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Official guidance has treated a Heat Index below 91°F as "lower" risk for outdoor work. But when investigators from the Occupational Safety and Health Administration (OSHA) and CDC's National Institute for Occupational Safety and Health (NIOSH) looked back at 14 heat deaths among outdoor workers, six had happened below that level. Their conclusion: a Heat Index of 85°F is a better point to start taking extra precautions.
The cases
OSHA's occupational medicine office reviewed 66 heat-illness consultations from 2011 to 2016. After excluding indoor incidents, cases near a heat source and those missing information, 25 outdoor cases remained: 14 deaths and 11 nonfatal illnesses. For each, investigators looked at the worker's risk factors (such as obesity, diabetes, high blood pressure, heart disease and certain medicines or drugs), whether the worker was acclimatized to heat (new on the job within two weeks, or back from more than a week away, counted as not), the workload and the clothing. They estimated the environmental heat from the nearest NOAA weather station.
Two ways to measure heat
| Wet bulb globe temperature (WBGT) | Heat Index | |
|---|---|---|
| What it combines | air temperature, humidity, wind and radiant heat (often sunlight) | air temperature and humidity — what it "feels like" |
| Designed for | the workplace; the basis of NIOSH's recommended exposure limits, which also account for workload, acclimatization and clothing | the general public, assuming light clothing, shade and a light breeze |
| Limits | not always available at worksites | ignores direct sun, still air, work clothing and hard work; often taken from broadcast forecasts |
What they found
- The WBGT-based limits worked. Heat stress exceeded NIOSH's recommended limits in all 14 deaths and in 8 of the 11 nonfatal illnesses — suggesting the limits protect most workers.
- The Heat Index was often "moderate." The median was 91°F, ranging from 83°F to 110°F. It was below 91°F in 12 of the 25 cases, including 6 of the 14 deaths.
- 85°F was the floor. Among workers in a single layer of normal clothing, no case occurred below a Heat Index of 85°F, and four deaths and four nonfatal illnesses happened between 85°F and 90°F.
Other data point the same way: 14 percent of moderate to severe heat illnesses at a military training base, and at least 25 percent in Washington agriculture and forestry workers, occurred below a Heat Index of 90°F, and some heat hospitalizations reported to OSHA came below 80°F.
What employers should do
Measure heat stress through the workday, preferably by WBGT, and keep workers within the limits. Where WBGT is not available, use a Heat Index of 85°F as a screening threshold. A full heat-illness prevention program includes:
- an acclimatization schedule for new workers and for long-time workers during early-season heat waves;
- training for workers and supervisors on recognizing symptoms and on first aid, including aggressive cooling of a presumed heat-stroke victim before medics arrive;
- engineering and administrative controls to reduce heat stress, and medical surveillance;
- water, shade and rest breaks.
Limits
Acclimatization, workload and clothing may have been misclassified — long-time workers may not be acclimatized in an early heat wave; weather-station data may differ from conditions on site; the heat estimates had small errors; and the sample was small.
Sources
Based on "Evaluation of Occupational Exposure Limits for Heat Stress in Outdoor Workers — United States, 2011–2016," by Aaron W. Tustin and colleagues (OSHA; NIOSH), Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; a work of the United States government in the public domain.
Лицензия: CC0 1.0 (общественное достояние) · По материалам www.cdc.gov
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