|The Occupational Safety and Health Act (OSH Act) requires employers to comply with hazard-specific safety and
health standards. In addition, pursuant to Section 5(a)(1) of the OSH Act, employers must provide their employees with a workplace
free from recognized hazards likely to cause death or serious physical harm. Emergency Preparedness Guides do not and cannot enlarge or
diminish an employer's obligations under the OSH Act.
Emergency Preparedness Guides are based on presently available information, as well as current occupational safety and health provisions
and standards. The procedures and practices discussed in Emergency Preparedness Guides may need to be modified when additional, relevant
information becomes available or when OSH Act standards are promulgated or modified.
During emergency response activities or recovery operations, workers
may be required to work in hot environments, and sometimes for
extended periods. Heat stress is a common problem encountered in
these types of situations. The following frequently asked questions
will help workers understand what heat stress is, how it may affect
their health and safety, and how it can be prevented.
Where might I be exposed
to heat stress?
Any process or job site that is likely to raise the workers deep core temperature
(often listed as higher than 100.4 degrees F (38°C)) raises the risk of heat
Operations involving high air temperatures, radiant heat sources, high humidity,
direct physical contact with hot objects, or strenuous physical activities have
a high potential for inducing heat stress in employees. Indoor operations such
as foundries, brick-firing and ceramic plants, glass products facilities, rubber
products factories, electrical utilities (particularly boiler rooms), bakeries,
confectioneries, commercial kitchens, laundries, food canneries, chemical plants,
mining sites, smelters, and steam tunnels are examples of industrial locations
where problems can occur. Outdoor operations
conducted in hot weather, such as construction, refining, asbestos removal, hazardous waste site activities, and emergency response operations, especially those that require workers to wear
semi-permeable or impermeable protective clothing, are also likely to cause heat
stress among exposed workers.
Are there additional causal factors for heat stress?
Age, weight, degree of physical fitness, degree of acclimatization,
metabolism, dehydration, use of alcohol or drugs, and a variety of medical
conditions such as hypertension all affect a person's sensitivity
to heat. However, even the type of clothing worn must be considered.
Prior heat injury predisposes an individual to additional injury.
Individual susceptibility varies. In addition, environmental
factors include more than the ambient air temperature. Radiant
heat, air movement, conduction, and relative humidity all affect
an individual's response to heat.
What kind of heat disorders and health effects are possible and
how should they be treated?
Heat Stroke is the most serious heat
related disorder and occurs when the body's temperature regulation fails
and body temperature rises to critical levels. The condition
is caused by a combination of highly variable factors, and its
occurrence is difficult to predict. Heat stroke is a medical
emergency that may result in death. The primary signs and symptoms
of heat stroke are confusion; irrational behavior; loss of
consciousness; convulsions; a lack of sweating (usually); hot, dry skin; and
an abnormally high body temperature, e.g., a rectal temperature
of 41°C (105.8°F). The elevated metabolic temperatures
caused by a combination of work load and environmental heat,
both of which contribute to heat stroke, are also highly variable
and difficult to predict.
If a worker shows signs of possible heat stroke, professional
medical treatment should be obtained immediately. The worker
should be placed in a shady, cool area and the outer clothing should
be removed. The worker's skin should be wetted and air movement
around the worker should be increased to improve evaporative
cooling until professional methods of cooling are initiated and
the seriousness of the condition can be assessed. Fluids should
be replaced as soon as possible. The medical outcome of an episode
of heat stroke depends on the victim's physical fitness and the
timing and effectiveness of first aid treatment.
Regardless of the worker's protests, no employee suspected of
being ill from heat stroke should be sent home or left unattended
unless a physician has specifically approved such an order.
Heat Exhaustion signs and symptoms are headache, nausea, vertigo,
weakness, thirst, and giddiness. Fortunately, this condition
responds readily to prompt treatment. Heat exhaustion should
not be dismissed lightly. Fainting or heat collapse which is often associated with heat
exhaustion. In heat collapse, the brain
does not receive enough oxygen because blood pools in the extremities.
As a result, the exposed individual may lose consciousness. This
reaction is similar to that of heat exhaustion and does not affect
the body's heat balance. However, the onset of heat collapse
is rapid and unpredictable and can be dangerous especially if workers are operating
machinery or controlling an operation that should not be left
unattended; moreover, the victim may be injured when he or she
faints. Also, the signs and symptoms seen in heat exhaustion
are similar to those of heat stroke, a medical emergency.
Workers suffering from heat exhaustion should be removed from
the hot environment and given fluid replacement. They should
also be encouraged to get adequate rest and when possible ice
packs should be applied.
Heat Cramps are usually caused by performing hard physical labor
in a hot environment. These cramps have been attributed to an
electrolyte imbalance caused by sweating. Cramps appear to be caused by the lack of
water replenishment. Because sweat is a hypotonic solution (±0.3%
NaCl), excess salt can build up in the body if the water lost through sweating
is not replaced. Thirst cannot be relied on as a guide to the need for water;
instead, water must be taken every 15 to 20 minutes in hot environments. Under
extreme conditions, such as working for 6 to 8 hours in heavy protective gear, a
loss of sodium may occur. Recent studies have shown that drinking commercially
available carbohydrate-electrolyte replacement liquids is effective in
minimizing physiological disturbances during recovery.
Heat Rashes are the most common problem in hot work environments
where the skin is persistently wetted by unevaporated sweat.
Prickly heat is manifested as red papules and usually appears
in areas where the clothing is restrictive. As sweating increases,
these papules give rise to a prickling sensation. Heat rash papules
may become infected if they are not treated. In most cases, heat
rashes will disappear when the affected individual returns to
a cool environment.
Heat Fatigue is often caused by a lack of acclimatization.
A program of acclimatization and training for work in
hot environments is advisable. The signs and symptoms of heat
fatigue include impaired performance of skilled manual, mental,
or vigilance jobs. There is no treatment for heat fatigue except
to remove the heat stress before a more serious heat-related
What kind of
engineering controls can be utilized?
General ventilation dilutes hot air with cooler air
(ideally, bringing in cooler outside air) and in is the most cost
A permanently installed ventilation system usually can handle large
areas or entire buildings. Portable or local exhaust systems
may be more effective or practical in smaller areas.
Air treatment/air cooling differs from
ventilation because it reduces the temperature of the air by removing the heat
(and sometimes humidity) from the air. Air conditioning is a method of air
cooling which uses a compressed refrigerant under pressure to remove the heat
from the air. This method is expensive to install and operate. An alternative to
air conditioning is the use of chillers to circulate unpressurized cool water
through heat exchangers over which air from the ventilation system is then
passed. Chillers are more efficient in cooler climates or in dry climates where
evaporative cooling can be used. Local
air cooling can be effective in reducing air temperature in specific
areas. Two methods have been used successfully in industrial
settings. One type, cool rooms, can be used to enclose a specific
workplace or to offer a recovery area near hot jobs. The second
type is a portable blower with built-in air chiller. The main
advantage of a blower, aside from portability, is minimal set-up
Another way to reduce heat
stress is to cool the employee by increasing the air flow or
convection using fans, etc. in the work area. This is generally
only effective as long as the air temperature is less than the worker's skin
less than 95 degrees F dry bulb). Changes in air speed can help
workers stay cooler by increasing both the convective heat exchange
(the exchange between the skin surface and the surrounding air)
and the rate of evaporation. This does not actually cool the
air so moving air must impact the worker directly to be effective.
Heat conduction blocking methods
include insulating the hot surface that generates the heat
and changing the surface itself. Simple devices such as shields,
can be used to reduce radiant heat, i.e. heat coming from hot
surfaces within the worker's line of sight. Polished surfaces
make the best barriers, although special glass or metal mesh
surfaces can be used if visibility is a problem With some sources
of radiation, such as heating pipes, it is possible to use
both insulation and surface modifications to achieve a substantial
reduction in radiant heat.
What administrative or work practice controls may be used?
Acclimatize workers by exposing them to work in a hot environment
for progressively longer periods. NIOSH (1986) suggests that
workers who have had previous experience in jobs where heat levels
are high enough to produce heat stress may acclimatize with a
regimen of 50% exposure on day one, 60% on day two, 80% on day
three, and 100% on day four. For new workers who will be similarly
exposed, the regimen should be 20% on day one, with a 20% increase
in exposure each additional day.
Replace Fluids by providing cool (50°-60°F) water or
any cool liquid (except alcoholic beverages) to workers and encourage
them to drink small amounts frequently, e.g., one cup every 20
minutes. Ample supplies of liquids should be placed close to
the work area. Although some commercial replacement drinks contain
salt, this is not necessary for acclimatized individuals because
most people add enough salt to their summer diets.
Reduce the physical demands by reducing physical exertion such
as excessive lifting, climbing, or digging with heavy objects.
Spread the work over more individuals, use relief workers or
assign extra workers. Provide external pacing to minimize overexertion.
Provide recovery areas such as air-conditioned enclosures and
rooms and provide intermittent rest periods with water breaks.
Reschedule hot jobs for the cooler part of the day, and routine
maintenance and repair work in hot areas should be scheduled
for the cooler seasons of the year.
Monitor workers who are at risk of heat stress, such
as those wearing semi-permeable or impermeable clothing when
the temperature exceeds 70°F, while working at high metabolic
loads (greater than 500 kcal/hour). Personal monitoring can be
done by checking the heart rate, recovery heart rate, oral temperature,
or extent of body water loss.
To check the heart rate, count pulse for 30 seconds at the beginning
of the rest period. If the heart rate exceeds 110 beats per minute,
shorten the next work period by one third and maintain the same
The recovery heart rate can be checked by comparing the pulse
rate taken at 30 seconds (P1) with the pulse rate taken at 2.5
minutes (P3) after the rest break starts. The two pulse rates
can be interpreted using the following criteria.
|Heart rate recovery pattern
P1 and P3
|High recovery (Conditions may require further study)
|No recovery (May indicate too much stress)
Check oral temperature with a
clinical thermometer after work but before the employee drinks
water. If the oral temperature taken under the tongue exceeds
37.6°C, shorten the next work
cycle by one third.
Measure body water loss by weighing the worker on a scale
at the beginning and end of each work day. The worker's weight
loss should not exceed 1.5% of total body weight in a work day.
If a weight loss exceeding this amount is observed, fluid intake
Develop a heat stress training program, and incorporate into health and safety plans at least
the following components:
- Knowledge of the hazards of heat stress;
- Recognition of predisposing factors, danger signs, and
- Awareness of first-aid procedures for, and the potential
health effects of, heat stroke;
- Employee responsibilities in avoiding heat stress;
- Dangers of using drugs, including therapeutic ones, and
alcohol in hot work environments;
- Use of protective clothing and equipment; and
- Purpose and coverage of environmental and medical surveillance programs and the advantages of worker participation in such
What Personal Protective Equipment is effective in minimizing
Reflective clothing, which can vary from aprons and jackets
to suits that completely enclose the worker from neck to feet,
can reduce the radiant heat reaching the worker. However, since
most reflective clothing does not allow air exchange through
the garment, the reduction of radiant heat must more than offset
the corresponding loss in evaporative cooling. For this reason,
reflective clothing should be worn as loosely as possible.
In situations where radiant heat is high, auxiliary cooling
systems can be used under the reflective clothing.
Auxiliary body cooling ice vests, though heavy, may accommodate
as many as 72 ice packets, which are usually filled with water.
Carbon dioxide (dry ice) can also be used as a coolant. The
cooling offered by ice packets lasts only 2 to 4 hours at moderate
to heavy heat loads, and frequent replacement is necessary.
However, ice vests do not tether the worker and thus permit
maximum mobility. Cooling with ice is also relatively inexpensive.
Wetted clothing such as terry cloth coveralls or two-piece,
whole-body cotton suits are another simple and inexpensive
personal cooling technique. It is effective when reflective
or other impermeable protective clothing is worn. This approach
to auxiliary cooling can be quite effective under conditions
of high temperature, good air flow, and low humidity.
Water-cooled garments range from a hood, which cools only the
head, to vests and "long johns," which offer partial
or complete body cooling. Use of this equipment requires a
battery-driven circulating pump, liquid-ice coolant, and a container.
Although this system has the advantage of allowing wearer mobility,
the weight of the components limits the amount of ice that can
be carried and thus reduces the effective use time. The heat
transfer rate in liquid cooling systems may limit their use to
low-activity jobs; even in such jobs, their service time is only
about 20 minutes per pound of cooling ice. To keep outside heat
from melting the ice, an outer insulating jacket should be an
integral part of these systems.
Circulating air is the most highly effective, as well as the
most complicated, personal cooling system. By directing compressed
air around the body from a supplied air system, both evaporative
and convective cooling are improved. The greatest advantage
occurs when circulating air is used with impermeable garments
or double cotton overalls. One type, used when respiratory
protection is also necessary, forces exhaust air from a supplied-air
hood") around the neck and down inside an impermeable
suit. The air then escapes through openings in the suit. Air
can also be supplied directly to the suit without using a hood
in three ways: by a single inlet, by a distribution tree, or
by a perforated vest. In addition, a vortex tube can reduce the temperature of circulating air. The cooled air
from this tube can be introduced either under the clothing
or into a bubble hood. The use of a vortex tube separates the
air stream into a hot and cold stream; these tubes also can
be used to supply heat in cold climates. Circulating air, however,
is noisy and requires a constant source of compressed air supplied
through an attached air hose. This system tethers the worker
and limits his or her mobility. Additionally, since the worker feels
comfortable, he or she may not realize that it is important
to drink liquids frequently.