The “heat index” combines humidity and air temperature to quantify what conditions feel like to the human body. It is an approximate guide, meant for the general public, and assumes that a person is wearing light clothing and walking in a shaded area with a light breeze—it does not account for direct sunlight, stagnant air, work clothing, or strenuous activity.
A better way to gauge whether workers are safe in the heat, accoriding to the National Institute for Occupational Safety and Health, is to follow recommended occupational exposure limits for heat stressthat specify the maximum combination of environmental heat (measured as the wet bulb globe temperature [WBGT] and metabolic heat). Because WBGT incorporates 4 environmental factors—air temperature, relative humidity, wind speed, and radiation—it is the recommended workplace environmental heat metric. But how well does it work in real (hot) life?
Researchers from the CDC reviewed 25 cases of outdoor occupational heat-related illness investigated by the Occupational Safety and Health Administration between 2011-2016. For each incident, Occupational Safety and Health assessed personal risk factors and estimated WBGT, workload, and the workers’ acclimatization status (that is, how new he/she was to the job’s environment).
Fourteen of the cases were fatal. Heat stress exceeded exposure limits in all 14 deaths and 8 of 11 nonfatal illnesses. Each incident occurred during outdoor work. Of the 25 workers, 12 had at least 1 predisposing personal risk factor. Workload was moderate to very heavy in 13 of the fatal cases; the fourteenth worker was unacclimated.
The Heat Index ranged from 83° to 110° F, with a median of 91°. Notably, 4 nonfatal illnesses and 4 deaths occurred when the Heat Index was between 85° and 90° F. The researchers also cite other cases of serious illness when the heat index was below 90°, including 14% of moderate-to-severe heat-related illnesses at a US military training installation.
The sensitivity of the NIOSH exposure limits was 100% for detection of fatal heat stress and 73% for nonfatal cases. The study findings suggest that the recommended limits are safe for most workers. However, because working in high heat can be risky, the CDC advises that when the Heat Index is 85° F or higher, employers should take additional precautions, such as:
- designating someone to monitor heat conditions;
- providing extra protection to new workers;
- scheduling frequent breaks in a cooler location and supplying water; and
- adjusting schedules and workload to stay below established heat stress limits.