Injuries and Illness, Personnel Safety

Heat, Humidity, and Five Other Factors That Contribute to Heat Stress

Heat-related hazards injured 2,061 and killed 884 people in 2019, according to the National Safety Council. As responsible employers recognize and develop policies or plans to reduce risk to employees who must work in hot and humid conditions, more than just heat and humidity need to be considered.

Heat stress

The Occupational Safety and Health Administration (OSHA), the National Institute of Occupational Safety and Health (NIOSH), and many other organizations recognize the risk of working in high heat and humidity conditions and have issued guidance to help employers protect their employees. Like other workplace hazards, evaluating the specific factors that can contribute to a heat-related injury is essential for reducing risk.

Heat index and Wet Bulb Globe Temperature

In the summer months, many local forecasters switch from talking about the wind-chill factor to announcing the daily heat index. The National Weather Service uses the heat index as a way to describe how hot it really feels outdoors.

The heat index can be used to begin to determine heat risk for indoor workers. It is based upon an equation that includes heat and humidity as well as conditions including light winds (or fans that produce a breeze) and shade.

To accommodate for outdoor working conditions, the U.S. military, sports organizations, and others rely on the Wet Bulb Globe Temperature (WBGT) to measure heat stress. The WBGT measurement includes working in hot, humid conditions; in direct sunlight; with and without air movement.

Both the heat index and the WBGT provide starting points for employers to gauge when they need to put plans and polices into action. But they are just that: starting points. Work-specific conditions that aren’t figured into the heat index or WBGT could make even those temperature calculations unsafe.

Radiant, convection, and conductive heat

Furnaces in iron and steel foundries and heat from the sun are two example of radiant heat. Sometimes, workers can be shielded or shaded from radiant heat; other times it isn’t practical. Radiant heat is a concern because the human body can radiate heat if the area or surfaces around it are cooler than the body. However, if the area or surfaces around the body are hotter, the body absorbs heat—making it more difficult, or impossible, to maintain a normal core (internal) body temperature and increasing risk.

Convection is the transfer of heat through moving air. When the temperature is lower than 95°F, wind, or air flowing from a fan will cool the body and help perspiration evaporate faster. If the temperature is hotter than 95°F, convection has the opposite effect and will warm the body—making fans an ineffective control measure.  

The body can also lose or gain heat through conduction. Clothing, tools, the ground, benches, seats, or any other surface that a person directly rests against, touches, sits, or lays on can create or reduce conductive heat, depending upon its temperature.


The first few hot days of the summer season are often more intolerable to workers than days later in the season that may be even hotter than those first few hot summer days. This is due to a lack of acclimation. Workers who are new to the geographic area, and even those who have lived all of their lives in an area that has seasonal temperature extremes need time to get used to hotter temperatures—especially if they’ve spent the past several weeks in more moderate temperatures.

People tolerate heat differently, and it can take anywhere from two to 10 days for people to fully acclimate to working in hot and humid conditions. But that doesn’t guarantee that after two weeks everyone will be prepared for the entire summer. Recovering from an illness, returning from an extended vacation and changing work shifts may all require an employee to reacclimate.

In the late spring and early summer, rest periods may need to be extended while employees acclimate. It may also mean adjusting the heat and humidity thresholds that trigger those more frequent rest periods to accommodate workers who aren’t yet acclimated.

Physical Activity

Shoveling coal into a furnace, climbing a utility pole, harvesting crops, working on a production line and delivering packages are all examples of strenuous activities that require energy and generate heat in the body. That heat needs to be dissipated to maintain a normal body temperature.

The more physically demanding the activity, the more heat the body generates. When workers will be performing tasks that involve heavy labor, consider adding up to 15° to the heat index or WBGT and increasing the frequency of breaks so that workers can rest and bring down their core temperatures.

Personal protective equipment

When personal protective equipment (PPE) is required to protect an employee from a recognized hazard, it is irresponsible to stop requiring it when the temperature rises. Whether it is a hard hat, gloves, long sleeves, FR coveralls, or a fully encapsulated chemical protective suit: it has a purpose.

Recognize the additional risks that all required PPE may have, such as limiting air movement that cools the body, or increasing sweating—which can increase the rate of dehydration. Like strenuous activity, wearing PPE may necessitate more frequent rest breaks.

Health factors

It shouldn’t be a surprise that no two workers are the same. Even if two workers started the same day, went through the same training and acclimation process and are performing the same work task, they may not react to hot, humid environments the same way.

Age, gender, weight, physical fitness, and health issues can make workers more or less susceptible to heat stress injuries. People who have previously experienced a health illness are also more susceptible.

Illnesses—especially those that involve high fever, vomiting, and diarrhea—can temporarily make a person more vulnerable as can consuming alcoholic or caffeinated beverages, and being dehydrated. While many of these factors are difficult to control, making both employees and supervisors aware of how they can contribute to a heat illness can help to reduce risk.

Identifying environmental and health factors beyond temperature and humidity provides a more realistic view of the risk workers face when working in hot indoor and outdoor environments. This view allows employers to create heat illness prevention policies and plans that include adequate rest periods, work schedules, and hydration that reflect site-specific needs.

Karen D. Hamel, CSP, CIT, WACH, is a regulatory expert, trainer, and technical writer at HalenHardy. She is presenting the EHSDA webinar “Since You Can’t Turn Off the Sun, Turn On Your Heat Illness Prevention Program” on August 24.

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