Heat Stress Monitoring in Warm Weather

Best Practices for Assessing Heat Exposure Hazards in the Workplace

Environmental heat exposure poses a serious risk to workers, making accurate assessment essential to prevent heat-related illnesses and even fatalities. While many workplaces depend on basic temperature readings or the heat index, these methods often overlook key factors that contribute to heat stress. 

Understanding Heat Stress: More Than Just Temperature

Environmental heat is more than just temperature.  Four factors contribute to heat stress in workers:

• Air temperature
• High relative humidity makes it difficult for the body to cool itself through sweating
• Radiant heat either from sunlight or artificial heat sources such as furnaces
• Lack of air movement doesn't help workers cool off

An effective environmental heat assessment should account for all of these factors.  OSHA recommends the use of wet bulb globe temperature (WBGT) monitor to measure workplace environmental heat. [1]

Limitations of Other Measurement Methods

• Standard thermometers only measure air temperature, ignoring humidity, radiant heat, and wind
• Heat index combines air temperature and humidity but is measured in the shade and doesn’t account for wind, sunlight, radiant heat, or workload. It doesn’t reflect the true heat stress experienced by workers, especially in direct sunlight or near heat sources. This is a less desirable substitute and shouldn’t be relied upon for accurate hazard assessment
• Air (dry bulb) temperature ignores relative humidity and other factors

The Problem with Weather Reports

Local weather reports and meteorological data from observation stations may not reflect the actual conditions at a specific worksite due to:

• Distance from the weather station allowing for potential error increases with distance

• Unique worksite features, such as:

  • Indoor work – weather reports cannot gauge indoor conditions

  • Direct sunlight – weather services measure in the shade; direct sun can increase the heat index by up to 13.5°F

  • Heat sources – hot equipment or heat-absorbing surfaces can result in increased temperature

  • Wind blockage – structures like trenches or buildings can block air movement

  • Reflective materials – water, metal, etc., can reflect sunlight onto workers

At worksites with the above features, weather reports are unlikely to provide accurate heat estimates of environmental heat. Employers should use an on-site measurement such as WBGT. [1]

The Role of the Heat Index

The heat index does not measure worksite heat as accurately as WBGT. Employers shouldn’t rely solely on the heat index for hazard assessment, though it may be useful as part of a broader hazard evaluation. Notably, heat-related illnesses and fatalities have occurred at heat index values below public advisory thresholds. The NIOSH/OSHA Heat App uses the heat index, a screening tool. It does not replace a more accurate WBGT-based hazard assessment which is the core tool used by occupational health professionals (ACGIH 2017, NIOSH 2016). [1]

Conclusion

For accurate assessment and effective management of heat exposure hazards, on-site measurement using WBGT meters is the best practice. Relying solely on temperature readings, the heat index, or weather reports can underestimate risks and endanger workers. Employers should implement WBGT monitoring to ensure a safe work environment and protect workers from heat-related harm.

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Reference:
[1]  OSHA, “Safety and Health Topics > Heat,” U.S. Department of Labor Occupational Safety and Health Administration, [Online]. Available: https://www.osha.gov/heat-exposure/hazards.
       [Accessed May 12, 2025].