CDC Report Suggests Heat Index of 85°F as Screening Threshold for Preventing Heat-related Illness
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 July 6 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. WBGT is the preferred measure because it incorporates air temperature, relative humidity, wind speed, and radiation or sunlight, all of which contribute to heat stress. If WBGT is not available, the researchers recommend using a heat index threshold of 85°F to screen for hazardous heat. In June 2017, OSHA and NIOSH jointly released an app for smartphones that displays the current heat index in the user’s location. The app is available for both Apple and Android devices.  According to NIOSH, employers should ensure that workers are acclimatized before beginning work in hot environments by gradually increasing time spent outdoors over a seven-to-fourteen-day period. For new workers, the agency recommends that work in hot environments should not exceed 20 percent of the workday. Employers should schedule frequent rest breaks and encourage workers to drink a cup of water every 15 to 20 minutes. To read the article in MMWR, visit CDC's website. 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.
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The CDC researchers determined that the heat index was less than 91°F for six of the14 fatalities they studied.