One of the most severe health injuries caused by over exposure to heat is known as a heat stroke. This is usually a result of dehydration which affects the body's ability to control its temperature. When the core body temperature in higher than 40.5 degree Celsius, complications in the central nervous system occur resulting in a heat stroke. A heat stroke can cause brain damage and damage to other internal organs. In extreme cases, it can also result in death. Although this condition is mostly known to affect people over the age of fifty, younger people, who spend most of their time outdoors during the summer months can be prone to it as well.
The main symptoms of this illness are fainting, dizziness, terrible headaches, lack of sweat (even in high temperatures), hot and dry skin, weakness or muscle cramps, nausea and vomiting, quickened heartbeat (either strong or weak), difficulty breathing, confusion and disorientation, seizures, loss of consciousness and sometimes even a coma.

When you suspect that someone is having a heat stroke it is best to get them medical help as soon as possible. If for some reason you are unable to do so, it is best to move the patient to a cooler, air-conditioned room or into the shade and remove any unnecessary clothing. Next, you should take their temperature and try to reduce it to 38.3 or 38.8 degree Celsius. You can do this by fanning them, or wetting their skin (use a sponge, garden hose etc). You can also apply ice packs to areas of their body that have the most blood vessels such as armpits, neck, back and groin to help reduce the body temperature. If needed, you can submerge the patient in an ice bath. 

Heat strokes usually affect those who live in places without proper air circulation, people who do not drink enough water, have chronic diseases such as diabetes, heart, lung, kidney diseases etc and those who drink excessive amount of alcohol. People who live in urban areas are more likely to suffer from this.

The best way to deal with this illness is to prevent it by staying in temperature controlled environments, wearing light coloured loose fitting clothing, using sunscreen with a sun protection factor or 30 or more, drinking lots of fluids, taking proper precautions before heading outdoors (especially for exercise) and if possible, canceling outdoor activity during the extra-hot summer days. Remember health and safety comes first!

Reference: www.webmd.com