BY JAN NULL
Special to the Mercury News

In recent weeks, there have been several tragic stories about heat-related deaths of children in cars, the elderly and even well-conditioned athletes. These events demonstrate that heat is the most deadly weather phenomenon in the United States, causing an average of 144 deaths per year. According to statistics from the National Weather Service, each year heat kills more people than floods, lightning, tornadoes, cold weather or hurricanes.

These numbers only account for fatalities directly related to heat. There are undoubtedly many more deaths indirectly related to hot weather, especially among the elderly during heat waves.

For example, in 1980 a severe heat wave in the Midwest killed more than 1,250 people. Officials from the federal Centers for Disease Control and Prevention estimated another 10,000 deaths were related to heat stress. Here in the Bay Area, there were nine heat-related deaths in June 2000 when temperature readings ranged from 100 degrees to 110 degrees.

Additionally, there is a great deal of truth in the expression: ``It's not the heat, it's the humidity.'' Not only is high humidity uncomfortable, it is also unhealthy. Heat-related illness occurs when a person's body can't properly cool itself by perspiring. Under extremely hot or humid conditions, sweating can't compensate enough to stave off a rapid rise in body temperature. The body's inability to cool itself is exacerbated when the humidity is high.

Ultimately, very high body temperatures may damage the brain or other vital organs.

The recent death of Minnesota Vikings player Korey Stringer was a prime example of this phenomenon. Stringer suffered heatstroke at training camp July 31, when the temperature was in the low 90s. But the relative humidity that day was about 50 percent and the ``heat index,'' which combines the two factors, was more than 100 degrees.

The heat index is a measure of the apparent temperature and relates to the body's ability to cool itself. It is calculated from the ambient temperature and the relative humidity. A table showing this relationship can be found online at http://ggweather.com/101/hi.htm, and there is an online calculator at http://ggweather.com/calc.htm.

A number of factors affect the body's ability to cool itself during extremely hot weather. Besides humidity, other conditions that can increase risks from heat include age, obesity, fever, dehydration, heart disease, mental illness, poor circulation, sunburn, prescription drug use and alcohol use. Infants and children up to four years of age and people older than 65 are also at high risk.

In the Bay Area, high humidity is often less of a concern that it is in other parts of the country. But we have other concerns to keep in mind -- our frequent sunshine and our car culture.

Extreme heat can arise easily in enclosed vehicles. A number of studies have shown that the temperature in an enclosed car or truck can be more than 30 degrees above the outside temperature after less than 20 minutes, and more than 40 degrees within a half-hour.

On a recent 80-degree day in San Jose, after an hour in the sun, my car was a toasty 136 degrees. This reading was taken with the thermometer out of direct sunlight. If the thermometer is in direct sun, the results are even more dramatic -- I recently recorded a rise from 80 to 103 degrees in only 5 minutes!

Keep this in mind when considering leaving a child or pet in a car for even a minute or two. Already this year, at least 27 children have died in cars due to excessive heat. State Sen. Jackie Speier, D-San Mateo, has sponsored a bill (SB 255) that is working its way through the Legislature. It would place penalties on people who leave a child who is 6 years old or younger unattended in a vehicle.

Q  I've heard people talk about 90-degree temperatures and 90 percent relative humidity. What would the heat index be under these conditions? Tom Robinson - Redwood City

A  Actually, these 90-90 conditions are a myth and do not occur naturally. For this to happen the dew point, the temperature that moist air must be cooled to for condensation to occur, would have to be at least 87 degrees -- and the highest that I have ever seen is about 82 degrees. With a temperature of 90 degrees and a dew point of 82, the relative humidity would be only 77 percent. This would translate to a heat index of 111. If it were possible to get to 90 degrees and 90 percent relative humidity, the heat index would be 122 degrees.

Q   I am interested in heat index conditions across the country. Are there any sites that show the heat index in other parts of the country? The Web sites we have found provide good information about what the heat index is, but they don't give current readings.  Gordon Kass - Los Gatos

A  There is a very good Web site that shows not only a current map of the heat index of the nation, but it also has forecast heat index values for the next 24 hours. See http://wxweb.meteostar.com/public/heatindex/index.shtml.