"ER physicians will tell you that they (obese people) are the ones collapsing," said Thomas Adams, a Michigan State University physiologist.

Federal health officials list obesity as a risk factor for heat-related illness, but health warnings generally focus on the dangers to children, the elderly and the socially isolated. Those are the groups considered most in danger of fatal heat stroke, health officials said.

But non-fatal heat exhaustion may hit obese people more quickly than thinner folk, said Adams, author of the 1993 book 'Guidelines for Surviving Heat and Cold.'

Fat is a natural insulator that traps core body heat, experts said. While that may be a boon in the winter, it's a burden for someone stuck in summer heat.

The body cools itself by circulating blood to dissipate heat through the skin. A heavy person's heart must pump harder to circulate blood on a hot day.

When a person is standing up, the most difficult place to circulate blood to is the brain. So inadequate circulation can cause someone to be lightheaded and to faint, Adams explained.

People also cool off by sweating. An obese person who cannot dissipate heat through circulation will sweat more, meaning they lose water and body salts to become more quickly dehydrated, he added.

The risk of heat-related death seems to rise with the degree to which a person is overweight, but scientists haven't determined at what level of being overweight the danger truly escalates, said George Luber, an epidemiologist with the U.S. Centers for Disease Control and Prevention.

In the years 1999 through 2003, about 1,200 U.S. deaths were reported in which heat-related illness was a major factor, according to a CDC report published last week. The report did not examine obesity as a risk factor, but found cardiovascular disease was an underlying cause of death in 57% of those cases and diabetes was in 3% of those cases.

"People who are obese tend to suffer more from cardiovascular disease," noted Luber, who co-authored the report. "Obesity kind of represents several risk factors rolled into one."

Everyone is at risk during a severe heat wave, but it makes sense for obese people to be cautious, said Dr. Arthur Kellermann, a professor of emergency medicine at Atlanta's Emory University.

"If they read this article and say 'Well I am kind of heavy, I should be careful,' that's good," Kellermann said.