Harvard Health Blog

Summer's heat is as predictable as winter's chill. Heat-related illnesses--and even deaths--are also predictable. But they aren't inevitable. In fact, most are preventable. Staying hydrated is the key.

No matter what the season, your body functions like a furnace. It burns food to generate chemical energy and heat. Some of the heat is used to keep your body temperature in the high 90s. The rest you have to get rid of. The body has two main ways of getting rid of excess heat:

1. Radiation. When the air around you is cooler than your body, you radiate heat to the air. But this heat transfer stops when the air temperature approaches body temperature.

2. Evaporation. Every molecule of sweat that evaporates from your skin whisks away heat. But as the humidity creeps above 75 percent or so, there's so much water vapor in the air that evaporation becomes increasingly difficult.

Most healthy people tolerate the heat without missing a beat. It's not so easy for people with damaged or weakened hearts, or for older people whose bodies don't respond as readily to stress as they once did. Damage from a heart attack can keep the heart from pumping enough blood to get rid of heat.

A number of medications can limit the body's ability to get rid of excess heat. These include beta blockers, which slow the heartbeat; diuretics (water pills), which can make dehydration worse by increasing urine output; and some antidepressants and antihistamines, which can block sweating. A stroke, Parkinson's disease, Alzheimer's disease, diabetes, and other conditions can dull the brain's response to dehydration.


There are three different levels of heart-related illness:

1. Heat cramps. These painful muscle spasms are usually triggered by heavy exercise in a hot environment. Inadequate fluid intake is usually the culprit. The remedy: slow down, tank up with water, stretch and gently massage the tight muscle, and get out of the heat.

2. Heat exhaustion. When body temperature begins to climb, physical symptoms such as weakness, headache, nausea, muscle cramps, profuse sweating, and flushed, clammy skin may appear. Heat exhaustion also affects mental clarity and judgment, which may appear as confusion or lethargy. Drinking water is essential. A cool shower or bath, ice packs to the skin, or other strategies to lower body temperature are also important.

3. Heat stroke. There are two distinct forms of heat stroke. Classic heat stroke tends to affect people who can't escape the heat, or can't physically cope with it. Exertional heat stroke strikes individuals who do vigorous physical activity in the heat, such as youthful football players at a summer training camp, firefighters battling a summer blaze, Marine recruits, and weekend warriors.

Heat stroke is a medical emergency. It starts out looking like heat exhaustion, but its symptoms are more severe, and they progress more quickly, as lethargy, weakness, and confusion evolve into delirium, stupor, coma, and seizures. Body temperature rises drastically, often exceeding 105 degrees or 106 degrees.

Heat stroke is a killer because it damages the heart, liver, kidneys, brain, and blood clotting system. Survival depends on prompt transfer to a hospital for aggressive treatment.


It can be hard to tell where heat exhaustion ends and heat stroke begins. Both can be mistaken for a summer "flu," at least at first. Be on the lookout for:

1. Nausea or vomiting

2. Fatigue

3. Headache