CDC Report Suggests Heat Index of 85°F as Screening Threshold for Preventing Heat-related Illness

Published July 5, 2018

​In cases where employers do not have access to wet bulb globe temperature measurements for outdoor work sites, a Heat Index of 85° Fahrenheit (29.4° Celsius) could be used as a screening threshold to prevent heat-related illness, according to an article in the latest issue of CDC’s Morbidity and Mortality Weekly Report. Researchers made their determination following a review of data for 25 outdoor occupational heat-related illnesses investigated by OSHA between 2011 and 2016, 14 of which were fatal.

The researchers set out to determine whether NIOSH exposure limits for heat stress were protective of workers, and found that “the sensitivity of the NIOSH exposure limits was 100% (14 of 14) for detection of fatal heat stress and 72.7% (eight of 11) for detection of conditions that caused nonfatal illness.”

NIOSH has two sets of exposure limits for heat stress: recommended alert limits, or RALs, for workers who are not acclimatized to work in hot environments; and recommended exposure limits for acclimatized workers. Both RALs and RELs are sliding-scale limits based on environmental and metabolic heat loads.

Researchers reconstructed the conditions for each of the 25 heat illness cases by referring to archived climatologic data at local stations of the National Oceanic and Atmospheric Administration, estimating heat index using an NOAA algorithm, and estimating WBGT at the time of each incident through use of a heat and mass transfer model. The median Heat Index—a measurement intended for the general public that purports to reflect what heat “feels like”—for the 25 incidents was 91°F (33.3°C).

Guidance from OSHA, which does not have a permissible exposure limit for heat stress, states that a Heat Index of less than 91°F is associated with a lower risk of heat-related illness. But the CDC researchers determined that the Heat Index was less than 91°F for six of the 14 fatalities they studied.

The researchers recommend that employers measure heat stress throughout the work day, preferably through use of WBGT. If WBGT is not available, the researchers recommend using a Heat Index threshold of 85°F to screen for hazardous heat.

Related: One of the authors of the CDC report, Brenda Jacklitsch, authored an article on protecting workers in hot environments in the April 2016 issue of The Synergist.