Evaluation of Occupational Exposure Limits for Heat Stress in Outdoor Workers — United States, 2011–2016

Aaron W. Tustin, MD; Glenn E. Lamson, MS; Brenda L. Jacklitsch, PhD; Richard J. Thomas, MD; Sheila B. Arbury, MPH; Dawn L. Cannon, MD; Richard G. Gonzales; Michael J. Hodgson, MD


Morbidity and Mortality Weekly Report. 2018;67(26):733-737. 

In This Article

Abstract and Introduction


Heat stress, an environmental and occupational hazard, is associated with a spectrum of heat-related illnesses, including heat stroke, which can lead to death. CDC's National Institute for Occupational Safety and Health (NIOSH) publishes recommended occupational exposure limits for heat stress.[1] These limits, which are consistent with those of the American Conference of Governmental Industrial Hygienists (ACGIH),[2] specify the maximum combination of environmental heat (measured as wet bulb globe temperature [WBGT]) and metabolic heat (i.e., workload) to which workers should be exposed. Exposure limits are lower for workers who are unacclimatized to heat, who wear work clothing that inhibits heat dissipation, and who have predisposing personal risk factors.[1,2] These limits have been validated in experimental settings but not at outdoor worksites. To determine whether the NIOSH and ACGIH exposure limits are protective of workers, CDC retrospectively reviewed 25 outdoor occupational heat-related illnesses (14 fatal and 11 nonfatal) investigated by the Occupational Safety and Health Administration (OSHA) from 2011 to 2016. For each incident, OSHA assessed personal risk factors and estimated WBGT, workload, and acclimatization status. Heat stress exceeded exposure limits in all 14 fatalities and in eight of 11 nonfatal illnesses. An analysis of Heat Index data for the same 25 cases suggests that when WBGT is unavailable, a Heat Index screening threshold of 85°F (29.4°C) could identify potentially hazardous levels of workplace environmental heat. Protective measures should be implemented whenever the exposure limits are exceeded. The comprehensive heat-related illness prevention program should include an acclimatization schedule for newly hired workers and unacclimatized long-term workers (e.g., during early-season heat waves), training for workers and supervisors about symptom recognition and first aid (e.g., aggressive cooling of presumed heat stroke victims before medical professionals arrive), engineering and administrative controls to reduce heat stress, medical surveillance, and provision of fluids and shady areas for rest breaks.

OSHA's Office of Occupational Medicine and Nursing receives consultation requests from OSHA area offices to address medical questions that arise during OSHA worksite inspections. A master list of these consultations was used to identify 66 heat-related illness consultations during 2011–2016. Three consultations with missing information, 32 indoor incidents, and six that occurred near a heat source were excluded because accurate retrospective heat exposure assessments were not possible. The remaining 25 records were reviewed to assess workers' personal risk factors, heat acclimatization status, workload, and clothing. Personal risk factors considered in this report were obesity (body mass index ≥30 kg/m2), diabetes, hypertension, cardiac disease, and use of certain medications[1] and illicit drugs. Workers were considered unacclimatized if they had started a new job within the preceding 2 weeks or if they had recently returned from an absence of >1 week. Workload was classified as light, moderate, heavy, or very heavy, according to ACGIH guidelines.[2]

Archived climatologic data (i.e., temperature, humidity, wind speed, and sky conditions) were obtained from the nearest National Oceanic and Atmospheric Administration (NOAA) weather station. WBGT at the time of each incident was estimated using a validated heat and mass transfer model,[3] and Heat Index was computed via a standard NOAA algorithm.* In cases in which the worker's clothing likely impaired heat dissipation (four), clothing adjustment factors[2] were added to the estimated WBGT to determine the effective WBGT (WBGTeff). Total heat stress was compared with the applicable NIOSH exposure limit (i.e., the Recommended Exposure Limit for acclimatized healthy workers or the Recommended Alert Limit for workers who were unacclimatized or had personal risk factors). The sensitivity of the exposure limits was defined as the percentage of cases where heat stress met or exceeded the applicable limit.

The sample consisted of 25 heat-related illnesses that occurred during outdoor work, 14 (56.0%) of which were fatal (Table 1). Approximately half (12 of 25) of workers had at least one predisposing personal risk factor. Workload was moderate, heavy, or very heavy in 13 of 14 fatalities; the remaining fatality involved light workload in an unacclimatized worker. Estimated WBGTeff and Heat Index did not differ significantly across categories of workload or acclimatization status (Table 2). The range of WBGTeff was 79°F–94°F (26.1°C–34.4°C). 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.

The median Heat Index was 91°F (33.3°C) and ranged from 83°F to 110°F (28.3°C to 43.3°C). The Heat Index was <91°F (32.8°C) in 12 of 25 cases, including six of 14 fatalities. Among workers wearing a single layer of normal clothing (21), the minimum Heat Index was 85°F (29.4°C), and four of nine nonfatal illnesses and four of 12 fatalities occurred when the Heat Index was between 85°F (29.4°C) and 90°F (32.2°C).