The moment you step into oppressive heat, the body senses life-threatening danger and starts fighting to keep things cool.

The heart beats faster as it increases the flow of blood to the skin, trying to keep critical internal organs from overheating. But if your core temperature continues to rise, drastic measures kick in. Sweat starts dripping — then pouring — from your glands so that evaporation can cool the body.

But if humidity leaves the sweat with no place to go and it simply drips off the skin, “your internal temperature will skyrocket,” said Matthew Ganio, a researcher at UT Southwestern Medical Center and Texas Health Presbyterian Hospital`s Institute for Exercise and Environmental Medicine. “Eventually it could lead to organ damage and death.”

Heat waves do more than make us cross and sluggish. Searing temperatures kill more people in the U.S. than hurricanes, lightning, tornadoes, floods and earthquakes combined.

People over the age of 60 are most vulnerable to suffocatingly hot conditions. But if you`re not fit, if you`re overweight, or if you suffer from heart disease, diabetes or respiratory problems, you`re also at high risk because these conditions can hamper the body`s ability to regulate its core temperatures in extreme heat.

Fatal heatstroke occurs 3.5 times more frequently in overweight or obese adults than those of average body weight, according to research published last year in the Canadian Medical Association Journal.

Those living with diabetes also have significantly higher rates of heat illness and death during heat waves than the general population, in part because they may have nerve fibers that don`t signal the blood vessels to dilate. This could decrease the amount of blood brought to the skin`s surface to dissipate heat, according to the review. Some evidence also shows people with diabetes may have a reduced ability to sweat.

In a heat wave, stress on the heart can be exacerbated by dehydration as the body`s core temperature rises. “To get the blood flow out to the skin, our cardiovascular system has to work hard,” said Lacy Holowatz, a professor of kinesiology at Penn State University who researches thermoregulation, or the body`s ability to regulate its temperature.

“For every one-degree Celsius rise in core temperature, a typical person`s heartbeat goes up 30 beats per minute,” she said. “So heat is a stress on the cardiovascular system, even without exercise.”

Heat exhaustion, the mildest form of heat-related illness, can develop in those who are exposed to high temperatures over several days and haven`t adequately replaced the water and salt they lose when they sweat. Common warning signs include cramps, fatigue, dizziness and nausea.

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