“When it’s humid, I’m drenched,” says Patricia Christie, a lecturer in MIT’s Experimental Studies Group who teaches The Chemistry of Sports. Some research studies do suggest that the human body sweats more as humidity increases, while others suggest that sweat eventually decreases. But what’s really sopping Christie is that the sweat’s just not evaporating as fast.
Normally, the body cools itself by opening pores on the skin and releasing water and salts. As the water evaporates, it transfers the body’s heat to the air. Because water has a high latent heat, which is the heat required to change liquid water to vapor, this process usually carries away enough heat to do a good job of cooling the body.”It’s a fabulous system,” says Christie.
But the rate at which water—or in this case, sweat—evaporates depends on how much water is already in the air. On dry days, sweat evaporates quickly, which means it also carries away heat faster. On humid days, when the air is already saturated with water, sweat evaporates more slowly.
This explains why it feels so much hotter in high humidity. When relative humidity reaches a high enough level, the body’s natural cooling system simply can’t work. Sweat evaporates very slowly, if at all, and the body heats up. In extreme cases, people begin to suffer from heat cramps or heat stroke, which is basically organ failure as the body begins to cook itself.
A metric called the heat index provides warnings for weather conditions that will make heat stroke more likely. For instance, the body experiences 88-degree (Fahrenheit) weather with 85 percent humidity as if it were 110 degrees. Working outside, even in the shade, is dangerous in these conditions. At 40 percent humidity or lower, however, 88 degrees feels like 88 degrees, and gardening is once again a perfectly safe activity.
To help athletes and laborers stay cool in extremely hot and humid conditions, engineers have developed special clothing that wicks moisture away from the skin. Wearing these fabrics “is like standing in a wind tunnel,” says Christie. The clothing pulls sweat off the skin through tiny channels in the fabric and deposits it on the outside of the fabric where it evaporates. Fabrics that do not wick moisture away from the skin, such as cotton, simply soak up the moisture and retain it—leaving you feeling soggy and hot.
“Wicking fabric will also keep you warmer in winter,” says Christie. Since wicking clothing keeps the skin dry, the body stays warmer because dry skin doesn’t transfer heat to the air as easily. “It’s great to have wicking stuff so you don’t have the cold, wet material sitting on the skin and sucking away all of the heat.” — Elizabeth Dougherty
Thanks to J. Mohan Rao of Bangalore, India, for this question.
(This question is an updated repost from May 10, 2011.)