What Is Thermal (Heat) Stress and Strain?

Heat stress is the heat load to which a worker may be exposed from combined metabolic heat from physical exertion, environmental conditions, and clothing. The body’s response to heat stress is heat strain. When heat strain occurs, the body becomes overheated which can lead to distress or serious illness. A growing problem in the workplace, heat stress can endanger worker health, safety, and productivity. While high indoor or outdoor temperatures on the job are a common cause of heat stress, other factors in the workplace that can contribute to heat stress include wearing personal protective equipment (PPE)/heavy clothing, intense physical work, and lack of heat acclimatization to a particular environment. Occupational and environmental health and safety (OEHS) experts work with employers to assess sources of workplace heat stressors and recommend strategies to mitigate these risks for employees.

Common Misconceptions About Heat Stress

Understanding your risks for heat-related illnesses – and the symptoms resulting from heat stress exposure – is vital to safeguarding your health. To better assess your personal risk from heat stress, OEHS experts recommend identifying and dispelling some of the most common misconceptions about this health threat.

Misconceptions

Misconception #1 – Heat-related injuries only occur in outdoor heat.

FACT – While outdoor heat is a key contributor to heat-related illnesses, any environment can produce heat stress. Indoor workplaces with poor ventilation can pose a problem for workers, high physical exertion, as well as heat stressors on the job such as high physical exertion, limited rest breaks, and productivity or economic incentives that do not allow workers to work at their own pace). In fact, you do not need to be in a high ambient temperature to experience heat stroke.

Misconception #2 – Clothing does not play a role in heat stress.

FACT – Wearing Personal Protective Equipment (PPE) can contribute to heat stress in workers. For example, first responders, such as firefighters, may be more exposed to heat stress during non-firefighting activities by performing work in heavy, insulating gear.

Misconception #3 – Sweating is not a symptom of exertional heat stroke.

FACT – Exertional heat stroke is a serious medical condition that requires immediate medical attention, and OEHS experts warn that having “dry, hot skin” is not a symptom of exertional heat stroke. Workers who succumb to exertional heat stroke are performing physical work during time of collapse and will present with sweating. The symptom of “dry, hot skin” occurs in a different form of heat stroke known as classical heat stroke, which occurs in the absence of any physical exertion and commonly occurs during heatwaves in the elderly and very young (infants) who are unable to regulate their body temperature properly.

Misconception #4 – Heat exhaustion always precedes heat stroke.

FACT – Heat-related illnesses do not act on a continuum. In other words, workers will not experience heat syncope or heat exhaustion symptoms prior to succumbing to heat stroke. An exertional heat stroke can occur without the presentation of signs or symptoms of other heat-related illnesses.

Misconception #5 – Age is not a risk factor for heat stress.

FACT – While heat-related illnesses can strike people of all ages, the very old and the very young are most susceptible to heat stress.

Misconception #6 – Hydration is the most important prevention strategy.

FACT – While staying hydrated is important in preventing heat stress-related illnesses, it is not the most important strategy. Heat-related illnesses are driven by increases in core body temperature and other heat stress prevention strategies such as work to rest ratios, activity modifications dictated by environmental conditions, and heat acclimatization are often more effective and will prevent continuous rise of core body temperature or even result in a reduction of core body temperature. The most effective strategy is to include multiple heat stress prevention strategies such as hydration, activity modification, body cooling, and heat acclimatization to reduce rise of core body temperature.

Workplaces Presenting the Greatest Risk

Heat Stress Outdoors

Studies show that workers experiencing some form of heat stress do not perform their job as efficiently as workers who are not impacted by this form of heat exposure. In fact, one Australian study reported that approximately 1,214 workers surveyed were 35% less productive on days they indicated experiencing heat stress. The impact of occupational heat stress is far-reaching with no signs of slowing down. It is estimated that heat stress could bring down global productivity levels by the equivalent of 80 million full-time jobs by the year 2030.

While heat stress commonly occurs in hot outdoor temperatures, there are many workplace situations that can lead to heat-related injuries, including:

  • High outdoor heat exposure
  • Poor ventilation
  • Wearing Personal Protective Equipment (PPE)
  • Low accessibility to fluids
  • Heat stressors on the job such as high physical exertion, limited rest breaks, productivity or economic incentives that do not allow self-pacing (i.e., working at your own pace)
  • Poor personal physical fitness
  • Workers unacclimatized to heat

Given these wide-ranging situations, it is easy to see how nearly any workplace can pose a certain level of risk from heat stress. However, the following industries are examples of environments that pose the greatest threat to worker health:

  • Agriculture
  • Construction
  • Oil & Gas
  • First responders/firefighters
  • Utility companies
  • Manufacturing plants
  • Postal workers (most OSHA-reported severe injuries)
  • Athletes (football lineman most susceptible to heat stress due to conditioning issues)

How to Protect Yourself From Heat Stress on the Job

Research has shown that in the United States alone, more than 700 heat-related fatalities occur on average per year, making exposure from environmental heat sources the country’s leading cause of weather-related deaths. Another analysis of the top causes of exertional injuries and fatalities examining OSHA data among laborers in the U.S. found that heat-related injuries accounted for 91.9% of the exertional injuries reported in OSHA’s Severe Injury Report database from 2015-2020.

So how can you protect yourself from heat stress on the job? OEHS experts offer these evidence-based tips:

  • Hydrate
  • Seek shade
  • Rest with adequate breaks
  • Use body cooling products
  • Follow company-based guidelines
  • Utilize environmental monitoring tools
  • Self-pacing during work
  • Slowly acclimatize to work environment in the heat

Frequently Asked Questions

Are certain workers more at risk for heat stress?

Older workers are the most susceptible to heat stress, but young children are also at greatest risk for heat-related illness. In addition, very highly motivated young workers (i.e., those workers who power through their work without taking rest or water breaks in workplace situations described above) are also at risk. Workers are also at greater risk if they are:

  • Physically inactive
  • Unacclimatized to work in the heat
  • Suffering from medical conditions such as diabetes, hypertension, and obesity
  • Pregnant
  • Taking medications associated with poor core body temperature regulation
  • Recovering from or experiencing an illness (i.e., respiratory infection)
How much water should I consume to stay hydrated on the job?

There is no one-size-fits-all formula when it comes to staying hydrated. Fluid needs are personal from one individual to another based on a person’s sweat rate, the amount of strenuous activity, and strain. A good rule of thumb is to drink water or drinks with electrolytes regularly when performing strenuous work in hot environments, and especially when you are feeling thirsty.

I work outdoors, and the heat in the summer months is unbearable at times. Is there a quick resource I can use to assess my heat stress risk daily?

Yes, OEHS expert volunteers of the AIHA’s Thermal Stress Working Group are creating a free Heat Stress Mobile App that functions as a resource for heat stress by educating and alerting employees and employers of heat stress, strain, and the best solutions to address heat in an occupational setting. The new app, which will be available for download in June 2024, will include multiple functionalities:

  • Heat Index and Wet Bulb Globe Temperature (WBGT)
  • Timers
  • Symptoms
  • First Aid
  • Risk Factors
  • More Tips
  • FAQs
  • Feedback & Contact Us

This new app is not a replacement for the OSHA app and is not affiliated with OSHA.

If my employer doesn’t have protocols in place for dealing with heat stress, how I can address the need for these resources as a worker health and safety issue?

AIHA recommends that all employers evaluate the heat stress risks in their workplaces and share evidence-based strategies for minimizing these risks with all employees. If your employer does not have heat stress protocols in place, you can download the AIHA’s free Heat Stress Mobile App to assess your daily heat stress risks and share this resource – and this website content – with your supervisor or human resources manager.

Who does AIHA represent?

AIHA is the association for scientists and professionals committed to preserving and ensuring occupational and environmental health and safety in the workplace and community.

Resources by Type

Support Documents
  • Coming Soon!
Articles
  • Morrissey, Margaret C. et al. “Analysis of Exertion-Related Injuries and Fatalities in Laborers in the United States.” International Journal of Environmental Research and Public Health, 2023, 20, 2683.
  • Morrissey, Margaret C. et al. “Impact of occupational heat stress on worker productivity and economic cost.” American Journal of Industrial Medicine, 2021; 1-8.
  • Morrissey-Basler, Margaret C. et al. “Perceived challenges and barriers for females working in the heat.” Journal of Occupational and Environmental Hygiene, 06 Oct. 2023.
  • Morrissey, Margaret C. et al. “Heat Safety in the Workplace: Modified Delphi Consensus to Establish Strategies and Resources to Protect the US Workers.” GeoHealth, 2021.
Publications

Healthier Workplaces and Schools eBook

The recent pandemic has taught us many important lessons about the spread of infectious diseases like COVID-19, RSV, and measles in indoor environments. As a result, we must take advantage of what we have learned to help ensure our places of work are and continue to be safe places for our workers, customers, and communities. Many of the steps you can take to help keep buildings and the people in them safer and healthier are simple and effective and can be adjusted to meet the current situation or infectious disease risk that exists. In this e-book, we will walk you through the different approaches to help mitigate the risk of spreading airborne infectious diseases depending on your workplace setting.

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