The effects of heat stress range from simple discomfort to life threatening illnesses such as heat stroke. Aside from the environmental factors (such as temperature, humidity, air movement, and radiant heat), the persons work regimen and clothing requirements are also factors that determine the risk of heat strain. This paper describes the following three steps to managing heat stress in your workplace:
Step1: Evaluate heat stress to Identify personnel who have a high risk of heat strain. The Wet Bulb Globe Temperature (WBGT) screening criteria established by the American Conference for Governmental Industrial Hygienists (ACGIH) is the most commonly used method of monitoring of potential heat stress.
Step 2: Monitor and limit heat strain for high-risk personnel. One option of monitoring heat strain is to use an ear thermometer to monitor the persons core temperature.
Step 3: Implement heat stress control strategies. Consider a combination of engineering controls, administrative and work practice controls, and personal protective equipment to manage heat stress. This step should also be incorporated into Steps 1 and 2 as appropriate.
Step 1: Evaluate Heat Stress
Heat stress is the net heat load to which a worker may be exposed from the combined contributions of metabolic rate required of the work performed, environmental factors (i.e., air temperature, humidity, air movement, and radiant heat exchange), and clothing requirements. Heat strain is the overall physiological response resulting from heat stress. In order to identify tasks or environments that have a high risk of heat stress, the first step is to determine if heat stress screening criteria established by the ACGIH is exceeded. Heat strain (physiological) monitoring is recommended for workers determined to have excessive heat stress.
Heat Stress Screening Criteria
The WBGT is the most widely used heat stress index. Only three measurements are needed to calculate the WBGT globe temperature, dry bulb temperature, and natural wet bulb temperature. The WBGT is determined by the following equations, depending on if measurements are taken indoors (without a solar load) or outdoors (with a solar load):
Outdoors (with solar load): WBGT = 0.7(WT) + 0.2(GT) + 0.1(DT)
Indoors (without solar load): WBGT = 0.7(WT) + 0.3(GT)
WT = Natural wet bulb temperature
GT = Globe temperature
DT = air temperature (dry bulb)
The following table contains the WBGT screening criteria (in C) for heat stress recommended by the ACGIH:
The WBGT assumes that the worker is fully acclimatized, light clothing is worn by exposed workers, and employees have adequate water and salt intake. Full-heat acclimatization requires up to three weeks of continued physical activity under heat-stress conditions similar to those anticipated for the work. Acclimatization loss begins when the activity under those heat-stress conditions is discontinued. A noticeable loss in acclimatization occurs after four days. A worker can be considered acclimatized with a recent history of heat stress exposure (i.e., five of the last seven days).
In order to use the WBGT index as the screening criterion, the work pattern and demands must also be considered. As the metabolic rate increases, the WBGT screening criteria must be adjusted. Examples of metabolic rate categories include the following:
- Resting: Sitting quietly, sitting with moderate arm movements, etc.
- Light: Sitting with moderate arm and leg movements, standing with light work at machine or bench while using mostly arms, using a table saw, standing with light or moderate work at machine or bench and some walking about, etc.
- Moderate: Scrubbing in a standing position, walking about with moderate lifting or pushing, walking on level at 6 kilometers per hour while carrying a three-kilogram weight load, etc.
- Heavy: Carpenter sawing by hand, shoveling dry sand, heavy assembly work on a noncontinuous basis, intermittent heavy lifting with pushing or pulling, etc.
- Very heavy: Shoveling wet sand.
The WBGT also assumes that the worker is wearing light clothing. Depending on the type of clothing ensemble, additions to the measured WBGT may be needed. For summer work uniform, ACGIH recommends that 0 be added to the measured WBGT. For cloth (woven material) overalls, the ACGIH recommends that +3.5 be added to the measured WBGT. For double-cloth overalls, the ACGIH recommends that +5 be added to the measured WBGT.
Step 2: Monitor and limit heat strain
Monitoring and Limiting Heat Strain
Monitoring signs and symptoms of heat-stressed workers may be needed when clothing may significantly reduce heat loss and/or when WBGT screening criteria are exceeded. According to the ACGIH, excessive heat strain is indicated when one or more of the following occur:
- Sustained (several minutes) heart rate is in excess of 180 beats per minute (bpm) minus the individuals age in years (i.e., 180 age), for individuals with normal cardiac performance.
- Body core temperature is greater than 101.3 F for acclimatized individuals or greater than 100.4 F for unacclimatized workers.
- Recovery heart rate at one minute after a peak work effort is greater than 110 bpm.
- There are symptoms of sudden and severe fatigue, nausea, dizziness, or lightheadedness.
An individuals exposure to heat stress should be discontinued when any of the above conditions or symptoms occurs. Also, an individual may be at greater risk if profuse sweating is sustained over hours, weight loss over a shift is greater than 1.5% of body weight, or 24-hour urinary sodium excretion is less than 50 milli-moles.