Heat waves cause the most weather-related deaths and the very young, very old and very sick are most at risk, U.S. health officials remind.

"Hyperthermia is an abnormally high body temperature caused by a failure of the heat-regulating mechanisms of the body to deal with the heat coming from the environment,"

officials at the National Institute on Aging, part of the National Institutes of Health, said in a statement. "Lifestyle factors can include not drinking enough fluids, living in housing without air conditioning, lack of mobility and access to transportation, overdressing, visiting overcrowded places and not understanding how to respond to hot weather conditions."

Older people, particularly those with chronic medical conditions, should stay indoors on hot and humid days, especially when an air pollution alert is in effect or go to a cooling center, officials said.

Health-related factors that might increase hyperthermia risk include:

-- Being dehydrated.

-- Age-related changes to the skin such as impaired blood circulation and inefficient sweat glands.

-- Heart, lung and kidney diseases, as well as any illness that causes general weakness or fever.

-- Reduced sweating, caused by medications such as diuretics, sedatives, tranquilizers.

-- Taking several drugs for various conditions. It is important, however, to continue to take prescribed medication and discuss possible problems with a physician.

-- Being substantially overweight or underweight.

-- Drinking alcoholic beverages. (UPI)