Surviving a summer scorcher.

With the dog days of August upon us, it's important to take every precaution against heat stroke. Many people consider the sweltering heat an uncomfortable aggravation, but for others it is a life-threatening event. Of all meteorological disasters, heat waves claim the most lives. According to the Centers for Disease Control and Prevention, approximately 400 Americans die each year due to summer's heat and humidity. Furthermore, the National Weather Service declares that excessive heat is the number one weather-related killer, causing more fatalities per year than floods, lightning, tornadoes, hurricanes, winter storms and extreme cold. Elderly people living in heat trapping, brick apartment buildings without air conditioning are the most vulnerable. Also at risk are young children.

So what exactly constitutes a heat wave? A heat wave is an extended period of unusually hot and humid weather, normally lasting from a few days to more than a week. According to the Weather Channel's criteria, a heat wave occurs when a minimum of 10 states experience 90-degree-plus temperatures, with some areas experiencing temperatures that are also at least five degrees above normal for two days or more. Sometimes, however, the actual 90-degree-plus temperatures can seem like 100-degree-plus temperatures when humidity is considered. During a heat wave, take measures to protect yourself and also to check in on elderly or disabled friends, family and neighbors.

Excessive heat can kill by taxing the human body beyond its abilities to cope with extremes. First, the heat attacks the normal cooling system of the body, sending a signal to the hypothalamus gland that the blood temperature is above 98.6 degrees, which leads to increased blood flow. Blood vessels dilate so the blood can circulate more quickly, closer to the skin's surface. The excess heat is then vented from the skin into the air, but only if the air temperature is lower than your body temperature. The hypothalamus also calls millions of tiny sweat glands into action. When sweat is evaporated into the air, it provides additional cooling. But too much humidity in the atmosphere prevents sweat from evaporating. On an intensely hot and humid day, people will drip with perspiration, even as they stand idly at the bus stop or sit on a park bench.

Extreme heat can undermine physical wellbeing so slowly and subtly that the dangers are not apparent until people have succumbed to the effects. The body's response to overexposure to high temperature and high humidity could lead to a severe sunburn, which is not only painful but also reduces the skin's ability to release excess heat, making the body more susceptible to heat-related illness. Exposure can also lead to heat cramps. These muscle pains and spasms, most often in the extremities, are caused by heavy exertion, which triggers loss of water through heavy perspiration.

Another dangerous effect is heat exhaustion, a mild form of shock marked by heavy sweating, weakness, cold and clammy skin, a weak pulse, fainting and vomiting. Heat stroke (also called sunstroke) occurs when body temperature exceeds 105 degrees. It is a truly life-threatening condition, during which the skin becomes hot and dry, there is a rapid and irregular pulse, perspiration stops, and the person loses consciousness.

Use extreme caution, experts warn, when it is between 95 and 105 degrees because possible heat cramps and heat exhaustion with prolonged exposure can occur. When the heat index is above 105 degrees, heat strokes are possible and heat exhaustion and heat cramps are likely.

The American Red Cross suggests wearing lightweight, light-colored clothing. It is also a good idea to wear hats or to use an umbrella. Drink water. Carry water or juice with you and drink continuously even if you do not feel thirsty. Avoid alcohol and caffeine, which dehydrate the body. Avoid using salt tablets unless directed to do so by a physician. Eat small meals and eat more often. Avoid high-protein foods, which increase metabolic heat.

Slow down. If you must work out strenuosly in the outdoors during a heat wave, do it during the coolest part of the day, which is usually in the morning between 4 and 7 a.m. Stay indoors when possible. If air conditioning is not available, stay on the lowest floor out of the sunshine. Remember that electric fans do not cool, they simply circulate the air.

Victims of heat-related illness should be moved to a cool place, given cool water to drink and ice packs or cool wet cloths should be applied to the skin. If a victim refuses water, vomits, or loses consciousness, call 911 immediately.

(The American Red Cross, www.redcross.org; The Centers for Disease Control and Prevention, www.cdc.gov)