Air conditioning not only keeps you cool during the summer heat, it may also keep you alive.

A new study of California residents suggests that people with air conditioners at home are less likely than their neighbors without the technology to develop serious heat-related illnesses -- including pneumonia, cardiac disease and heat stroke -- during temperature spikes.

Although the differences were generally small, given the population size in the nation's largest state, the public health effects of air conditioning could be substantial, the researchers say.

Policies designed to encourage the use of air conditioning at home, particularly central systems, might reduce illnesses and death linked to heat, said study co-author Dr. Rupa Basu, an epidemiologist at the California Environmental Protection Agency.

We don't want to say that (more air conditioning) is all we need, said Dr. Basu, who noted that greater use of ACs could strain the electrical supply to the point of being harmful. But I think that it would be beneficial to have this as at least one of the ways of minimizing temperature effects.

In addition to causing heat stroke, high temperatures can aggravate breathing problems, such as asthma and pneumonia, and force the heart to work harder as the body tries to cool itself by moving blood nearer to the skin.

Heat waves are well known to increase rates of death in stricken areas. In California, for example, a July 2006 heat wave has been blamed for nearly 150 deaths, and possibly two to three times that many, according to state epidemiologists who studied the event. They found that every 10-degree F jump in temperature led to a nine-percent increase in deaths each day, findings they reported last year in the journal Environmental Research.

In the latest study, the same researchers compared hospitalization rates and the use of air conditioning in California between 1999 and 2005. They estimated use -- as opposed to prevalence -- of air conditioners through a detailed 2004 state survey of household appliances.

To account for California's widely variable climate -- which ranges from scorching Death Valley to the snowy peaks of the Sierras -- and the effects of geography on local temperature the researchers divided their study population into more than 100 zones.

As in their earlier study, they found that every 10-degree rise in temperature on a given day was associated with higher rates of hospitalizations for pneumonia, stroke, diabetes, heat stroke and many other conditions. But use of AC reduced the chances of ending up in the hospital.

For some conditions, such as respiratory ailments, the protection was relatively modest, with hospitalizations dropping half a percent for every 10-percent increase in AC use. But for heat stroke, every 10-percent rise in AC use led to nearly a 15-percent decrease in hospitalizations.

The benefits of AC appeared limited to central air rather than window units. However, that difference might simply reflect the fact that few people in the study reported having the window units at home, Dr. Basu said.