NIOSH defines heat stress as the sum of the heat generated in the body (metabolic heat) plus
the heat gained from the environment (environmental heat) minus the heat lost from the body
to the environment, primarily through evaporation [NIOSH 2013]. Many bodily responses
to heat stress are desirable and beneficial because they help regulate internal temperature
and, in situations of appropriate repeated exposure, help the body adapt (acclimatize) to the
work environment. However, at some stage of heat stress, the body’s compensatory measures
cannot maintain internal body temperature at the level required for normal functioning. As
a result, the risk of heat-related illnesses, disorders, and accidents substantially increases.
Increases in unsafe behavior, which may lead to accidents, are also seen as the level of
physical work of the job increases [NIOSH 1986].
Many heat stress guidelines have been developed to protect people against heat-related
illnesses. The objective of any heat stress index is to prevent a person’s CBT from rising
excessively. The World Health Organization concluded that, “it is inadvisable for CBT to
exceed 100.4°F or for oral temperature to exceed 99.5°F in prolonged daily exposure to
heavy work and/or heat.” Additionally, a CBT of 102.2°F should be considered reason to
terminate exposure even when CBT is being monitored [NIOSH 1986]. This does not mean
that an employee with a CBT exceeding those levels will necessarily experience adverse
health effects; however, the number of unsafe acts increases as does the risk of developing
heat stress illnesses [NIOSH 1986].