Dying from the heat is an all-too-common occurrence in the summer despite advances in technology and physical conditioning, medical authorities say.
Each year, according to the U.S. Centers for Disease Control and Prevention, about 300 Americans die from extreme heat every year, most of them elderly and infants.
That's a higher death toll in a typical year than from floods, tornadoes, hurricanes, lightning and earthquakes combined.
For athletes under the age of 40 -- like Korey Stringer, a Pro Bowl offensive tackle for the Minnesota Vikings who died Wednesday after being overcome by the heat at training camp -- heat illness is the second most common cause of death after heart failure.
Stringer, 27, died a day after collapsing in Mankato, Minn., on the hottest day of the year in the region.
Heatstroke occurs when the body's internal thermostat rises uncontrollably, usually above 106 degrees within a few minutes. Victims stop sweating -- which is the body's way of using evaporation to cool the blood just below the skin surface, lowering body temperature.
As the internal temperature rises, the brain and other vital organs begin to fail.
Hot weather is the obvious prerequisite. However, body weight, clothing and humidity all contribute by preventing sweat from evaporating, said Dr. Stephen Smith, emergency physician at Hennepin County Medical Center in Minneapolis.
Everyone's heat threshold is different, Smith said.
''Even if they tell you it's only 90 degrees, if it feels hot -- it's hot,'' Smith said. ''You've got to trust your instincts.''
For athletes, whose instinct is to keep going, combining those factors in a competitive environment can be deadly.
News of Stringer's death raced through NFL training camps, where full- contact drills in hot weather are standard procedure. Players are reminded to drink fluids and the sight of trainers administering fluids intravenously is not uncommon.
''We do everything we can to prevent it,'' said Cincinnati Bengals trainer Paul Sparling. The Bengals practice in Georgetown, Ky. ''You have to be honest with yourself when your body starts telling you, you can't go anymore.
''Sometimes a guy is going out on the field dehydrated to begin with because he hasn't replaced all the fluids he lost from his last practice. Diuretics used to cut weight are going to dehydrate you. You're losing water instead of fat.''
Stringer, who weighed 335 pounds and struggled with his weight, was dressed in pads and participating in full-contact drills.
On Tuesday, the temperature was in the low 90s. Stifling humidity pushed the heat index as high as 110 degrees.
Team officials said at least five other players had heat symptoms during Tuesday's practice. Stringer also experienced problems on Monday, officials said.
During Tuesday's workout, Stringer vomited at least three times, but didn't summon a trainer until practice ended and he became disoriented. He was unconscious when he arrived at Immanuel St. Joseph's-Mayo Health System in Mankato, and had a temperature of over 108 degrees.
''The personality changes. You may look normal, but be acting strangely,'' Smith said. ''Reactions may be slower, your judgment is bad, you don't respond to the situation as you normally would -- it's a downhill spiral from there.''
Stringer's death came six days after a University of Florida football player died from heat stroke complications.
According to the National Center for Catastrophic Sports Injury Research in Chapel Hill, N.C., 18 high school or college players have died of heat-related causes since 1995.
After the heatstroke death of a high school athlete last summer, high school coaches and marching band directors in suburban Atlanta monitor playing conditions with a special device. A digital psychrometer measures temperature, humidity, wind and heat radiation from the ground. Workouts are stopped when a certain number is registered.
The devices, which cost about $100 apiece, are commonly used in industry and hospitals, but are not widely used in athletics.
CDC doctors say heatstroke is easily prevented by simply avoiding strenuous activity in hot weather and wearing loose clothing. Activity should be stopped if your heart pounds or you get dizzy.
Drink plenty of water or sports drinks, but avoid alcohol and soda.
People experiencing heat symptoms should seek shade and medical attention immediately. Until an ambulance arrives, they should be immersed in a tub of cold water or even sprayed with a garden hose.
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