The Facts About Heatstroke and the Heat Index
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The Facts About Heatstroke and the Heat Index

Heatstroke is dangerous and it can come on fast without you knowing it. Understanding what the heat index means can also prepare you and help you avoid heatstroke.

Heatstroke is dangerous and it can come on fast without you knowing it. Understanding what the heat index means can also prepare you and help you avoid heatstroke.

The Heat Index

During the winter we have the wind chill factor which tells us how the wind combined with the temperature can affect our skin and our health. During the summer we have what is known as the heat index.

The heat index is the combination of the air temperature and the humidity and how it affects our skin, body and health. The National Weather Service (NWS) will give you the heat index along with other readings every hour. The higher the humidity the higher the heat index will usually be. For example, if it is just 90 degrees F with a humidity of 60%, it will actually feel like it is 100°F. Raise the humidity to 70% and the heat index jumps to 109°F. So an air temperature of just 90°F can actually affect your body as if it was 109°F.

The heat index was devised for the shade and light winds, being in full sunshine can increase this heat index temperature by as much as 15°F.

This is why it is important to understand and pay attention to the heat index. A heat index at or above 105°F is in the danger category and can cause heatstroke. The NWS will issue excessive heat watches and heat warnings when dangerous heat is expected or occurring.

Heat and Heatstroke Kills People

Hot weather especially combined with higher humidity should not be taken lightly. In the United States, heatstroke is the number one weather related cause of death. In the US, a 10-year average shows that heat and heatstroke killed on average 237 Americans per year. In the 1980 heat wave, 1,270 Americans were killed and in the 1995 heat wave, more than 700 people died in Chicago alone. The United States isn't the only place where heatstroke is a concern. In 2003, a severe heat wave hit Europe where an estimated 50,000 people were killed by heatstroke and heat related health problems.

The Cause of Heatstroke

Heatstroke occurs when the body's ability to cool itself starts to fail. Our body keeps cool during excessive heat by sweating and circulatory changes. The circulatory changes are when the capillaries begin to dilate or open up to dissipate heat. This causes the sweat glands to draw water from the blood so it can put water on our skin to cool us. Our body will pump more blood to keep the body cool, this is why excessive heat can put a strain on the heart. A healthy heart is more able to keep up with the demand of the body's need to pump blood during heat. The older you are, the more susceptible to heat illness and heatstroke you are.

When the sweat on your body evaporates it turns into water vapor and this process takes energy which cools your skin and body, the energy of evaporation takes heat off of your body. When the humidity is high and the heat index is high, this evaporation might not occur or occur as quickly. Even though the sweat isn't evaporating at a normal rate under high humidity and temperature, your body continues to sweat. And the hotter your body is getting the more you are sweating.

When we sweat, we are losing water and minerals and the loss of these minerals causes an imbalance in our body which can lead to heat related illness including heatstroke.

NWS local observations for New Orleans. HX is the heat index.

Stages and Symptoms of Heatstroke

Heatstroke can begin with muscle cramps, headaches and fatigue brought on by the start of dehydration.

Stage two is heat exhaustion and is characterized with more water and mineral loss (electrolytes) through excessive sweating. The symptoms of heat exhaustion are excessive sweating, dizziness, disorientation, lethargy and vomiting. The skin sometimes can actually feel cool.

Stage three is heatstroke and the symptoms are the above symptoms and possibly a body temperature of 105°F, rapid shallow breathing, seizures and possibly coma. In children, the symptoms can include an inability to sweat any more.

How to Prevent Heatstroke

Excessive heat and sweating actually causes the volume of blood to drop along with minerals and the all important water. If this continues, dehydration starts with muscle cramps. These minerals include salt or sodium, potassium, magnesium and calcium. When dehydration continues it progresses into the various stages of heat illness and heatstroke.

This is why you have to drink plenty of water, even if you're not thirsty. We hear this so much that we ignore it anymore. We need to drink a lot of water on these hot days and nights especially when the heat index is high with high humidity. By drinking plenty of water we are replenishing exactly what our body needs to keep us cool. Use the following tips to prevent heatstroke.

  • Drink plenty of water
  • Avoid alcohol, alcohol is dehydrating and will make the heat worse.
  • Eat smaller and lighter meals. Eating a heavy meal or a meal high in animal protein will cause your body to become hotter while digesting the food.
  • Take breaks, find shade and rest until you cool down.
  • Dress lightly and in light colored clothes, wearing a hat can help keep the hot sun off of your head.
  • Go to an air conditioned mall, ice rink or friends house with air conditioning during a heat wave.

Heat alert on TV with heat index temperature. Photo by Humain/

Heatstroke Treatment

Heatstroke can come on you without you even realizing it, that is what makes it so dangerous. I will relate a story a friend recently told me about a camping trip in South Dakota. Everyone sitting around talking when they noticed a woman started to talk incoherently. There was no alcohol involved so they realized she was on the verge of getting heatstroke. She said she was fine and refused to drink water until she realized she couldn't stand up. They forced her to drink water, which she did throw up at first which can be a symptom. Finally she was able to drink enough water and the health crises passed. If this had continued they would have taken her to a hospital.

  • Give the person water or a drink that will replace the electrolytes. A drink like Pedialyte is very good as are some sports drinks.
  • Move to a cool place
  • Loosen or remove tight clothing
  • Cool the body with wet towels or ice bags. Put the ice bags on the wrists, ankles, armpits and the neck to cool the person.
  • Do not use rubbing alcohol since this can close the pores and prevent sweating.
  • Fan the person
  • Give the person a half a glass of water every 15 minutes and don't let the person drink too quickly.
  • Do not give the person alcohol or beverages that contain caffeine, this will make the condition worse.
  • If the person is showing signs of heatstroke call the local emergency number or get them to a medical clinic.

The Elderly, Children and Pets

We hear the warnings every summer and we hear of children or pets left inside boiling hot cars. Never leave a child or pet inside of a car and cracking the windows doesn't help. The inside of a car can heat up 50F or more in less than an hour reaching a temperature of 145F or higher. Children, the elderly and pets can get heatstroke quickly. During a heat wave, check on elderly relatives and friends.

© 2010 Sam Montana


Main article photo by Sister72/

National Weather Service

American Red Cross

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Comments (16)

Very sensible advice. I hate the heat and stay out of it. When I was a veterinary nurse we often had collapsed dogs in with heat stroke and that is in England!

Superb article, as always, Sam.

What a great example of how to share good, useful information.

Thank You. Lisa, I should have added also that people need to make sure their dogs water bowl is always filled with water outside. I know I can fill my dogs water dish and a minute later it is already empty.

Excellent and timely, Sam.

Great article. It gets awful hot here in GA... I've gone to walking indoors rather than outside because it is just too dang hot! I walked for 45 minutes in the air conditioned kitchen... just walking around my island there and that's how I get my exercise when it is so hot.

I noticed today that there are a lot of heat advisories for the southeast US from Louisiana all the way to SC. Some of the heat index temperatures are above 105. That is too dangerous to exercise in. It sounds like it is even to hot and humid early in the morning to exercise outside in these conditions. I don’t do well at all in humidity living in Colorado my whole life.

Excellent work Sam as well as timely!

Sam, having spent the first 40 years of my life in a place where temperature can go up to 122 degrees F in summer, I endorse all what you said. Such educational article help people to avoid danger before it is too late to act. By the way, 118 in the horizontal table should be 108

You are right, the chart does have an error. The chart is from the US NWS. I look at the New Dehli temperature once in a while and I am sure I couldnt handle it. When reading about heatstroke, one article mentioned that people who live in these climates like in India, their diet is a salty diet, is that true in India.

Sam, it is true that our diet is loaded with salt. What is more, many people often carry a small packet of salt with them and administer it to themselves or others if there are symptoms of sunstroke. I have been the victim of it twice, and each time there were compassionate people (in the University) who gave me a tumbler full of water with salt.

It's good you have that reminder on display. For those who do not have the privilege such as yours, drinking water is best. Prevention is better than cure.

Very informative, and hopefully the science behind this dangerous condition will help people realize that yes, it can happen to them, too. Don't even get me started about the dog thing--what's the matter with people?

Great work on an important subject.

Superbly done, Sam, and very timely too. I think that we are in the midst of our first heat wave of the season here I Colorado Springs with the temperature being right around 96 to 98 degrees almost everyday for a week now.

Thank you Jerry. The current heat wave is almost one for the record books. The 105 in Denver today ties the hottest temperature ever recorded and the fourth day in a row of 100 or higher, five days in a row is the record. The low humidity and a breeze help us compared to those with high humidity. For your area, you can go to the Pueblo National Weather Service web site, scroll down and you can click on Colorado Springs for your local weather. and also for Colorado Springs  normals and records.