Your body can produce a half gallon of sweat every hour in a hot environment.

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What causes heat stroke?

­Summer temperatures in the United States can climb above 100 degrees Fahrenheit (38 de­grees Celsius), making heat stroke a big probl­em. Heat stroke can be fatal in many cases be­cause it happens so quickly -- there is not much time to react.

Let's say th­at it really is 100 degrees F outside. The human body wants to stay at 98.6 degrees F. The only way to stay at 98.6 is to sweat. By putting moisture on the skin and letting it evaporate, your body can cool itself very effectively and keep its temperature in the proper range.

­Sweat works really well as long as there is plenty of water in your body -- it takes water to manufacture sweat. If you run out of water, sweat stops and your body rapidly overheats. It turns out that it is extremely easy to run out of water -- your body can produce 0.5 gallons (2 liters) of sweat every hour in a hot environment. Unless you are drinking water at the same rate, you will dehydrate and then stop sweating. Your internal thirst meter often is not sensitive enough when you need that much water (and it has been said that by the time you feel thirsty, you're already dehydrated), so you have to keep drinking regardless of how thirsty you feel.

The other thing that can lead to heat stroke is very high humidity, which keeps sweat from evaporating.

In either case -- be it the lack of sweat or the inability to evaporate it -- the core body temperature can rise very quickly if it is hot outside. Once the core gets to 106 degrees F, it is a serious problem. Symptoms include red, hot, dry skin (the body dilates skin blood vessels to try to release heat, making the skin red, and the dryness comes from lack of sweat), rapid heart rate, dizziness and confusion. The dizziness and confusion come from the high body temperature, which affects the brain.

For children and pets, one way for heat stroke to happen suddenly and unexpectedly involves a hot car or a hot room in a house. Cars are especially dangerous. At HowStuffWorks we did the following experiment:

  1. We turned on the air conditioner in a car at 3:30 p.m. on a sunny, hot summer afternoon in Raleigh, NC.
  2. We waited until the interior of the car cooled to a comfortable 75 degrees F.
  3. We turned the engine off.

Within 15 minutes, the interior temperature of the car was 110 degrees F. This temperature is quickly fatal.

The reason the temperature rises so high and so fast is because the interior of a car is an excellent solar oven that uses the greenhouse effect to trap heat. Sunlight heats the sheet metal of the car, and it streams in through the windows to heat the interior. It turns out that glass is completely transparent to visible light but opaque to infrared light -- and infrared light is the heat that is trying to radiate back out of the interior. So the temperature rises rapidly, to the point where you often cannot touch the steering wheel without getting singed. Leaving the window cracked is not going to help -- it is never safe to leave a child or pet in a parked car for any length of time.

The only solution for heat stroke is to cool the person down. You can:

  • Try to get the person to drink water if the person is conscious.
  • Soak the person's entire body in cool water.
  • Sponge cool water onto the person's body.
  • Apply ice packs to the head, neck, armpits and groin.

If not treated, heat stroke can be fatal in less than an hour.