There’s a reason you seek out shade in the summer, and it’s not just a matter of comfort. It’s one of survival. Our internal organs thrive at a core temperature of 98.6 degrees Fahrenheit. When the ambient temperature (what’s felt on the skin when you’re sitting in your house or walking to work) is 73 degrees, your body hums along in neutral; your core temperature stays put without extra work. But when the ambient temperature soars, it shifts the core temp with it, kicking off a feedback loop aimed at resetting your internal reading. Here, find out why these changes occur, and learn how to keep cool from head to toe.
As ambient temperatures rise and fall, the brain’s hypothalamus acts like a thermostat, constantly receiving messages from temperature-sensing nerve cells in the skin (thermoreceptors) and in turn sending signals out to other organs, muscles, and blood vessels to make adjustments. The result: Sweat glands emit sweat; blood is shunted to the skin; muscles get fatigued. A 2012 Experimental Physiology paper noted that animals resisted exercise when brain temperature was artificially raised, even as their bodies were kept cool, suggesting that it’s the brain that calls the shots.
Beat the heat: To cool the brain, cool the body, says Lars Nybo, a professor of human physiology at the University of Copenhagen, in Denmark. Wearing a broad-brimmed hat can be an effective way to do this—it shields the face, which is densely populated with thermoreceptors, as well as the scalp and the neck. All three are significant absorbers of radiant (direct) heat from the sun. Slurries (crushed-ice drinks) may help, too, says a 2012 BMC Medicine paper. Like most liquids, they replenish water lost by sweat. And as the ice melts in your body, it absorbs heat from nearby tissues.
Sweating is your body’s best defense against heat. Liquids absorb heat when they turn into vapor, so when sweat evaporates, it takes the warmth on your skin along with it. On a hot day, sweat glands (tubelike skin structures) can emit about 16 ounces of sweat if you’re sitting and 2½ gallons if you’re running a marathon, says Douglas Tomczak, Ph.D., a research and development manager for deodorants at Unilever. As the skin cools, so does the blood circulating beneath it and, in turn, the internal organs.
Beat the heat: Skin feels coolest when you optimize sweat evaporation while shielding it from the blazing sun and concrete. That’s why when you sit outside on a hot, bright day you’ll feel better in a light, loose blouse made of cotton (or some other breathable fabric), rather than a tank top. The blouse, along with a hat, could cut in half the total amount of heat—both radiant and from the air—felt on your skin, says Nybo. Blocking evaporation (for instance, by wearing polyester) will not only make you feel hotter but could also make you break out. Heat rash—tiny red, itchy bumps—emerges when sweat plugs pores, says New York–based dermatologist Whitney Bowe. Treat it by wearing light, breathable fabrics during the day and at night. Staying in air-conditioned rooms or getting a fan to help the air circulate may also help. If your skin is uncovered and you’re still breaking out, you may have a sun rash, which can look the same as a heat rash but is more common in early summer, when the skin isn’t accustomed to sunlight, says Lawrence E. Gibson, a dermatologist at the Mayo Clinic in Rochester, Minnesota. Experts believe that it may be triggered by an allergen inside the body that’s activated by ultraviolet rays. Prevent the condition with sunscreen, and soothe it with a topical steroid cream, like hydrocortisone. It will probably disappear in a few hours.
Cued by the brain, your heart beats harder to move the warm blood from the internal organs to the skin's surface so that heat can dissipate.
Beat the heat: Hydrate diligently. Blood plasma, which holds blood cells in solution, is about 92 percent water. Drink 16 ounces of water an hour before an activity, then three ounces (two big gulps) every 20 minutes while outside, so that the blood can stay viscous and move easily to the skin’s surface, says Jian Cui, Ph.D., an associate professor of medicine at Penn State University in Hershey, Pennsylvania.
As the organs lose oxygen (because the blood carrying it is flooding the skin’s surface), you breathe faster and harder, says Lisa R. Leon, Ph. D., a research physiologist with the U.S. Army in Natick, Massachusetts.
Beat the heat: Do outside activities—gardening, jogging—early. Afternoon air has higher levels of ozone, a gas that is created when heat and UV rays mix with pollutants and oxygen. It causes airway irritation, which American Lung Association adviser Norman Edelman, M.D., calls “sunburn for your lungs,” and leads to less efficient oxygen intake.