Staying Cool in Hot Weather
It is important to stay cool in hot weather. This will maximize athletic performance, maintain comfort, prevent injuries, and prevent heat related injuries. Heat related illnesses have a variety of symptoms from mild to severe. Prevention is much better than treating and includes:
1) Acclimatization. (To avoid confusion this does not refer to acclimatization to altititude) Initial adaptation occurs after a few days of exposure to warm weather, so travelers' are less likely to get heat illnesses a few days after arrival at a warm climate

2) Fluids. Maintaining hydration is most important. People should be urinating a clear stream (should be able to read a newspaper through it!). If urine is yellow or concentrated they must drink more water. Thirst is not a reliable indicator. If you are thirsty then you are already dehydrated. Dehydration directly decreases athletic performance. So extra fluids not only prevent heat related illnesses, it also improves physical performance under heat.

Salt supplementation should be generally discouraged. We usually get enough salt in our diet. In conditions of excessive sweating extra table salt should be enough on food. Salt tablets can cause lots of stomach upsets and should be avoided Electrolytes like Gatorade may be used with water. Likewise athletes should not skip
meals. If not hungry, have fluids. Caffeine and alcohol will have diuretic affect and hasten dehydration. If someone is moderately or severely dehydrated it may take 1 to 2 days to get re-hydrated even with rest and access to fluids. It is more important to avoid any dehydration and be ahead of the game.
3) Clothing
. Loose, light colored clothing are recommended and a hat. The hats should be functional with an adequate brim to reduce glare and give shade.

4) Physical fitness
helps aid in acclimization. Extremes of age such as very young, very old and obesity have more trouble with heat illness. These individuals should be recognized as being more susceptible to heat problems, and be more conservative in their activities. Some medications make acclimatization to heat more difficult.

Types of Heat Illnesses
Heat Cramps

Severe muscle spasms usually in the large muscles (legs, abdomen) caused by combination of electrolyte loss, hyperventilation and dehydration. Give the person rest and oral fluids with sodium. Gentle massage of cramped muscle helps. Activity may be resumed after rest.

Heat Rash
(Prickly heat, or miliaria)
Seen after prolonged sweating in humid areas. Sweat glands become inflamed and then blocked. Usually appears on trunk and extremities but excluding palms also, further decreased heat tolerance due to lack of sweating mechanism. Keep areas clean and limit exercise and heat exposure.

Heat Syrncope (fainting from heat)
Seen after strenuous work in a hot environment when patient faints. Treat with rest, lying down and liberal fluids.

Heat Exhaustion
Symptoms - weakness, inability to work, headache, mild confusion, nausea, faintness, anorexia, Dyspnea (shortness of breath), and rapid pulse. Skin may be warm or cool with sweating. This may lead to heat stroke which is defined where the core body temperature is 40.5 and can cause severe kidney, liver and nervous damage.

Heat Stroke

A continuation of heat exhaustion symptoms along with cardiovascular shock, mental changes and elevated temperature. Heat stroke is a full medical emergency and needs to be treated aggressively inside a hospital with frequent monitoring.

Heat Edema
Swelling of feet from heat. Will improve with fluids and rest

Is caused by both Ultra Violet A and B (UVA and UVB) are both found in sunlight. Sunscreen should be effective against both types and should be applied 15-30 minutes before exposure. Sunscreen undergoes a chemical binding to the skin making it effective during this time. It is important that it not be caked on and left over skin. It should always be rubbed into skin.

Clothing also has a SPF factor similar to sunscreen. Black tightly woven cloths (denim) give the best protection while loose cotton the least. The weave of clothing becomes looser when wet so that any sun-blocking benefit is lost when a t-shirt becomes wet. So it is possible to burn very easily wearing wet clothing.

When sunburn has happened local erythema, pain and swelling happen because of the cell medicated damage that continues to occur as enzymes and other inflammatory chemicals are leaked.

This can be partially blocked by taking ASA (Aspirin) orally or by cream as well as Aloe Vera and cool compresses which help neutralize these chemicals leading to a faster recovery from the sunburn. Individuals who have allergies to ASA (Aspirin) should not take it although cool compresses and aloe Vera will still work very well.
A generalized condition called Sun Sickness occurs when a large sunburn results in large amounts of these "mediators of inflammation" are in the blood leading to a malaise and achy feeling. This can be improved by lots of fluids rest and ASA.

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Reference :
Wilderness and Travel Medicine, Medicine for the Outdoors, Wilderness Medical Society Practice Guidelines, Sports Medicine, Secrets How to Survive on Land and Sea