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What is Heat Fatigue?

By D. Jeffress
Updated Mar 03, 2024
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Heat fatigue is one of several warning signs of heat-related stress and illness. It can precede a potentially life-threatening condition called a heatstroke. Working, exercising, or even simply spending too much time outdoors in very hot weather can lead to symptoms of heat fatigue. A person might start sweating profusely and develop a mild, widespread skin rash. Mental confusion and physical sluggishness progressively worsen if an individual does not seek a cooler environment and medical care. It is essential to recognize the early signs of heat fatigue to prevent severe health complications.

Body temperature is naturally regulated by processes that adjust blood circulation and excrete sweat. Under comfortable conditions, most people maintain body temperatures of about 98.6 degrees Fahrenheit (about 37 degrees Celsius). When hot weather and physical activity raise internal body temperature, sweating helps to cool the skin and the rate and amount of blood pumped throughout the body rises to compensate for exhausted organ systems. Extreme heat and activity can overwhelm regulating functions, resulting in body temperatures in excess of normal ranges and symptoms of heat fatigue.

In most cases, heat fatigue affects people who do not take the proper precautions when venturing outside in hot climates. Symptoms of heavy sweating, thirst, and tiredness can begin to arise within a matter of hours if a person does not take regular breaks and stay sufficiently hydrated. As more and more water is lost through sweat, a person can experiences muscle weakness, cramps, and hand tremors. Mental confusion, concentration problems, and poor decision making are characteristic of late-stage heat fatigue and can be signs of serious illness. If medical help is not sought, heat fatigue can lead to fainting, seizures, coma, and permanent damage to the heart and lungs.

A person who exhibits heat-related illness symptoms should be brought indoors immediately and given cold water to promote rehydration. When symptoms are minor and a person seems to recover quickly, a trip to the emergency room may not be necessary. Any further physical activity should be avoided for one or two days to make sure symptoms do not return. If fainting occurs or severe sluggishness is obvious, the problem must be addressed at an emergency room.

At the hospital, doctors can provide cooling blankets or ice packs and intravenous fluids. Breathing, heart rate, and concentration ability are carefully monitored and treated as necessary. Most people who receive immediate treatment are able to recover without lasting consequences, though they may need to make lifestyle changes and take extra precautions to prevent recurring episodes.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
By anon342878 — On Jul 24, 2013

Well I am wondering about something related.

I've played hockey for 20 years during winter and inline hockey during the summer for my first season this year. And I noticed something which pretty much follows me all my life.

Sports under heat - not directly sun of course - exhausts me 10 times faster - not even comparable performance as during winter.

I do feel fatigue on warm days and I recall when go for jogging I start hyperventilating after five minutes during the summer - and almost never on winter or cold days.

Can that drastic performance decrease be related to some condition or illness? I do like summer, just for your information, but it really made me feel beaten up.

By donasmrs — On Jul 20, 2012

@fBoyle-- Yea, I think that's possible. Does it take you a long time to gather your energy again after the gym? Do you have to go and take a nap?

I remember when I had heat fatigue, my doctor told me that more and more young people are having this issue lately. He said one of his young patients recently was jogging outside and fainted from heat fatigue.

I also believe that not everyone might have the same tolerance for heat. So it's definitely possible that you might be experiencing heat fatigue, but go to your doctor to make sure it's not something else.

Meanwhile, I think you should take it easy. Stay away from heat and excessive exercise, and drink three to three and a half liters of water a day.

By fBoyle — On Jul 19, 2012

Do you guys think it's possible to have heat fatigue while working out in an air conditioned gym?

I love working out at the gym, but for the past two months, I leave the gym abnormally exhausted. I make sure to drink water throughout and I know I'm not pushing myself too much. The only explanation I can come up with is that my body's temperature increases too much while working out and tires me out.

Has anyone had this happen to them before? And do you think some people just have less tolerance to heat than others?

By stoneMason — On Jul 19, 2012

I had heat fatigue once, although I never went to the hospital for it. My mom and I were visiting my aunt who lives in a small town in Texas. We took a walk one day and ended up walking uphill for about forty-five minutes under the sun. Then we reached some shops and decided to do some window shopping so we were still outside.

After about an hour, I started feeling horrible. I felt extremely hot, was sweating profusely, and felt like I was about to faint. My mom wasn't aware of it at first, but then she realized that I had sat down on the ground and apparently, my face was extremely red.

She got me into a restaurant right away so I could cool down. I had a bottle of water there and rested for about an hour. My mom made me have food too in case my blood sugar was low. Being in a cool place helped a lot. After resting there, we had my aunt pick us up and went home immediately.

Ever since, I refuse to walk under the sun. I don't want to go through that ever again. It's really scary and I'm sure if I hadn't rested and gotten hydration, I would have landed in the hospital that day.

By OeKc05 — On Jul 18, 2012

I hate that clammy feeling that my skin gets when I get too hot. I know that sweating is good for me, because it causes cooling to combat the heat, but it makes me feel so gross.

I generally try to avoid working in the heat, but sometimes, I have no choice. I had to clean out the old utility room in my carport in the summer because we were about to move, and this room had next to no ventilation. I nearly suffocated in there, since it was even hotter than the yard.

I actually got chills from all the sweat I had produced. Once I started feeling cold, I knew that I was in trouble. I had to stop for the day.

By cloudel — On Jul 17, 2012

@wavy58 – Isn't that the scariest feeling? I had some pretty severe heat fatigue last summer, and I also hyperventilated. Because I could not breathe regularly, I started to see purple spots, and I nearly lost consciousness.

I was so glad that my friend was with me. We had been working in the flower garden in August, and even though I knew that it was too hot outside to be doing this, I had no other time to do it.

I was determined to get the weeds removed from around the flowers. I know what usually causes fatigue for me, and that is a combination of yard work and heat. However, I also knew that it needed doing badly, so I had just set my mind to it.

My friend helped me to the house and gave me ice water. She told me to shower and lie down, and she would not let me do any more work that day.

By wavy58 — On Jul 17, 2012

@StarJo – That would require quite an adjustment! I have lived in the deep South all my life, so I'm used to the weather here, but I can imagine how hard it would be for someone from a cold climate to deal with it.

Like your boyfriend, I have only experienced minor heat fatigue symptoms. I think probably everyone who has ever lived here has experienced them at some point. It just gets too hot and muggy to avoid this.

For me, the main indicator of heat fatigue is heavy, rapid breathing. Once I start laboring to breathe, I move indoors or to a cool spot. I once hyperventilated a bit because I stayed out in the heat for too long, and I have learned my lesson.

By StarJo — On Jul 16, 2012

I used to worry a lot about my boyfriend when he worked in construction and demolition. The summers where we live have temperatures of over 100 degrees frequently, and it also gets very humid.

He moved here from New York, where they have to worry more about the freezing temperatures and snow than heat exhaustion. So, it took awhile for his body to adjust, but thankfully, he started working here in winter, so his body had time to acclimate.

He has never developed a rash or fainted, but he has experienced dizziness and fatigue from the heat. He went and sat in an air-conditioned truck for about ten minutes, and that was all it took to get him back to normal.

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