On hot and humid days, your skin might feel sticky and overheated, and the air can feel so heavy that breathing becomes a chore. But what is it about humidity that can make the outdoors feel so stifling?
The answer is that high humidity can make us feel hotter than we do on drier days. That's because humidity — the amount of water vapor (moisture) air possesses — can make it difficult to shed excess body heat through sweat, according to the National Oceanic and Atmospheric Administration's (NOAA's) National Weather Service.
Ordinarily, as sweat beads onto the surface of our skin, heat from our bodies evaporates that sweat into the air, thereby cooling our skin. Humidity, however, prevents sweat from evaporating as readily, because the surrounding air already has a high moisture content and can't absorb much more. And the less that moisture evaporates from our skin, the more uncomfortably warm we feel.
High temperatures enhance humidity's ability to thwart temperature regulation, because warm air can hold more moisture than cool air can. According to the NOAA National Centers for Environmental Information, a 1-degree Fahrenheit (0.55-degree Celsius) rise in temperature equals as much as a 4% increase in atmospheric water vapor. This explains why humidity feels more uncomfortable in summer than in winter, even if the humidity levels happen to be identical.
Not only is hot and humid air stuffy and sticky on the skin, it's also a chore to breathe. But its "heaviness" isn't due to its high water vapor content, according to The Washington Post's Capital Weather Gang. The water vapor molecules that displace some of dry air's nitrogen and oxygen are actually less dense, meaning that moist air is actually lighter than dry air; however, because water vapor ousts small amounts of these gases, there's less oxygen in humid air to breathe in. What's more, our bodies are already taxed by being overheated, so it feels like it takes more work than normal to simply breathe.
While there's no set humidity threshold above which general comfort level begins to deteriorate, NOAA typically considers relative humidity (RH) levels of 50% or more, and dewpoints (a more direct measure of humidity) above 65 F (18 C) to be uncomfortably high.
The good news is that our bodies can adapt somewhat to high temperatures and high humidity — though, probably not quickly enough if, say, you were vacationing for a week in a tropical locale. "It takes an average of nine to 14 days to fully acclimate, depending on one's fitness [level], body size and prior acclimation," Larry Kenney, a professor of physiology and kinesiology at Penn State, told Live Science in an email.
One of the first stages of heat acclimation is the dilation of blood vessels, which allows more blood to flow near the skin's surface, where excess heat can easily vent into the air outside of the body. Meanwhile, these dilated vessels translate into a lower heart rate and provide more fluid for sweating, Kenney said. By days three to five, sweat rates start to increase, he added. But this adaptation is only temporary. Similarly to how physical fitness and strength is lost if one skips a month at the gym, thermal adaptation can wane if it isn't exercised.
As for the question of what temperature-humidity combo is beyond human tolerance, Kenney told Live Science this query drives his current research. With average temperatures and humidity levels projected to continue rising in some places due to climate change, an answer will likely present itself before you can utter the words, "It's not the heat; it's the humidity."
Originally published on Live Science.