Cih - Thermal Stress 2013

Front 1

1. heat strain’s risks: Discomfort Disorders Diseases Death:
the Four Ds, a physiological continuum
2. heat strain physiology and human thermoregulation
biological effects of heat stress
human thermal balance and heat exchange
sentinel adverse health effect events
3. problem industries, occupations, conditions, jobs
4. risk factors (Hazard + Exposure = Risk)
environmental parameters (heat stress)
medical issues !
biological considerations, human factors
job factors
5. measurements and workplace and worker evaluations, instruments
6. standards, guidelines, regulations
7. myths and tenaciously held erroneous beliefs
8. occupational and environmental case studies
9. control methods and prevention strategies
10. indoor thermal comfort zones; environmental heat “waves”
11. ten sure ways to kill football players and other athletes
12. cold stress and strain (hypothermia)

Back 1

Heat Stress is the net heat load on the
body for all workers in the same environment
assuming identical exposures, same clothing,
metabolic rates, and work tempo.
Heat Strainis net physiological load
resulting from heat stress on an individual
worker. Heat strain will vary considerably
among workers in the same heat stressing
environment.
Heat Stress ≠ Heat Strain

Front 2

Principles of Thermoregulation
The seven mechanisms of human heat loss and/or gain are radiation, conduction,
convection, evaporation, respiration, excretion, and metabolism. An algebraic equation
relating these factors is:
S = M ± C1 ± C2 ± R1 ± R2 - E1 - E2
S = heat storage in the body
M = metabolic rate, total metabolism (anabolism + catabolism)
C1 = conduction: The direct transfer of heat to cooler objects or receipt of heat from
warmer objects when direct contact is made. Heat transfer is proportional to the
thermal gradient, surface area, and contact time.
C2 = convection: The transfer of heat is dependent on air velocity over the skin, air
temperature, and surface area.
R1 = respiration: Heat exchange is determined by the amount of heat needed to warm
inhaled air to body temperature or the amount of heat absorbed from air.
R2 = radiation: The transfer of electromagnetic heat (infrared radiation) to those cooler
parts of the environment not in contact with the body, or receipt of

Back 2

Heat is generated in the body by oxidation of food, metabolic and
muscular activity, and gain of heat from the environment. When a
person is not excessively heat strained, heat is lost from the body by:
Convection and conduction by about 25%. Convection is normally
more than conduction. When the dry bulb ≥ 95ºF, convection ceases
as a heat loss mechanism. When the dry bulb temperature exceeds
95ºF, the body gains heat from warmer air heating cooler skin.
Radiation is about 43%. This is a two-way street because some
environments (e.g., molten metal, glass) can put more radiant energy
into the body than the body can radiate to the environment. Every
object above absolute zero radiates (animate and inanimate).
Evaporation of moisture from the skin is about 30%. In some work
environments, this can be 90% or more of a person’s heat loss.
Exhaled air is about 2%. That is, exhaled air at 99ºF is 100% relative
humidity and holds more moisture than, for example, air inhaled at
70ºF and 40% rela

Front 3

Heat Stress Standards
1. OSHA: None (General Duty Clause applies)
2. Wet Bulb Globe Temperature
WBGT with infrared radiant heat exposure =
O.7 Tnwb + 0.2 Tgt + 0.1 Tdb
WBGT without infrared radiant heat exposure =
0.7Tnwb+ 0.3Tgt
3. Heat Stress Index (Belding & Hatch)
HSI = (Ereq/Emax) x 100
4. ACGIH Threshold Limit Values
5. Effective Temperature, ASHRAE, USWS, et al

Back 3

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