Heat Hazards
Protecting Workers in Hot Environments
Heat-related deaths, illnesses, and injuries continue to occur in workplaces, despite efforts at prevention. Data from the Bureau of Labor Statistics show that in 2010 there were 4,190 cases of injury or illness arising from exposure to heat in the workplace that resulted in one or more days of lost work. Both outdoor and indoor workers who are exposed to heat or who work in hot environments are at risk. To address the hazards of heat and hot environments, NIOSH published the Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments in February. The document can be found on the NIOSH website. Exposure to extreme heat can result in occupational illnesses caused by heat stress, including heat stroke, heat exhaustion, heat syncope, heat cramps, heat rashes, or death. Heat can also increase workers’ risk of injuries: it may result in sweaty palms, fogged-up safety glasses, and dizziness, and it may reduce brain function responsible for reasoning ability, creating additional safety hazards. Other heat injuries, such as burns, may occur as a result of contact with hot surfaces, steam, or fire. The original NIOSH criteria document on hot environments was published in 1972 and revised in 1986. These publications assessed the potential safety and health hazards encountered in hot environments, regardless of the workplace, and recommended a standard to protect workers from those hazards. Heat-related occupational illnesses and injuries occur in situations where the total heat load (environmental and metabolic) exceeds the body’s ability to remain balanced. In the 1986 document, NIOSH recommended sliding-scale limits based on environmental and metabolic heat loads determined from available relevant scientific data and industry experience. The experience of recent years, including the response to the Deepwater Horizon oil spill in 2010, has identified the need for updated guidance that reflects current research and findings. While the basic knowledge of heat balance and heat exchange remains largely unchanged, more information is available today on the biological effects of heat, such as the sweating mechanism, water and electrolyte balance, and dietary factors. And while measurements of heat stress have changed little over the years, the revised NIOSH document includes information about bimetallic thermometers and the psychrometric chart. Psychrometric charts, which show the relationships among dry bulb temperature, wet bulb temperature, relative humidity, vapor pressure, and dew point temperature, are especially valuable for assessing the indoor thermal environment. NIOSH evaluated its Recommended Alert Limits (RALs) for unacclimatized workers and Recommended Exposure Limits (RELs) for acclimatized workers and determined that both limits are still protective (see Figures 1 and 2). Most healthy workers exposed to environmental and metabolic heat below the appropriate NIOSH RALs or RELs will be protected from adverse health effects. However, consideration of the scientific data led to the decision to remove the ceiling recommendations. Many acclimatized workers live and work in temperatures above the ceiling limits without adverse health effects.
Heat Stress: Acclimatization in Workers (Open PDF in browser)
Disadvantages of being unacclimatized:
  • Readily show signs of heat stress when exposed to hot environments.
  • Difficulty replacing all of the water lost in sweat.
  • Failure to replace the water lost will slow or prevent acclimatization.
Benefits of acclimatization:
  • Increased sweating efficiency (earlier onset of sweating, greater sweat production, and reduced electrolyte loss in sweat).
  • Stabilization of the circulation.
  • Work is performed with lower core temperature and heart rate.
  • Increased skin blood flow at a given core temperature.
Acclimatization plan:
  • Gradually increase exposure time in hot environmental conditions over a period of 7 to 14 days.
  • For new workers, the schedule should be no more than 20% of the usual duration of work in the hot environment on day 1 and a no more than 20% increase on each additional day.
  • For workers who have had previous experience with the job, the acclimatization regimen should be no more than 50% of the usual duration of work in the hot environment on day 1, 60% on day 2, 80% on day 3, and 100% on day 4.
  • The time required for non–physically fit individuals to develop acclimatization is about 50% greater than for the physically fit.
Level of acclimatization:
  • Relative to the initial level of physical fitness and the total heat stress experienced by the individual.
Maintaining acclimatization:
  • Can be maintained for a few days of non-heat exposure.
  • Absence from work in the heat for a week or more results in a significant loss in the beneficial adaptations leading to an increase likelihood of acute dehydration, illness, or fatigue.
  • Can be regained in 2 to 3 days upon return to a hot job.
  • Appears to be better maintained by those who are physically fit.
  • Seasonal shifts in temperatures may result in difficulties.
  • Working in hot, humid environments provides adaptive benefits that also apply in hot, desert environments, and vice versa.
  • Air conditioning will not affect acclimatization.
Acclimatization in Workers