Both infection and heat stress can lead to dangerously high core body temperatures.

But how can you tell the difference these 2 things physiologically, and should they be treated differently?


With the rapid spread of the coronavirus (SARS-CoV-2/COVID-19), the Kenzen team wanted to make sure that anyone who might be continuing to work (or live) in hot climates is aware of the the differences between heat-related illnesses (e.g., exertional heat exhaustion or heat stroke) vs. fever, so that you can stay safe.

Body Temperature Regulation: The Basics

Your body wants to maintain it’s temperature around a ‘“set-point” where it’s most comfortable and works most efficiently. This set-point is typically around 97.9 to 98.8°F (~36.6-37.1°C). Of course, this set-point differs for each individual— based on how fit they are, whether they are heat acclimatized, and what time of day it is— but in general, your body tries to maintain its temperature within this narrow range.

This set-point is controlled by a region in your brain called the hypothalamus, where a group of cells (neurons) sense changes in your body’s temperature and then send out responses to adjust the temperature accordingly. There are also neurons in your skin that sense hot and cold, and then relay this information to the brain. Your brain combines all of this information together and if your temperature is outside of that set-point, it will cause your body to respond by either increasing (e.g., through shivering) or decreasing (e.g., through sweating) your temperature to get it back to normal.


So what happens with a fever?

  • A fever just indicates that your body temperature is higher than normal. This is typically above 100.4°F (~38°C) and occurs from an infection (either viral or bacterial) that is causes substances, called pyrogens, to leak outside of the invader cells.

  • These pyrogens increase the body’s set-point, which means that now instead of having a set-point of say 97.9°F, your new set-point is 103°F.

  • Because you have a new (higher) set-point, your body now thinks that a temperature of 97.9°F is “cold”, and so your body will try to increase your temperature.

  • This causes the typical responses when you’re cold, like 1) shivering (chills), 2) vasoconstriction (narrowing) of the vessels in your skin which makes your skin colder because there is less blood flowing to the skin, and 3) increased cellular metabolic heat production.

  • All of these responses cause your body temperature to go up. It continues to go up (in some cases for hours) until your temperature hits this new set-point of 103°F. At this point, you might feel OK because you’re at your “new normal.” However, as soon as you cross this point, or you take medications that inhibit the pyrogens, your body set-point goes back down, and now your body temperature is way too high.

  • What does your body do? That’s right, it tries to cool you down by sweating, vasodilation, etc… which is when you feel really hot and sweaty.

  • This cycle of shivering and sweating can continue for as long as you are fighting the infection.

How do you know if you have a fever?

If you ARE NOT working/exercising in the heat, and you feel chills or cold skin, have body aches, and/or feel weak & tired, you may have an infection that should be treated by a medical professional.

Note: If you ARE working in a hot environment, and your core temperature does not feel like it has been increasing steadily, you are intermittently getting chills, and/or you feel cold or achy, you might have something that is unrelated to heat stress.

What’s the best way to treat a fever?

Contact your doctor, stay home, drink plenty of fluids, and rest.

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What happens with heat-illnesses (like heat stroke)?

  • Unlike with a fever (from an infection), when you have a heat-related injury or illness, your body temperature (hypothalamic) set-point has not changed.

  • With heat stress, your core body temperature continues to increase past your set-point because your metabolic heat production (from working or exercising really hard in the heat) is exceeding your body’s ability to get rid of the heat you’re generating.

  • And once your core body temperature goes up past the set-point, then you start sweating to try to cool your body down. But remember, if it’s really humid and hot outside, then it won’t be as easy for you to dissipate that heat (because the sweat won’t evaporate).

  • So if you’re working hard in the heat, and you are producing heat faster than you can get rid of it, your body temperature will continue to climb.

  • Often, while working or exercising in the heat, core body temperatures can easily (& safely) exceed the “fever” criteria of 100.4°F. In fact, reaching this core body temperature is often necessary to acclimatize to the heat.

  • Most trained athletes & heat-acclimatized workers can safely reach and maintain core body temperatures of 101.3°F (38.5°C) without any damage to their body.

  • However, the main difference (between reaching this higher core temperature with heat stress vs. fever) is that this is a safe increase in core temperature that has not altered the brain’s set-point. And furthermore, your body is not too hot where you could get irreversible (organ) damage.

  • That being said, if people are not closely monitored for signs & symptoms of heat-related injuries & illnesses, these hotter temperatures can lead to heat stroke and even death.

How do you know if you have a heat-related illness?

Signs & symptoms can vary, but some of the first signs of heat exhaustion are really hot skin, very sweaty, feeling lightheaded or faint, difficulty continuing to work/exercise, and general weakness.

Exertional heat stroke means that your core body temperature is >104°F (40°C) and it is a serious medical emergency. Along with this extremely high core body temperature, the person will often exhibit changes in their behavior (e.g., aggressiveness, confusion, irritability), can collapse or faint, and is very weak.

The main point is that you will generally be feeling hot, sweaty, & faint with heat-related illness, more so than cold, chilly, & achy, like with a fever.

What’s the best way to treat heat-related illnesses?

For heat exhaustion: rest (for at least 15 min), drink water, and find shade or air conditioning. Continue to monitor your symptoms & if possible, track your body temperature throughout the day.

For exertional heat stroke: emergency cooling using an ice water bath is best, & call emergency medical services immediately. Remember: cool first at the site, and then transport to the hospital.

Stay safe & healthy, everyone!



Guyton, Arthur C., and John Edward Hall. Textbook of medical physiology. 12th edition. Philadelphia: Saunders, 2011.

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