Critical Factors for Heat Stress Assessment and Prevention
It’s the Heat—And the Humidity
Heat-related illnesses are insidious conditions that can affect almost anyone with little warning, especially since symptoms may affect one’s ability to recognize the danger. The danger is pervasive: according to the Bureau of Labor Statistics, exposure to environmental heat was associated with 3,120 cases of lost workday injuries or illnesses and 49 fatalities in 2018. However, many cases likely go unreported. 

The problem goes beyond staying out of the sun, staying hydrated, and taking regular breaks for recovery. Imagine someone exercising vigorously within an air-conditioned gym. She is physically fit, drinking plenty of water, and wearing loose, breathable clothing. Could she be at risk of heat stress? Yes—although if the exerciser were outside in the sun, in high humidity, the development of heat stress would likely occur much sooner. All that is needed for the onset of heat stress is for the energy and blood demanded by the muscles to compete with the body’s cooling mechanisms. 
Heat stress is a complicated issue and one occupational health and safety professionals must not overlook, as a worker can become heat stressed even when the temperatures are mild. We must consider heat stress as a multifactorial condition that arises due to many circumstances. Our focus here is on the factors strongly associated with the development of heat stress, especially those we can control as professionals. We will discuss the importance of each factor and show how they can be used collectively to assess heat stress, as well as identify and implement effective preventive measures.  CRITICAL FACTORS Seven main factors are associated with heat stress: temperature, air velocity, humidity, radiant heat, clothing, metabolic rate, and acclimatization. Two additional factors—body weight and work-rest schedule—affect metabolic rate. Still other factors contribute to heat stress for certain susceptible individuals. The contribution of each factor varies depending on the work scenario. Understanding this, and being able to recognize and evaluate heat stress factors, are first steps in preventing and controlling heat-related illnesses. Temperature Elevated ambient temperatures affect the body’s ability to dissipate heat. Heat flows from a warmer body to a cooler one by means of conduction, convection, or radiation. If the surrounding air is warmer than the body, heat transfers into and accumulates within it. The body responds by increasing skin temperature, which may reverse the transfer and aid in heat dissipation, but the body has its limitations. Air Velocity Air velocity affects both evaporative cooling and convective heat transfer. When the air temperature is less than skin temperature (approximately 35 C or 95 F at rest), increased air velocity increases sweat evaporation and the removal of heat from the body. However, if the air temperature is significantly above skin temperature, then air movement in fact increases the rate of heat transfer from the air to the body, the same mechanism that makes a convection oven effective.

Humidity The relative humidity (RH) ranges from zero to 100 percent and is a measure of how much water vapor is in the air compared to how much it can hold. The primary relevance of RH to heat stress is its effect on the human body’s main mechanism of cooling, the evaporation of sweat from the skin’s surface. When the RH is less than 50 percent, sweat efficiently evaporates from the skin, cooling the underlying blood vessels. But when the RH is higher, the air cannot hold as much additional water vapor. Evaporation decreases, and with it, the cooling effect.  Radiant Heat Thermal radiation, a form of electromagnetic radiation, is dependent on the temperature difference between opposing surfaces, and often passes through the air without heating it significantly. For example, the sun radiates visible, ultraviolet, and infrared rays that can be absorbed by the body and other surfaces, and so do radiant heat sources such as open flames, furnaces, and molten metal or glass. If the radiant source is warmer than the body and there are no obstructions, then heat can be transferred to the body. Sitting around a campfire is an example of radiant heat.  Clothing Clothing has a variety of positive and negative effects on heat stress, depending on the situation. When air temperatures are significantly above skin temperature and the air is moving, clothing can reduce the convective transfer of heat to the skin. When air temperatures are below skin temperature, less clothing coverage and more breathable fabrics improve heat dissipation and evaporative cooling. Conversely, thicker clothing and multiple layers insulate the body and retain unwanted heat. Impermeable chemical protective clothing and suits can also affect the body’s cooling by preventing sweat from evaporating.
Metabolic Rate Metabolic rate is the amount of energy expended over time. Heat is a major byproduct of energy. Thus, the human body generates a lot of heat, especially during physical exertion. The higher the physical exertion, the higher the metabolic rate and heat generation.  One of the dangers of overexertion is that more body heat is accumulated than can be dissipated, leading to heat stress. Work-rest schedules can improve the situation by allowing the body time to eliminate excess heat. Guidelines by ACGIH and NIOSH are designed to help professionals regulate work-rest schedules based on metabolic rate and the other factors discussed here.  It is often overlooked that heavier people have a higher metabolic rate, and that most standards are based on data for a 70 kg (154 lb.) person. The ACGIH threshold limit values and the OSHA Technical Manual provide details on how to adjust metabolic rate based on body weight, and it is important to consider overweight and obese workers more at risk for heat stress.  Acclimatization The human body can adapt to hot conditions, becoming more efficient at heat dissipation and evaporative cooling, but this takes up to two weeks and starts to wane after as little as four days away from hot temperatures. Unacclimated workers are at a higher risk of heat stress. Both ACGIH and NIOSH guidelines have established criteria for acclimated and unacclimated workers, and the NIOSH resource provides further details on how to acclimate workers to hot environments.  ASSESSING HEAT STRESS The ACGIH TLVs and NIOSH’s Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments provide detailed guidelines on assessing heat stress. The following steps are common to each:

  1. Determine the wet-bulb globe temperature (WBGT), which considers temperature, air velocity, humidity, and radiant heat. WBGT is preferred over a standard heat index, which only reflects temperature and humidity. For outdoor work, Argonne National Labs developed a validated WBGT calculator that allows for pre-planning based on geolocation, time, and weather data. Real-time WBGT monitors can be used as well but are best for applications when conditions are stable and predictable. Pre-assessment and planning are essential for proper prevention and control. 
  2. Determine the WBGT-Effective using a clothing adjustment factor (CAF), when applicable. For example, use of double-layer woven clothing may increase the WBGT by 3 C or 5.4 F. Encapsulating vapor-barrier suits may increase the WBGT by 11 C or 19.8 F. The WBGT-Effective is the sum of the WBGT and CAF.
  3. Establish the metabolic rate, based on the level of physical exertion required by the task and the worker’s body weight. Both the ACGIH TLVs and the NIOSH criteria document provide details on determining metabolic rate.
  4. Determine the work-rest schedule for the work. For example, it might be 15 minutes of rest for every hour of work, or as high as 45 minutes of rest every hour under extreme conditions.
  5. Determine the occupational exposure limit using the metabolic rate, work-rest schedule, and acclimatization status. For acclimated workers, ACGIH provides a TLV and NIOSH provides a recommended exposure limit. For unacclimated workers, these become an action limit or recommended alert limit (RAL).
  6. Compare the WBGT-Effective to the appropriate OEL.
  7. Take preventive measures when the WBGT-Effective exceeds the OEL, as these workers would be at risk of developing heat stress.
PREVENTIVE MEASURES When the heat stress assessment shows that workers are at risk, the best approach is to review the assessment to identify and control those factors contributing most to the WBGT-Effective or OEL. The solutions will vary from one situation to the next, and in some complex cases innovation and an internal task force will be required to find them. The ACGIH and NIOSH resources provide lists of basic solutions. The following examples correspond to the critical factors described above. Those marked with an asterisk (*) can lower the WBGT-Effective or increase the OEL, thus reducing risk.
Putting It Into Practice
The following scenario demonstrates how implementing preventive measures based on the results of a heat hazard assessment can reduce the risk of heat stress. Scenario: Five unacclimated workers are to plant hundreds of small trees and shrubs using hand shovels. They are working outside continuously from 8 a.m. to 5 p.m. with only short water breaks and one hour for lunch. It is partly cloudy with a solar irradiance of 980 watts per square meter (W/m2). (For more information about solar irradiance, see Chapter 4 of the OSHA Technical Manual.) Their work attire includes pants, long-sleeve shirts, leather gloves, and a hardhat. Half of the workers weigh about 200 lbs.

Hazard Assessment: Using the National Weather Service, a maximum temperature of 72 F was forecasted at 2 p.m., with a relative humidity of 69 percent and 10 mph winds. Based on the workers’ geolocation and work conditions, the WBGT-Effective is 72.5 F. However, this is well above the NIOSH RAL of 67 F. Despite low temperatures, the workers are at risk of heat stress. Contributing Factors: From the assessment, the identified heat stress hazards include sun exposure, high humidity, strenuous physical exertion, and unacclimated workers.  Solutions: The feasible corrective actions include replacing the majority of hand digging with the use of a powered auger, decreasing the metabolic rate and also improving productivity; providing shade from the sun to reduce radiant heat exposures; and adjusting the work schedule to avoid work between 2 and 4 p.m. Result: Reassessment under the new work conditions results in a maximum temperature of 70 F at 1 p.m., a significant reduction in solar irradiance, and reductions in metabolic rate. The new WBGT-Effective is 66.7 F, which is below the also improved NIOSH RAL of 69 F. The workers are no longer at increased risk of heat stress under these conditions, and the project time is reduced as well.
Temperature • Reschedule work for cooler times of the day.* • Avoid strenuous work between 2 and 4 p.m., the hottest parts of the day.* • When possible, preassemble products indoors under controlled conditions before outdoor installation or construction.*  Air Velocity • Increase air velocity using fans when air temperatures are below skin temperature and humidity is low.* • Shield against wind when air temperatures are significantly above skin temperature.*
Humidity • This factor may be more difficult to control for as outside humidity typically peaks near sunrise and drops midday. The WBGT calculator from Argonne National Laboratory can help determine what times of day are best for strenuous outside work.* • Control and lower humidity in indoor settings.* • Use local exhaust ventilation to capture and control humidity sources.* Radiant Heat • Provide shade from the sun.* • Provide radiant barriers between sources and workers.* • Provide workers with lighter-colored clothing. • Provide workers with protective clothing that acts as a barrier to radiant heat. Clothing • Reduce clothing layers when safe and feasible.* • Adjust clothing coverage based on the relationship between ambient and skin temperatures. • Provide lighter-colored, breathable clothing and increase skin coverage for intense sun exposures. • Provide cooling devices, such as vortex cooling devices, on supplied-air suits. Metabolic Rate • Lessen manual labor and physical exertion using powered tools or other engineering controls.* • Adjust work-rest schedule to include more frequent and longer rest breaks.* Acclimatization • Gradually acclimate new and returning workers to hot environments.* • Reduce physical exertion for unacclimated workers.* SUSCEPTIBLE INDIVIDUALS Additional protective measures may be necessary for unacclimated workers, those with higher metabolic rates due to body weight, or those wearing chemical protective clothing such as hazmat suits. In addition, workers with various medical conditions (see the NIOSH criteria document) or a history of heat illness are more susceptible. Medical surveillance and physiological monitoring are recommended under these circumstances to ensure workers are adequately protected. Monitoring body temperature or heart rate recovery can catch early stages of heat stress. Workers can also monitor their body weight and urine color as indicators of possible dehydration. The NIOSH criteria document provides details on these different strategies.  TAKEAWAYS Heat stress is a complex condition that requires focus on more than just temperature extremes. A thorough heat stress assessment that accounts for all these factors is a critical first step toward the prevention and control of heat-related illnesses and fatalities. A proper assessment can help identify factors we can address and control. However, we must also recognize that medical surveillance and physiological monitoring may be necessary with some workers, such as those with a prior history of heat-related illness and certain medical conditions, including obesity.   ROBERT N. PHALEN, PHD, CIH, FAIHA, is an associate professor and program chair of Occupational Safety and Health at University of Houston-Clear Lake in Houston, Texas. CATHERINE L. BESMAR is a graduate student in Occupational Safety and Health at University of Houston-Clear Lake in Houston, Texas. She served as a research assistant on an OSHA Susan Harwood heat stress training grant, has conducted research in the field, and has given many presentations on the topic. Send feedback to The Synergist.

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ACGIH: 2019 TLVs and BEIs, “Heat Stress and Strain” (2019).  Argonne National Laboratory: Wet Bulb Globe Temperature (WBGT) Version 1.2, 2008, available via the OSHA Technical Manual, Section III, Chapter 4, “Heat Stress.” BLS: Injuries, Illnesses and Fatalities, “Fatal occupational injuries by industry and event or exposure, all United States, 2018,” Table A1. BLS: Injuries, Illnesses and Fatalities, “Number of nonfatal occupational injuries and illnesses involving days away from work by event or exposure leading to injury or illness and selected sources of injury or illness, private industry, 2018,” Table R33. Energy Procedia: “Estimation of Global Solar Radiation Using Three Simple Methods” (2013). Journal of Occupational and Environmental Hygiene: “Modeling the Wet Bulb Globe Temperature Using Standard Meteorological Measurements” (October 2008). NIOSH: Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments (February 2016). OSHA: OSHA Technical Manual, Section III, Chapter 4, “Heat Stress.” The Synergist:Heat Hazards: Protecting Workers in Hot Environments” (April 2016).
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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