Hot weather kills more Americans than all other natural disasters combined, and the casualties continue to climb despite decades of warnings about how to recognize the signs of heat stress and take prompt corrective action.

With climate change, some experts predict ever-worsening summer heat waves and even more related illnesses and deaths. The Natural Resources Defense Council estimates that excessive heat caused by climate change could kill more than 150,000 Americans by the end of the century in the 40 largest cities.


Extreme heat claims an average of 117 lives each year, but the real incidence is likely far higher. In addition, about 1,800 people die from illnesses made worse by heat, the council estimates.

“Death rates from many causes rise during heat waves that are related to heat but not reported as such,” said Dr. Christopher B. Colwell, director of emergency medicine at Denver Health Medical Center. “Lots of deaths that occur during heat waves are attributed to natural causes like heart attacks, kidney disease or respiratory disease.”

Especially at risk are the elderly, young children, athletes of all ages and weekend warriors whose bodies are not adapted to heat stress.

“As common as the problem is, it’s not common enough to grab people’s attention until it hits close to home,” Colwell said in an interview.

Even a high-profile death, like that of Korey Stringer, 27, a Minnesota Vikings offensive tackle who suffered heatstroke after a summer morning practice in 2001, has not prompted all coaches to take necessary precautions.

“Many coaches have held practices in the heat for years and no one died, so they think a bigger deal is being made of the problem than it really is,” Colwell said.

In the six years before Stringer’s death, 19 high school and college players died from heatstroke, according to researchers at the University of North Carolina. Too often, a player suffering from heat exhaustion, the first stage of a potentially life-threatening heat illness, is sent back on the field after a brief rest instead of being benched for the day, or longer.

While deaths of healthy young athletes tend to be well publicized, the elderly are much more likely to succumb to extreme heat. Colwell explained that with age, the body’s ability to cool itself declines. Among other changes, blood vessels don’t dilate as readily to allow heat to escape, a problem made worse by conditions like congestive heart failure and peripheral vascular disease.

Many older people without air conditioning or fans may not know when to get out of the heat, or they may be physically unable to leave an overheated dwelling.

Dehydration, a common problem among the elderly as well as among younger people who exercise strenuously, raises the risk of heat illness by diminishing the body’s ability to lose heat.

Medications taken by many older people also increase their vulnerability to heat stress, among them beta blockers prescribed for high blood pressure and anticholinergics used to treat lung problems and urinary incontinence.

Other drugs, too, can contribute to a hypersensitivity to heat, including lithium, tricyclic antidepressants, antihistamines and antispasmodics. Recreational drugs, like cocaine, amphetamines, PCP and alcohol, can be a problem as well.

Heat illness often occurs several days into a heat wave, as the effects on the body accumulate.

The body normally operates within a rather narrow temperature range. If body temperature rises above 105 degrees Fahrenheit, enzymes begin to break down and normal metabolic processes are disrupted. When Stringer collapsed, his temperature registered above 108 degrees.

Heat radiates from the body when blood vessels are maximally dilated and the air temperature is lower than body temperature. But the most effective natural coolant is sweat; as it collects on the skin and evaporates, it draws heat from the body.

The risk of heat illness rises with the heat index, a combined measure of air temperature and relative humidity. When the humidity is high (or too much clothing is worn), sweat simply rolls off the skin without evaporating and cooling it.

Coaches, take note: Depending on athletes’ ages, intensity of activity and degree of acclimatization, you should consider canceling practice and games when the heat index exceeds 105, according to experts. City dwellers are most at risk during heat waves because paved surfaces, tall buildings and minimal tree cover enhance heat absorption, creating a “heat island.”

Heat illness is a form of hyperthermia, defined as a rise in core body temperature. But it does not respond to fever-reducing medications, making it extremely important to recognize heat exhaustion, an early sign of trouble. Common complaints include fatigue, dizziness, weakness, headache, nausea and muscle cramps.

Colwell explained that the brain’s cerebellum is especially sensitive to heat, which explains the early signs of a heatstroke: unsteady gait, confusion and disorientation. Heatstroke, characterized by a rise in body temperature above 104 degrees, has a death rate as high as 50 percent. Symptoms typically include a change in mental status, like delirium, seizures or even coma.

Among the elderly, heatstroke most often develops gradually, over several hot days. But among otherwise healthy people engaged in strenuous exercise, it tends to occur suddenly, within minutes to hours, which demands particular attention to early symptoms.