As much as people enjoy the warm summer months, high temperatures can be hard on the human body. “As mammals, we live close to the thermal edge of life and death,” says Craig Heller, a physiologist and biology professor at Stanford University. “We run at 37°C [98.6°F], and only a couple of degrees above that puts us into heat illness and heat stroke.”
Not every part of the body is the same temperature, however, “and blood flow determines where the heat is distributed,” Heller says.
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What part of the body cools down the fastest? And can we use that knowledge to cool down more quickly when it’s hot outside?
Focus on your core
In order to prevent the negative health effects of high temperatures, scientists say the most important goal is to reduce your core temperature. The most effective way to do this is to apply cooling methods, like a cold towel or ice water, to as large of a surface area as possible.
“Cooling a body segment is not going to do much good when it comes to trying to reduce core temperature, which is the key determinant of heat-related illnesses,” says Ollie Jay, a professor of thermal physiology at the University of Sydney.
Read More: Why You Sweat So Much at Night—And What to Do About It
As the body warms up, it tries to get rid of heat by opening up blood vessels closer to the skin and sending more blood to those areas. This moves heat away from the core toward the surface of the skin, where it can dissipate from the body. Putting cold water or ice on the skin helps speed up this process, and cools down the body more quickly when water evaporates off the skin.
Jay recommends pouring cool water over as much of the body as possible or placing a towel with ice on the chest for a minute or two at a time every 10 minutes until you feel more comfortable.
Target your hands and feet
If taking a dip or wrapping yourself in a cold towel isn’t immediately feasible, “then cooling a limb, for example, is probably a good idea,” Jay says. Basic physics can help determine where to start. Every object has what is called specific heat, which is how much energy is required to heat it up or cool it down by 1°C. Objects with large surface areas and smaller masses have lower specific heats. This means the extremities—like the hands and feet, which have a lot of skin but not a lot of mass—are the most efficient at cooling down.
Ice-cold water is best, “and the more skin surface area that’s covered, the faster you’re going to cool,” says Douglas Casa, a professor of kinesiology at the University of Connecticut and CEO of the Korey Stringer Institute, a nonprofit at the university dedicated to heat-stroke prevention.
Read More: How to Spend Time Outside if You Hate Getting Sweaty
Another unique aspect of these areas is that they’re essentially hairless—and Heller has found that the parts of your skin without hair can quickly heat up and cool down. “The palms of your hands and the soles of your feet are radiators,” he says. This is, in part, because there are large networks of blood vessels in these areas that can quickly exchange heat through the blood.
Know when it’s time to step things up
If you’re truly overheated, cooling off one body part at a time may not be sufficient. Most negative effects from heat, like heat stroke or heat illness, come from when a person’s core temperature reaches dangerous levels, or the heat has put excess strain on their heart.
It’s not always obvious to a person when they’ve reached this point. How hot someone feels is not always connected to how hot their core temperature is.
“Behavior is driven by how hot you feel, and health is driven by how hot you are,” Jay says. “You can have wildly different skin temperatures for a given core temperature.” For example, applying a cold towel to the neck and face can cool arteries heading toward the brain—which gives the body a signal that it is actually colder than it is. Some areas, like the face, have a lot of receptors to detect temperature—so cooling them may make someone feel like they are cooling off quickly when they aren’t.
One way to know it’s time to cool off is to keep an eye out for signs of heat exhaustion: symptoms like nausea, cramps, dizziness, and an elevated heart rate. If a person experiences any of these, they should get out of warm environments, seek medical attention, and use methods to immediately cool off.
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