Thermal Stress

Thermal stress is a significant physical agent in many working

environments. Just considering routine work out-ofdoors, air temperatures

between –20 to 110 F are expected over different regions of the United

States. Other countries may reasonably expect temperatures beyond that

range. Human-made environments from freezers to ovens extend the range

of thermal environments in which work is expected. Because tasks must be

performed under adverse thermal conditions, this chapter provides guidance

for recognition, evaluation and control of work in thermal extremes.

DEGREES OF THERMAL STRESS

Conceptually, work can occur in one of five zones along the continuum of

thermal stress. In the middle is the comfort zone. Here, most people would

report thermal sensations as being acceptable (neither hot nor cold). In the

comfort zone, the demands for physiological adaptation are modest and

productivity should be the greatest. The comfort zone is described at the end

of this chapter to provide information to health and safety professionals who

may be asked to evaluate the thermal conditions with comfort as a goal.

On either side of the comfort zone are the discomfort zones for heat and cold

stress. Under these conditions, most people should be able to safely work

without experiencing a disorder related to the stress (i.e., heat-related or

cold-related disorders). They will report sensations of cold or heat,

productivity and quality of work may decrease, and the risk of accidents may

increase. The goal of most evaluation schemes for occupational heat and

cold stress is to limit exposures to the discomfort zone.

The health risk zone for heat and cold stress are the outer zones of the

thermal stress continuum. The physiological adaptations have reached their

limits and work capacity is severely limited. In the health risk zone, the

likelihood of heat and cold stress-related disorders increases markedly.

Health and safety professionals should manage exposures in the health risk

zone. Management of exposures in the health risk zone is the principal

theme of this chapter.

Of course, there are no firm boundaries to these zones because the

boundaries depend on the environment, individuals, and season as well as

many unknown variables. But we should try to control the thermal stress

factors for the less tolerant workers in order to minimize the risk of injuries

and illness to the lowest reasonable level. The major emphasis on evaluation

and control is placed on the transition from the discomfort zone to the health

risk zone for both heat and cold stress.

THERMAL BALANCE

Model of Thermal Balance

Three factors influence the degree of thermal stress. The most obvious factor

is the climatic conditions of the environment. The other two factors are work

demands and clothing. The tradition for more than 40 years is to describe

thermal balance by an equation with major avenues of heat exchange

between the body and the environment represented by a term in the equation.

(There is no uniformly accepted version but the reader will not have

problems reconciling different versions as they are found.)

Most versions of the heat balance equation use ± instead of +, especially in

front of R and C. The purpose is to emphasize that the heat exchange

represented by R, C, K, and Cresp can be in either direction. A more rigorous

sign convention is used in this chapter. A positive value for any of these

terms (as opposed to the sign in front of the term) means that the heat is

gained by the body and a negative value means that heat is lost from the

body. The values for M and (M + W) can only be positive. The values for W,

Eresp and E are always negative, meaning that there is only heat loss

associated with these terms. Each term has the unit of energy per unit of

time; that is, the terms represent rates of energy transfer. The international

units (SI units) are watts, and other units that are reported include kcal/h,

kcal/min and Btu/h. Sometimes the rates are reported as normalized values

to body surface area.

S––HEAT STORAGE RATE

If the value for S is zero, the body is in thermal equilibrium, and heat gain is

balanced by loss from the body. If S is positive, the body is gaining heat at

the rate indicated by the value of S. If the value of S is negative, the body is

losing heat, and body temperature is decreasing.

M––METABOLIC RATE

Chemical reactions occur continuously inside the body. These serve to

sustain life (basal metabolism) and meet the demands of work (muscle

metabolism). As muscle metabolism increases to meet work demands, the

rate of energy conversion from chemical energy to kinetic energy increases.

Because the energy conversion from chemical energy to kinetic energy is

inefficient, increased metabolism results in increased rates of heat gain to the

person. The rate of metabolism depends directly on the rate and type of

external work demanded by the job.

W––EXTERNAL WORK RATE

W is the amount of energy that is successfully converted from internal

chemical energy to mechanical work on external objects. This route of

energy transfer is called external work and it does not contribute to body

heat. The rate of external work depends directly on forces applied against

external resistance and distance moved. W is usually about 10 percent of M.

R––RADIANT HEAT EXCHANGE RATE (RADIATION)

Solid bodies of different temperatures have a net heat flow from the hotter

surface to the cooler surface by electromagnetic radiation (primarily infrared

radiation). The rate of heat transfer by radiation depends on the average

temperature of the surrounding solid surfaces, skin temperature and clothing.

C––CONVECTIVE HEAT EXCHANGE RATE (CONVECTION)

The exchange of heat between the skin and the surrounding air is referred to

as convection. The direction of heat flow depends on the temperature

difference between the skin and air. If air temperature is greater than skin, C

is positive and heat flows from the air to the skin. If the air is cooler than the

skin, C is negative and heat flows from the body. The rate of convective heat

exchange depends on the magnitude of the temperature difference, the

amount of air motion, and clothing.

K––CONDUCTIVE HEAT EXCHANGE RATE (CONDUCTION)

When two solid bodies are in contact, heat will flow from the warmer body

to the cooler body. The rate of heat transfer depends on the difference in

temperatures between the skin and the solid surface, the thermal

conductivity of the solid body that the person contacts, and clothing that may

separate the person from the solid surface.

CRESP––RATE OF CONVECTIVE HEAT EXCHANGE BY

RESPIRATION

The fact that air is moved in and out of the lungs, which have a large surface

area, means there is an opportunity to gain or lose heat. The rate of heat

exchange depends on the air temperature and volume of air inhaled.

ERESP––RATE OF EVAPORATIVE HEAT LOSS BY RESPIRATION

The large surface area of the lungs provides an opportunity to lose heat by

evaporation. The rate of heat exchange depends on the air humidity and

volume of air inhaled.

E––RATE OF EVAPORATIVE HEAT LOSS

Sweat on the skin surface will absorb heat from the skin when evaporating

into the air. The process of evaporation cools the skin and in turn the body.

The rate of evaporative heat loss depends on the amount of sweating, air

movement, ambient humidity, and clothing.

Because W, K, Cresp and Eresp are small relative to the other routes of heat

exchange in industrial applications, they are usually ignored. When

calculating heat storage, Equation 1 becomes Equation 2 as a general

statement of heat balance.

Excessive heating or cooling of a small portion of the skin can occur when it

comes in contact with a hot or cold surface. The contact can be either

intentional or incidental. Injury occurs when there is sufficient heat gain to

cause a burn or heat loss to cause the tissue to freeze (or at least become

very cold for a period of time). In these cases, the local storage rate (Slocal)

becomes important.

where K is conductive heat transfer between the skin and an object, and D is

the rate of heat transfer to or from the local area by conduction through the

local tissue and by the heat supplied or removed via local blood flow.

Factors Affecting Thermal Balance

As mentioned at the beginning of the discussion on thermal balance, three

factors play an important role. They are the climatic conditions of the

environment, work demands, and clothing. Climatic conditions are widely

used to describe the degree of stress, as seen in casual descriptions by air

temperature, relative humidity, and wind chill. They are not the only

determinant of thermal stress.

The role of metabolic rate in heat balance is very important because it is a

substantial contributor to heat gain. In heat stress, metabolic rate can add 10

to 100 times more heat to the body than radiation and convection combined.

In cold stress, metabolic rate affects heat balance on the same order as

radiation and convection losses.

Clothing is also a major contributor to thermal balance. Clothing has three

characteristics: insulation, permeability, and ventilation.

Insulation is a measure of the resistance to heat flow by radiation,

convection, and conduction. The greater the amount of insulation, the less

the rate of heat flow from the warmer temperature to the cooler temperature.

During heat stress exposures, it reduces heat flow by radiation and

convection. It also reduces heat flow by conduction if a person has a

substantial portion of the body in contact with a warm surface. Insulation

plays a very important role (1) in preventing burns by contact with a hot

surface and (2) in cold stress. In cold stress, it is used to reduce heat losses

by convection and radiation as well as conduction, and it prevents cold

injury to local tissues in contact with cold surfaces.

Permeability is a measure of the resistance to water vapor movement

through the clothing. It is a factor in thermal stress because it influences the

amount of evaporative cooling that can be achieved. Permeability is related

to both insulation characteristics and the clothing fabrics. Generally, as

insulation increases, permeability decreases. In addition, some clothing

fabrics designed as a contamination barrier can reduce the magnitude of

permeability. This means that there may actually be a trade-off between the

risks of heat stress and the risks from skin contact with harmful chemicals.

New protective clothing fabrics are entering the market that provide

protection against some chemical hazards while permitting water vapor

transmission. These new fabrics provide a greater range of opportunity to

find a balance between prevention of chemical exposure and prevention of

heat stress.

Clothing ventilation is the third factor. Depending on the nature of the fabric,

garment construction, and work demands, ambient air can move through the

fabric or around the garment openings. Clothing ensembles that support the

movement of air can enhance evaporative and convective cooling; while

those that are designed and worn to limit such movement, limit evaporative

and convective cooling. A good example of using ventilation characteristics

to regulate heat balance is arctic parkas with drawstrings around the waist,

cuffs, and hood. As metabolism heats a person, cooling can be achieved by

loosening some of the closures to increase the amount of air flow

(ventilation) under the clothing.

HEAT STRESS

Remembering that thermal stress is a combination of environmental, work,

and clothing factors, heat stress is a combination that tends to increase body

temperature, heart rate, and sweating. These physiological adaptations are

collectively known as heat strain. Figure 12–1 is a schematic representation

of the physiological responses to heat stress.

Looking first at metabolism, the heat generated by muscular work heats the

deep body tissues, which means that there is a tendency for core temperature

to increase. Blood circulating through the core picks up heat energy, and the

warmer blood is directed to the skin where the blood is cooled. The cooler

blood returns to the core to pick up more heat energy. The skin is the site of

heat exchange with the environment. Convection and radiation depend on

temperature differences between the skin and the environment; and the net

heat exchange by R+C can be either positive (heat gain) or negative (heat

loss). In addition, the skin secretes sweat onto the surface. As the water

evaporates, it removes more heat energy from the skin, cooling the skin

surface.

Under ideal conditions, the body balances heat gains with losses so that the

storage rate, S, is zero. This is accomplished by increasing the sweating rate

until evaporative cooling is sufficient to remove the heat generated by

metabolism plus any heat gained from (or lost to) the environment through R

+ C. The required evaporative cooling is denoted as Ereq. (Remember that

the value of Ereq is negative because heat flow is away from the body.) Then

Equation 2 becomes

Thus, Ereq marks the degree of physiological adjustment required to

establish a thermal equilibrium between the body and the environment so

that the body does not store heat. In many heat stress exposures, M is the

dominant term, and Ereq increases to meet additional cooling requirements

of the work demands.

Heart rate is another important physiological parameter in assessing heat

strain because it reflects the demands on the cardiovascular system to move

blood (and heat) from the core to the skin. The total blood flow through the

heart is proportional to the metabolic rate and inversely proportional to the

temperature difference between the core and the skin. As work demands and

metabolic rate increase, cardiac output increases, as seen in the heart rate.

Sometimes skin temperature increases because evaporative cooling is

limited or the net heat gain from R + C is high. As the skin temperature

increases toward core temperature, more blood must be delivered to the skin

to achieve the same rate of cooling.

Finally, sweat rate (and total sweat volume) is another important measure of

physiological strain. The greater the level of heat stress, the greater is the

sweat loss. The body has a natural ability to increase the tolerance to heat

stress exposures through a process called acclimation (sometimes called

acclimatization). As people become acclimated, they are able to sweat more

and therefore increase their cooling capability. With increased cooling, heart

rate and core temperature are lower for the same work conditions.

The following material on heat stress describes recognition, evaluation, and

control of heat stress as it may affect the whole body. At the end, there is

information on special topics including contact with hot surfaces and

breathing of hot air.

Recognition of Heat Stress

Heat stress in the workplace can be recognized in terms of workplace risk

factors and in terms of the effects it has on workers. The workplace risk

factors, broadly stated, are hot environments, high work demands, and

protective clothing requirements. These factors are the traditional

considerations in the evaluation of heat stress, and the details are in the

section on evaluation of heat stress. In essence, if the workplace is generally

considered as being hot through subjective judgment of workers and

supervisors, then heat stress may be present. If the demands for external

work are high (e.g., high metabolic rate), heat stress may be a factor in

environments that are considered comfortable by casual observers (those not

exerting themselves in the environment). Clothing is the third factor. While

light-weight, loose-fitting, cotton clothing is the ensemble of choice during

exposures to heat stress, many workplaces require protective clothing that

decreases permeability and ventilation and increases insulation. The added

weight of personal protection may increase the metabolic heat load and

therefore the level of heat stress.

The responses of workers are a good tool for the recognition of heat stress in

the workplace. At the extreme end are a pattern of heat-related disorders.

Intermediate markers are physiological adjustments and worker behaviors.

HEAT-RELATED DISORDERS

Heat-related disorders are manifestations of over-exposures to heat stress.

Table 12–A is a list of common or important heat-related disorders. The

table includes the signs a trained observer may see, the symptoms the person

may report, the likely cause of the disorder, first aid, and steps for

prevention. Figure 12–2 is a simple illustration of normal responses to heat

stress and how these responses may lead to a heat-related disorder.

Heat stroke is the most serious heat-related disorder. While it may be

relatively rare, it must be immediately recognized and treated to minimize

permanent damage. The risk of death is high in heat stroke. Heat exhaustion

is the most commonly seen disorder when treatment is sought. Dehydration

is a precursor to heat exhaustion, but it is usually not noticed or reported by

workers.

As part of the recognition process, the health and safety professional

examines reports to a medical or first aid facility. Because no specific heat-

related disorders are listed does not mean heat stress is not present. It is

worthwhile to examine the records for reports of faintness, weakness,

nausea, cramps, headaches, and skin rashes. If temperatures are taken, some

may be elevated. If urine samples are taken, some may have high specific

gravity due to dehydration. There may also be an increase in the number of

accidental injuries that are related to heat stress conditions.