min of exercise (as
determined by regression analysis) with and without PPE
(mean ± SD, p<0.001, paired t-test).
576 D. GRAHN ET AL.
min) while wearing PPE was approximately five times
that of DT
es
/min without the PPE ensemble
(0.064 ± 0.018C/min with PPE vs. 0.011 ± 0.005C/
min without PPE, p<0.001).
Protocol 2
Palm cooling reduced the rate of T
es
rise in subjects clad
in PPE during fixed load exercise at three intensity levels
(Figure 3). During vigorous exercise (power output
¼250 ± 73 W), DT
es
/min ¼0.095 ± 0.04C/min without
cooling vs. 0.063 ± 0.019C/min with cooling. During
moderate exercise (181 ± 78 W), DT
es
/min ¼
0.062 ± 0.014C/min without cooling vs. 0.044 ± 0.014C/
min with cooling. During light exercise (108 ± 40 W),
DT
es
/min ¼0.023 ± 0.012C/min without cooling vs.
0.016 ± 0.007C/min with cooling, (p<0.01, at
all workloads).
Protocol 3
Palm cooling reduced heat stress during rest in a hot
environment (T
a
¼45 ± 0.5C) (Figure 4). T
es
of PPE clad
individuals increased at a rate of 0.037 ± 0.008C/min with-
out cooling compared to 0.015 ± 0.005C/min with cool-
ing (p<0.001).
Protocol 4
The effect of palm cooling on T
es
of individuals clad in
PPE during light intensity exercise was independent of
the circulating water source (Figure 5). Both water circu-
lating systems reduced DT
es
/min to a comparable degree
(wearable system: DT
es
¼0.019 ± 0.012C/min without
palm cooling vs. 0.01 ± 0.009C/min with palm cooling,
tethering stationary systems: DT
es
/min ¼0.023 ± 0.01C/
min without palm cooling vs. 0.016 ± 0.007C/min with
palm cooling) (p<0.01 for both comparisons). The
choice of heat sink was not a significant factor (p¼0.09,
t-test assuming equal variance).
Discussion
The conditions in the Kerrytown, Sierra Leone, Ebola
Treatment Center were well documented and provide
a useful framework for the discussion of the portable
cooling system described in this article. The ETC in
Kerrytown, Sierra Leone, was in operation from
November 2014–September 2016. During that time
the monthly average day time high temperatures
ranged from 32C (August 2015) to 36C (December
2015) and nighttime lows ranged from 24–27C. The
risk of heat stress while wearing PPE in the ETC hot
zones limited work shift durations. Permissible work
periods were based on established guidelines for miti-
gating heat illness.
[11]
The United States Department
of Labor Occupational Health and Safety
Administration (OSHA) recommends an hourly work/
Figure 4. The effect of palm cooling on T
es
of subjects clad in
PPE during rest (power output ¼0 W) in a hot environment
(Ta ¼45 ± 0.5C, n ¼10). Left: T
es
vs. time (mean ± SD, n ¼10).
- significant treatment effect. Right: Rates of changes in T
es
(C/min) during the last 20 min of the trial with (C) and with-
out (N) palm cooling (mean ± SD, p<0.01 paired t-test).
Figure 5. Rates of change in T
es
during the last 20 min of
exercise with and without palm cooling using a wearable sys-
tem vs. the use of a tethering stationary heat sink
(mean ± SD). Treatment effects (cooling vs. no cooling) under
both conditions were significant (p<0.01, paired t-tests). The
type of heat sink did not influence treatment effect
(NS, p¼0.09).
Figure 3. The effects of palm cooling on rates of changes in
T
es
(C/min) during the last 20 min of light, moderate, and vig-
orous fixed load treadmill exercise by individuals clad in PPE
ensembles (mean ± SD, p<0.01, paired t-tests at all work-
loads). Open bars: no palm cooling. Closed bars: palm cooling.
JOURNAL OF OCCUPATIONAL AND ENVIRONMENTAL HYGIENE 577
rest schedule of 40 min of work and 20 min of rest for
individuals clad in chemical resistance suits engaged
in light work in a no-sun 33C environment. In con-
trast, there are no such work/rest schedule recommen-
dations for individuals clad in normal work clothes
under those environmental conditions. HCPs at the
Kerrytown, Sierra Leone ETC typically occupied the
hot zone for a maximum of 70 min during the daily
thermal peak periods.
[3]
Sufficient time for thermal
recovery (a minimum of 30 min) was enforced
between the doffing of the used PPE ensemble and
subsequent donning of a fresh PPE ensemble. The
adoption of wearable devices that enhance heat dissi-
pation capacity would reduce the need for limiting
work time of individuals clad in PPE and would also
decrease the number of PPE ensemble replace-
ments required.
The impetus for this project was to develop a prac-
tical solution to mitigate the heat stress problems
experienced by HCPs working at infectious disease
treatment centers such as the Kerrytown ETC during
the 2014–2015 Ebola virus outbreak in western Africa.
While the 2014–2015 Ebola outbreak has waned, a
wearable cooling system would be a valuable tool for
field applications in hostile work environments that
require the use of encapsulating protective outerwear.
If properly developed, future iterations of this wear-
able technology could dramatically impact the logistics
of the workforce in environments that require the use
of PPE.
The glabrous skin regions of the distal extremities
and face are the primary heat exchange surfaces of the
body. The direct arterial input from the heart and dir-
ect venous return to the vena cava creates an efficient
thermal conduit for transferring heat directly between
the body core and the external environment. Under
heat stress conditions, heat transfer from a single
hand to cool water can exceed 100 W.
[12,13]
Immersion of distal appendages in cool water has
been shown to be effective in reducing heat stress sub-
jects clad in military-style protective outerwear.
[14–21]
Heat transfer across the skin surface is dependent on
subcutaneous blood flow. The only regions of the dis-
tal appendages that contain vascular structures capable
of accommodating large subcutaneous blood flows are
the anatomically distinct glabrous skin regions. Thus,
the bulk of the heat transfer across the surface of the
distal appendages is via the glabrous skin regions. The
portable heat dissipation system described here utilizes
the glabrous skin thermoregulatory effector vascular
structures to maximize the thermal impact of treat-
ment on critical core regions of the body.
The results presented here demonstrate that a
wearable cooling system is a viable means for reduc-
ing the thermal load despite encapsulating outerwear.
However, a demonstration of feasibility in a controlled
laboratory setting is a far cry from a field ready piece
of equipment. Optimization of efficiency and utility
are critical for the transition into a field useful tool. A
short list of required design modifications includes:
optimizing heat sink temperatures, improving the
ergonomics of the interfaces, incorporating the system
into garments, potentially utilizing additional glabrous
skin regions, and reducing the size of the system. The
heat sink temperature is critical for optimizing the
system. The greater the differential temperature gradi-
ent across the heat exchange surface (i.e., the lower
the temperature of the circulating water heat sink) the
more efficient the heat transfer.
[17]
However, below a
critical temperature a local feedback response causes
vasoconstriction of the arteriovenous anastomoses that
supply blood to the subcutaneous radiator structures
and, as a result, effectively shuts down the heat trans-
fer. Also, a heat sink maintained at a temperature
below the vasoconstrictive threshold will, over time,
become uncomfortable and could compromise fine
motor function. We anticipate that circulating water
temperatures near 10C will prove to be optimal for
individuals clad in encapsulating outerwear conduct-
ing light to moderate workload activities.
Conclusions
Glabrous skin region cooling as an effective means for
extending fixed workload exercise in individuals clad
in encapsulating outerwear. A self-contained tempera-
ture-controlled water circulation system worn beneath
the encapsulating outerwear is a solution for tempera-
ture management in field applications.
Acknowledgments
The authors acknowledge and appreciate the support of
Vinh Cao and Patricia Seawell for the execution of the pro-
tocols as well as the undergraduate students who partici-
pated in the studies. Our thanks to DuPont and Camelbak
Products for generously supplying the PPE overalls and the
hydration packs (respectively). No external funding sup-
ported these studies.
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