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Heat, Humidity, and Hydration

Joseph Gerald (Jerry) Reves, M.D.
September 13, 2022
Senior man is drinking water after exercising.

The end of summer brings the greatest heat. It’s also a time when many of us are trying to squeeze in a vacation, often at places that are hot and humid. Wherever and whenever heat and humidity combine to create a high “heat index,” we must stay hydrated. When you walk out of your airconditioned house early in the morning and feel as if you’ve walked into a sauna, it’s certain the heat index is already high.

How we cool off in the heat

Our bodies are efficient at regulating our temperature. In fact, if the thermometer deviates much from 98.6 F, our body swings into action. Our cardiovascular system and sweat glands keep our temperature remarkably normal, even in hot, humid weather. Nevertheless, we must exercise caution when we exercise or engage in any physical activity during extreme heat and humidity. If our bodies are unable to cool down through sweating and other physiological responses, we can suffer a progression of increasingly menacing heat illnesses.

The remainder of this column highlights ways we can maintain adequate hydration when our bodies perspire profusely to cool us down. This is important because dehydration is the first threat to our ability to function in heat.


Who is susceptible to heat illness? Anyone who is exercising vigorously in hot (over 85 F,) and humid (over 65%) weather, especially in the sun, is potentially susceptible. Other factors that make us susceptible to heat illness are older age, heart disease, other chronic diseases, sunburn, obesity, sleep deprivation, alcoholism, and certain drugs. Finally, people who take beta-blockers, tricyclic antidepressants, aspirin, and diuretics are predisposed to heat disorders.


People who exercise or participate in routine activities in the heat and humidity, especially in the sun, will perspire. The more vigorous the activity or the higher the heat and humidity, the more we will perspire, which causes our bodies to lose vital body fluids and electrolytes (salt, potassium, and other important chemicals). When we lose more fluids and electrolytes than we take in, we become dehydrated.

Consequence of dehydration

What goes wrong when the body loses an excessive amount of fluid? The cardiovascular system tries to compensate with an increased heart rate, but the tendency to vasodilate makes it difficult to maintain a normal cardiac output and blood pressure. This response can lead to insufficient blood flow to the brain, causing dizziness and even fainting. This obviously is dangerous since losing consciousness can cause a fall with disastrous consequences. The signs and symptoms of dehydration include thirst, dry mouth, dry skin, fatigue, less urination, dizziness, and headache. Dehydration should be prevented but treated immediately it if occurs.

Prevention of dehydration

We can take several measures (See Table 1 below) to avoid becoming dehydrated. Most are well known and make common sense. The most important is to drink plenty of water and fluids that contain carbohydrates and salt. Be sure to drink 15 to 20 minutes before your workout and every 15 minutes while exercising. A typical person needs 8 to 9 glasses of fluid per day just to replace average fluid losses. The rule of thumb for calculating your daily fluid requirement in ounces, as advised by exercise physiologist L. Lee  Coyne PhD, is your weight in pounds divided by 2 ( Thus, a 160-pound man would need 80 ounces of fluid a day.

What fluids to drink?

Label of contents of Gatorade powder packet 
Figure 1. Label of contents of Gatorade powder packet. This powder can be mixed as directed or diluted with more water. Other sports drinks have a different formula that may include chloride and calcium with or without sugar. It is advisable to consult your physician about the appropriate sports drink for you based on your medical history.

Most of the time, water is sufficient to replenish fluids and is generally the fluid of choice for staying hydrated throughout the day.

However, too much water can be problematic. Sports drinks such as Gatorade that contain sodium, chloride, potassium, and perhaps calcium are recommended for people who are exercising or working in heat and humidity for more than one hour. These ingredients help replenish what is lost during perspiration. Gatorade can be diluted if you’re on a diet that restricts salt or sugar. Fruit drinks or smoothies are also good choices since fruits have a high water content. Finally, drinking skim milk or low-fat milk after vigorous exercise restores much-needed protein.

Avoid drinking alcohol to cool off or quench thirst. Alcohol is a vasodilator and diuretic that can deplete blood volume instead of augmenting it. Coffee and tea are also diuretics when consumed in large quantities. Drink cold liquid instead of room-temperature or warm liquid; it is absorbed better and not only cools the body, but also restores lost fluid.


The Bottom Line

Heat and humidity are a particular threat to older people who cannot compensate as well as younger people. It is essential to anticipate the greater fluid requirements for everyday activity and exercise in hot weather. Staying hydrated is one of the most effective strategies to combat the heat!

Table 1. Tips for Heat and Humidity

  1. Drink plenty of fluids containing carbohydrates and electrolytes when outside for extended periods.
  2. Wear light-colored clothing that wicks perspiration, such as synthetic materials as opposed to cotton.
  3. Get acclimated by increasing exercise or outside activities gradually each day in the heat.
  4. Don’t exercise strenuously in full sun during the hottest part of the day.
  5. Avoid sunburn.
  6. Shelter in air conditioning; that’s why it was invented.

MUSC Health has earned a "High-Performing Hospital" designation in geriatrics from U.S. News & World Report for 2018-2019