Tips for Reducing Workers' Heat Load
Dealing effecitvely with heat stress can increase workers' comfort, safety, and productivity.
- By Megan Torgrude, MPH, Don Garvey
- Jul 22, 2008
Heat stress can be a major concern in workplace
environments, potentially causing irritability,
low morale, absenteeism, shortcuts
in procedures, and unsafe behavior.1 In
extreme cases, heat stress, in the form of heat stroke, can
be fatal. The United States Bureau of Labor Statistics
data for the years 2003-2005 indicate an average of 31
worker deaths annually from exposure to “environmental
The main factors leading to heat stress include strenuous
physical activity, high air temperature, high humidity,
direct contact with hot materials, and radiant
heat sources. Some industries with these conditions include
foundries, bakeries, commercial kitchens, laundries,
chemical plants, mining sites, smelters, and more.
Seasonal potential for heat stress exists in many outdoor
operations, such as construction, asbestos removal, and
hazardous waste activities.Many of these functions also
require the use of semipermeable or impermeable protective
clothing, which adds to the heat stress burden.3
Heat Stroke Symptoms
Heat stress can lead to both heat exhaustion and heat
stroke. They have differing physical signs, which are
Symptoms of heat exhaustion include:
• Headaches, dizziness, lightheadedness, or fainting
• Moist skin
• Mood changes, such as irritability or confusion
• Upset stomach or vomiting
Symptoms of heat stroke include:
• Dry, hot skin with no sweating
• Mental confusion or losing consciousness
• Seizures or convulsions
Total heat load on a body is the combination
of environmental conditions, clothing,
and metabolic or workload factors. According
to the American Conference of
Governmental Industrial Hygienists
(ACGIH), the Wet Bulb Globe Temperature
(WBGT) can be useful in evaluating
the environmental contribution to heat
stress [ACGIH 2007]. In the indoor environment,
the WBGT is a combination of
wet bulb temperature (accounts for humidity)
and Vernon Globe temperature
(accounts for radiant heat sources). Outdoors,
a dry bulb temperature (to account
for solar heating) is also added. Each temperature
is weighted, and they are added together
using one of the following formulas:
• WGBT Indoors = (Wet Bulb Temp) 0.7 +
(Globe Temp) 0.3
• WGBT Outdoors = (Wet Bulb Temp) 0.7 +
(Globe Temp) 0.2 + (Dry Bulb Temp) 0.1
The WBGT can then be used with the
ACGIH Heat Stress and Heat Strain
Threshold Limit Value (TLV) to assess
worker exposure as part of an overall heatstress
The WBGT measurement may also help
to identify environmental conditions that can
contribute significantly to heat stress. That, in
turn,may help identify ways to reduce the environmental
heat load on a worker.
A heat stress monitor can quickly and
easily measure WBGT. It displays the overall
WBGT, as well as the individual temperatures
that comprise the WBGT reading.
When used in conjunction with the ACGIH
Thermal Stress TLV, the monitor is a valuable
tool for use in establishing and implementing
a heat-stress management program.
For more information on using WBGT
to assess heat stress and the risk to worker
health, consult the ACGIH Heat Stress TLV.5
As noted above, clothing and metabolic
workload also play a significant role in determining
a person’s overall heat strain.
Protective clothing can increase heat load
by reducing heat exchange with the environment
through reduced air movement
across the skin and inhibition of sweat
evaporation. Use of negative pressure airpurifying
respirators can increase the metabolic
workload because the worker must
supply the energy to draw air though the
Especially in a job that is already physically
demanding, either of these examples
can result in an increase in total heat load.
When looking for ways to reduce worker
heat load, methods to reduce each of these
factors (environment, clothing, and work
load) should be investigated.
Many workers are required to wear protective
clothing, such as high-visibility apparel
when working in traffic work zones, materials
handling areas, etc. Wearing additional
garments, such as reflective vests, can lead to
increased heat burden on the worker. Comfort
and visibility, however, can still be
achieved by incorporating reflective materials
directly into apparel such as t-shirts,
rather than requiring additional garments,
such as a vest, to be worn.
The design of the retro-reflective material
can help to improve moisture vapor
transmission, helping to keep workers drier
and cooler in certain situations. In addition,
workers don’t have to remember to put on
their reflective apparel because it is already a
part of their everyday work wear.
If respiratory protection is required, one
potential solution to reduce heat load is to
use a powered air-purifying respirator
(PAPR) or a supplied air respirator (SAR).
PAPRs use a battery and motor blower to
pull air through the respirator filter or cartridge
and blow it into the respirator headpiece.
SARs use a supplied air hose to deliver compressed breathing air to the
Both PAPRs and SARs help to reduce the
extra workload caused by breathing through
a non-powered respirator. They also have
the benefit of blowing air across the worker’s
face, which may supply a cooling effect.
Hood-type head pieces typically have an
inner shroud that is tucked under coveralls
or outer work clothing. The shroud can
channel air under the clothing, creating air
movement that may increase both convective
and evaporative cooling.
It should be noted, however, that PAPRs
do not cool the air. If a reduction in air temperature
is desired, a supplied air system
with a cooling vortex is required.
Cooling devices, or vortex tubes, may be
available as part of SAR systems. They are
powered solely from the pressurized air of a
compressor and are worn at the worker’s
waist.Vortex tubes can cool breathing air by
up to 50° F (28° C).Air-warming devices are
also available for cold work environments.
Workers can easily adjust the vortex to increase
or decrease cooling according to comfort
and changing work conditions.
With any respirator use, employers
should implement an effective respirator
program that complies with the requirements
of 29 CFR 1910.134 or other local
Excessive exposure to heat can seriously affect
worker health, safety, and productivity.Accurate
measurement of environmental conditions,
along with use of PPE that can minimize
or reduce worker heat load, can help to
reduce the risk of heat strain. Heat stress
monitors can help in evaluating the work environment,
while reflective products and
powered and supplied air respirators can help
minimize the heat load on workers.
1. National Institute for Occupational Safety and
Health (NIOSH). (1986) Working in Hot Environments
Publication No. 86-122. Cincinnati, OH.
2. Bureau of Labor Statistics (BLS). (2003-205)
Census of Fatal Occupational Injuries—Current and
Revised Data. www.bls.gov/iif/oshcfoi1.htm.
3. “Heat Stress.” Occupational Safety & Health Administration.
Accessed Feb. 29, 2008. www.osha
4. “OSHA Quick Card. Heat Stress.” Occupational
Safety & Health Administration. Accessed Feb. 29,
5. American Conference of Governmental Industrial
Hygienists (ACGIH). (2007) Threshold Limit Values
for Chemical Substances and Physical Agents and
Biological Exposure Indices. Cincinnati: American
Conference of Governmental Industrial Hygienists.
This article originally appeared in the July 2008 issue of Occupational Health & Safety.