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Heat Stress: Indoors vs. Outdoor

By Jessica Barrett
Published: May 7, 2019
Presented by Ergodyne
Key Takeaways

Indoor and outdoor heat aren't the same, and knowing the difference between the two is key to keeping workers safe from heat-related illnesses.

Heat stress is a major concern for workers in a wide range of industries. And while landscapers, construction workers, and farmers may be the first people who come to mind when you think about those affected by environmental heat, indoor workers can be affected, too. Those who work in hot indoor environments – like those faced by firefighters, bakers, and boiler room workers – are equally at risk of heat-related illness.

In this article, we’re going to look at the unique hazards posed by hot indoor and outdoor environments, how to identify heat stress, and steps you can take to prevent the onset of heat-related illnesses in both indoor and outdoor settings.

Indoor and Outdoor Heat Are Different

If you asked most people, they would tell you that heat is heat. In fact, indoor and outdoor heat are different. In our day-to-day lives, those differences don't matter too much, but when it comes to keeping workers safe, it's helpful to know exactly what sort of heat you're dealing with.


One of the main differences is the source of the heat. Indoor workers are often dealing with radiant heat from equipment (like ovens or boilers), while outdoor workers have to face the sun and its strong UV rays, as well as humidity that can’t be managed by adjusting a thermostat.

(Learn more about UV Risk in the Workplace.)

Everyone is familiar with the relief that comes with a cool breeze on a hot summer day. Indoor workers don’t have this luxury. That's another key difference. Air flow tends to be more plentiful in outdoor work environments, even on days when the breezes are relatively calm. Factory floors and boiler rooms, on the other hand, are stifling unless they have adequate ventilation and fans.

And while it’s critical to acknowledge these differences, it’s also important to remember that anyone working in a hot environment – indoor or outdoor – can be affected by heat stress.

Signs and Symptoms of Heat Stress

While the factors that bring on heat stress may be different in indoor versus outdoor work environments, the signs and symptoms that indicate a worker may be experiencing heat stress aren’t.

Signs to watch for include:

  • Muscle cramps
  • Shallow breathing
  • Increased heart rate
  • Cool, pale, clammy skin
  • Excessive sweating
  • Absence of sweat (this can indicate the onset of heat stroke, which is a medical emergency)
  • Weakness, fatigue, or dizziness
  • Headache and nausea
  • Fainting

So, if a worker is experiencing these symptoms, what should you do about it?

In many cases, the best course of action is to move the worker to a cool place (a shaded area outdoors or an air-conditioned one indoors), loosen or remove any unneeded clothing, and provide plenty of fluids (water is usually best). If you suspect heat stroke, however, get immediate medical assistance as the condition can be life threatening.

Solutions to Reduce the Risk of Heat Stress

The reality is that there really aren’t many ways to prevent work in hot environments. Summer days are often hot and humid, and certain workplaces require the use of heat. There are, however, a number of different ways employers can mitigate the risks and keep work environments comfortable enough to prevent the onset of heat-related illnesses.


These solutions fall into two key categories: engineering controls and administrative or work practice controls.

Indoor Work Environments

In indoor environments, one of the best ways to lower the risk of heat stress is to modify metabolic heat production. Engineering controls that can be implemented to this end include:

  • Increasing ventilation to the workspace
  • Bringing in cooler outside air
  • Using fans to maintain constant airflow
  • Reducing the temperature of a radiant heat source
  • Shielding workers from radiant heat sources
  • Insulating or covering hot surfaces to reduce radiant heat
  • Using air conditioning equipment to maintain comfortable temperatures

There are also a variety of different administrative controls that businesses can employ to further address the issue. These include:

  • Limiting exposure times using work/rest schedules
  • Enhancing worker heat tolerance through acclimatization
  • Providing plenty of water to keep workers hydrated
  • Providing body cooling garments

Acclimatization is especially important for new workers or workers returning from a period off work and consists of gradually increasing exposure to hot working conditions over a period of 7 to 14 days. During this time, workers should be monitored closely. Workers who are less physically fit generally require more time to acclimatize.

(Learn more about Indoor Temperature Control.)

Outdoor Work Environments

In outdoor environments, it's common for workers to be moving around frequently, and this can make it more difficult to implement engineering controls. Work practice controls, however, can be extremely effective. Some options include:

  • Acclimatizing workers to the heat
  • Creating a buddy system and training workers to identify the first signs of heat stress
  • Providing frequent rest breaks in shaded areas (with fans, if possible), and adjusting the frequency and duration of these breaks based on the temperature and humidity levels
  • Providing adequate water for workers to hydrate (coffee and soda are not recommended)
  • Altering work schedules to avoid working during the hottest periods of the day

Special Considerations

Whether indoor or outdoor, employers need to consider individual needs. Just as one size doesn’t fit all when it comes to PPE, a single approach is often not sufficient to address the varied needs of workers who are of different ages with different levels of health and fitness.

Glen Kenny, professor of physiology at the University of Ottawa, believes that current heat stress guidelines are inadequate to protect all workers. Older adults don’t deal with heat as well as younger adults, as the ability for the body to dissipate heat is compromised as we get age. “They [the guidelines] don’t take into account age-related changes, nor do they take into account an individual who may have a chronic health condition, such as diabetes or hypertension,” Kenny says.

When employers have older workers, workers experiencing health issues, or workers on certain types of medication, it’s particularly important to tailor heat solutions to fit their needs. These solutions may be as simple as providing certain workers with more breaks or rotating their duties so they aren’t always working in a hot environment. These are small modifications but they could make all the difference when it comes to their health and well-being.

Closing Thoughts

Hot work environments can pose a serious hazard to worker health. Heat stress and heat-related illnesses are no joke, and it’s incredibly important that employers take steps to mitigate the risks and address this workplace risk in a meaningful way.

Outdoor workers are subjected to heat, humidity, and the rays of the sun, while indoor workers must deal with radiant heat sources and hot environments without fresh air or a breeze to provide relief. In both cases, employers should implement engineering and administrative controls to keep workers cool and ward off heat stress.


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Written by Jessica Barrett

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Jessica is a freelance writer and editor from Toronto, Canada. She specializes in creating content for nonprofits and has written for organizations working in human rights, conservation, education, and health care. She loves traveling and food, speaks Spanish, and has two dogs, one of whom she rescued while living in Mexico.

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