Perhaps Irving Berlin knew July in Washington when he wrote the lyric "We're having a heat wave" more than 60 years ago.

The temperature's rising and it isn't surprising to anyone here. July is historically Washington's hottest month, and the area has already had its first urban heat advisory of the season, with more blistering weather on the horizon.

Heat waves, periods of abnormally high temperatures that can kill, grip at least one region of the United States nearly every summer, according to the National Weather Service. East of the Rockies, heat waves often come with high humidity. West of the Rockies, they usually are dry.

These periods of soaring temperatures can last merely several days or linger for weeks. In 1980, more than 1,250 people died when high temperatures hit parts of the southern United States. In 1995, heat waves were implicated in the deaths of more than 700 people in Chicago. Last year in Texas, more than 100 people died from a record month-long heat wave that gripped the state.

From 1976 to 1996, an average of 381 people died annually from heat-related illnesses, according to the federal Centers for Disease Control and Prevention in Atlanta.

To step up efforts to protect the public, the National Weather Service has begun a heat index system for classifying hot weather. The index takes into account both temperature and humidity, providing a more accurate assessment of the real effects of high temperatures, much the same way that wind chill more accurately gauges the dangers of cold during the winter.

According to the National Weather Service, heat waves occur when hot air masses become stationary and the heat stacks up in the atmosphere and produces at least two days in a row that register at least 105 degrees on the heat index.

Often prior to a heat wave, the National Weather Service issues an urban heat advisory, as officials did in early June for the Washington and Baltimore areas. Urban heat advisories are issued when the heat index edges close to 105 during the day, evening temperatures stay above 80 and the forecast for the following day is projected to be 100 to 105 on the heat index.

"The urban environment is more difficult because of all the concrete and older buildings, particularly those without air conditioning," said James Travers, the National Weather Service meteorologist-in-charge for this region. "We don't require quite as high a criteria" to issue an urban heat advisory as for a heat wave.

In the East, prolonged periods of heat are generally the result of air masses that form over the Gulf of Mexico or gather in the Caribbean and pump the heat up the Atlantic seaboard. A high pressure system, known as a Bermuda high, then often stalls over the Atlantic and traps the heat in place.

People at greatest risk of heat-related injuries are the young and the old. Infants and young children are unable to recognize when they become dehydrated, but watchful parents or caregivers can see that youngsters stay cool and get plenty of fluids, so they generally do not suffer the more severe heat complications.

It is the frail elderly who often are hurt by heat. During the 1995 Chicago heat wave, nearly all the deaths occurred among seniors.

"We found that the medically fragile elderly, who are usually socially isolated and living alone, are the most vulnerable to heat," said Chicago's deputy health commissioner, John Wilhelm, who headed an 18-member commission that investigated the fatalities.

For those who don't have air conditioning at home, the best way to protect against the ravages of high temperatures is to get out for at least several hours to a place that is cool, the commission found. That proved very difficult for many seniors who were too fragile or too isolated to travel to an air-conditioned shopping mall or movie theater or grocery store. "They had severe arthritis or advanced heart disease and couldn't be moved," Wilhelm said.

The commission found that the city's well-meaning efforts to protect this frail elderly population frequently failed. Government-sponsored cooling centers were unappealing and little used, even among those who could get to them. "Elderly people don't want to be carted somewhere," Wilhelm said. "They don't want to leave their pets behind and there's nothing at the cooling centers except a big air-conditioned room. It's not comfortable."

After the commission's report, Chicago developed a new action plan for heat waves, one that includes a city command center, a hot line and warning system from the National Weather Service and an extensive grass-roots effort to check daily on seniors who live alone.

"For the last four years, we have encouraged church and community groups, ward organizations, public health nurses, the Department on Aging, people who deliver Meals on Wheels and even mail carriers to identify this group of medically fragile elderly people," Wilhelm said. "Then when we have extremely hot or extremely cold weather, we can reach out to them. It's the real responsibility of being a citizen."

The commission found that giving an elderly person an hour or two of cool respite in a neighbor's air-conditioned apartment could make a significant difference in survival during a heat wave. So did providing sponge baths or something cool to eat and drink to the homebound elderly.

The Risks of Being Sedentary

No age group is exempt from the risks of heat illness. At the U.S. Army Research Institute of Environmental Medicine in Natick, Mass., scientists study the toll that heat takes on younger people. Among their findings is that being out of shape and overweight greatly increases the risk of heat-related health problems, from dehydration to heat stroke, a potentially fatal medical condition in which the body temperature goes above 106 degrees and people can lose consciousness.

Sedentary people face three times the risk of suffering a heat-related injury or death during a heat wave than those who are physically fit, according to Capt. William Latzka, a physiologist at the institute. "The same goes for people who are overweight," Latzka said. "They have about threefold increase in risk compared with normal-weight people. And if you are both overweight and unfit, the risk rises about eight times higher."

According to studies conducted at the institute, acclimation to the heat requires exposure to it for 60 to 90 minutes per day. The more time spent indoors during a heat wave, the slower the adaptation. "Roughly half of the body's major adaptions to heat occur in the first three to four days of exposure," Latzka said. "So if people can just modify their behavior in those first few days so that they don't overdo it, they will be about half-done."

As the air temperature rises, the body begins adapting by increasing heart rate and perspiration. It takes about 10 days of exposure to the heat to be fully adapted, Latzka said, and this protection does not last very long if temperatures return to normal, as they did after the short June heat advisory in Washington.

"But if you are sporadically exposed to heat, you can sustain your adaptation," Latzka said. "If you can get exposed to heat at least once a week, you can keep pretty good acclimation."

When It's Hot, Take a Drink

On hot days, extra fluids are needed to replace those lost from the body through perspiration. Two to four liters per day--about 68 to 135 ounces--are needed to maintain body fluids during balmy temperatures; four to six liters per day--about 135 to 203 ounces--can be required during a heat wave, particularly for those who spend a lot of time outdoors.

But exactly what kind of fluids should be consumed is often confusing. Some experts advise drinking a lot of water. Others swear by electrolyte-filled "sports drinks," such as Gatorade, which help replace sodium, potassium and chloride lost in perspiration. Still others recommend drinking anything that is not caffeinated, since caffeine is a diuretic that accelerates water loss.

To Latzka, the key to remaining well hydrated during a heat wave is simply to drink throughout the day, and especially at meals when the sodium, potassium and chloride in food helps the body to retain liquids.

Strong coffee or tea and other caffeinated beverages, such as colas, need not be avoided. "They still count as fluid replacement beverages," Latzka said. The point is not to drink too many of them, he said, since "you don't retain as much of the fluid in them as other beverages because of their diuretic properties."

As for Gatorade and other so-called sport drinks, Latzka said, they are probably unnecessary for most people who eat a well-balanced diet. Food contains adequate amounts of sodium, potassium and chloride. Drinking too much of the electrolyte drinks can also depress appetite, since they contain a fair amount of calories.

But for those who are exercising or working in the heat and want to quickly replenish fluids, sports drinks can be a good approach, Latzka said.

Besides, he said, "if you like the taste of something you're apt to drink more of it. If something is cold, you're apt to drink more of it. We tend to like things that are flavored and have sweetness. [Having] a variety of beverages also helps people drink more during the hot weather."

Resources

More information on heat-related illnesses is available on the Internet at:

* The National Weather Service, www.nws.noaa.gov/er/lwx/heat.htm

* The National Institute on Aging, http://www.nih.gov/nia/health/pubpub/hyperthe.htm

* American Family Physician, published by the American Academy of Family Physicians, www.aafp.org/afp/980901ap/barrow.html.

Recommendations for Beating the Heat

After more than 700 people died during a record heat wave in 1995, Chicago Mayor Richard M. Daley formed a commission to develop better preventive strategies during hot weather. The commission, led by Deputy Health Commissioner John Wilhelm, developed 13 simple recommendations to help people, particularly those who don't live with air conditioning, reduce the risk of heat-related illnesses during a heat wave:

* Drink lots of water and natural juices.

* Avoid going out into the blazing sun, if possible.

* Keep shades drawn and blinds closed but windows slightly open.

* Keep electric lights low or turned off.

* Take a cool bath or shower periodically or sponge the body with cool, wet towels.

* Take advantage of air conditioning in other buildings or in neighbors' homes when possible.

* Wear loose, light cotton clothing.

* Avoid alcoholic beverages and try to cut down on the use of caffeinated beverages such as coffee, tea and cola. Replace them with other beverages.

* Avoid heavy meals, but be sure to eat regularly.

* Avoid using your oven.

* Avoid physical activity during the hottest times of the day.

* Call family members and friends who might be at risk for heat-related illnesses. Check on elderly neighbors.

CAPTION: Blocks of ice, far left, used at the D.C. Caribbean Carnival recently didn't last long in the heat. Danny Collins, center photo, found a fun way to beat the heat last summer by taking advantage of a neighbor's hose. Quince Orchard Swim Club lifeguard Suzanne Kline, above, takes a quick dip to cool off after her shift in the lifeguard chair. Wesley Howard, left, is careful to drink lots of water and wears a special hat to protect himself from the sun during the Caribbean Carnival.

CAPTION: Four-year-old Tommy Roberts, of Fairfax, plays on the banks of the Potomac River with his dog Timber. Parents or other caregivers should watch children for signs of dehydration in hot weather.