Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 432

understand the hazards posed by temperature extremes, industrial hygienists must be familiar with
several basic concepts related to thermal energy and comfort. The most important of these are sum-
marized here:

• Conduction is the transfer of heat between two bodies that are touching or from one loca-
tion to another within a body. For example, if an employee touches a workpiece that has
just been welded and is still hot, heat will be conducted from the workpiece to the hand. Of
course, the result of this heat transfer is a burn.
• Convection is the transfer of heat from one location to another by way of a moving medium
(a gas or a liquid). Convection ovens use this principle to transfer heat from an electrode by
way of gases in the air to whatever is being baked.
• Metabolic heat is produced within a body as a result of activity that burns energy. All
humans produce metabolic heat. This is why a room that is comfortable when occupied by
just a few people may become uncomfortable when it is crowded. Unless the thermostat is
lowered to compensate, the metabolic heat of a crowd will cause the temperature of a room
to an uncomfortable level.
• Environmental heat is produced by external sources. Gas or electric heating systems pro-
duce environmental heat, as do sources of electricity and a number of industrial processes.
• Radiant heat is the result of electromagnetic nonionizing energy that is transmitted through
space without the movement of matter within that space.
• Most experts state that workers should not be permitted to work when their deep body
temperature exceeds 38°C (100.4°F).
• Heat is a measure of energy in terms of quantity.
• A calorie is the amount of energy in terms of quantity.
• Evaporative cooling takes place when sweat evaporates from the skin. High humidity
reduces the rate of evaporation and thus reduces the effectiveness of the body's primary
cooling mechanism.

15.8.3 b ody ' s r esponse to h eat
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 engaged in such operations. Such places include: Iron and steel foundries,
nonferrous 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.
Outdoor operations conducted in hot weather, such as construction, refining, asbestos removal,
and hazardous waste site activities, especially those that require workers to wear semipermeable
or impermeable protective clothing, are also likely to cause heat stress among exposed workers
(Spellman and Whiting, 2005).
The human body is equipped to maintain an appropriate balance between the metabolic heat it
produces and the environmental heat to which it is exposed. Sweating and the subsequent evapora-
tion of the sweat are the body's way of trying to maintain an acceptable temperature balance. This
balance can be expressed as a function of the various factors in the following equation.

H = M ± R ± C - E (15.16)

where
H = Body heat.
M = Internal heat gain (metabolic).
R = Radiant heat gain.