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:

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.

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.