Editor's note: This story was updated on July 26, 2019.

Summertime has arrived, and across the country, temperatures are soaring to record-breaking numbers. Here's what you need to know to stay safe when it gets hot.

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The hypothalamus in your brain is very sensitive to changes in core temperature, said Julien Periard, a physiologist at Aspetar who studies exercise performance in heat. Once your temperature—which for humans is 98.6 degrees Fahrenheit on average—rises about a degree above your baseline, the hypothalamus reroutes blood to the skin's periphery, which causes you to sweat.

Your body sweats because it wants to cool down. But that works only if the air outside is colder than your core temperature, which allows sweat to evaporate off your skin.

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If the outside environment is too hot, the sweat won't evaporate and you won't cool down—but you will dehydrate.

All that blood rerouting to your skin can also put other organs under strain. It means less blood for your heart, which has to work harder to maintain its regular output—which is compounded by the fact that higher temperatures stimulate your heart to beat faster. That means "the heart has to work significantly harder on a hot day versus a cool one," Periard said. "This usually means decreased performance that feels quite unpleasant."

Your other muscles are affected, too: When less blood is available, your muscles rely on energy production processes that don't need oxygen but require more sugar. That raises the chance that your body will run out of energy, slowing you down and leading your muscles to recover less quickly.

And the hotter it is outside, the higher your risk for serious illness or injury. Once it's over 104 degrees Fahrenheit, "the brain overheats and the central nervous system starts to go haywire," said Periard. "You might become confused, agitated, and dizzy. These are all telltale signs of heat stroke, and signals that you should seek medical attention immediately."

Staying safe

If you have to exert yourself in the heat, there are steps you can take to protect yourself from heat exhaustion and other serious illnesses, according to Lisa Leon, a scientist who studies heat stroke with the Department of Defense:

• Exert yourself less the hotter it is;
• Exercise earlier in the day, when it's cooler outside;
• Take more breaks and seek shade;
• Use sunscreen;
• Do not take nonsteroidal anti-inflammatory drugs, which can be more dangerous if your core temperature is elevated; and
• Seek medical help if you, or someone else, present with symptoms of heat stroke, which include dizziness, confusion, and disorientation.

Periard also recommends drinking a sports drink to replace your electrolytes—but cautions that you should drink only to thirst so you don't overhydrate (Fritz, "Capital Weather Gang," Washington Post, 7/28; Stulberg, Outside Online, 8/23; WebMD page, accessed 8/31; Park, CNN, 6/21).

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