The human body can’t handle excessive heat. The processes that keep us alive work best within a certain temperature window. That’s generally between about 36° and 37° Celsius (96.8° to 98.6° Fahrenheit), depending on the person.

If someone’s core body temperature goes higher, “the body’s primary response to heat is to try and get rid of it,” explains Jonathan Samet. He’s the dean of the Colorado School of Public Health in Aurora. To get rid of excess heat, blood vessels in the skin dilate, or expand. At the same time, the heart begins beating faster. That pushes blood flow to the skin. There, the blood can release heat to cool down. Meanwhile, sweating kicks in to cool the skin.

When people experience high temperatures again and again, their bodies can get better at shedding excess heat. That’s why someone can move from cold Minnesota to steamy Florida and get used to the higher heat and humidity.

But there is a limit to how much the body can adjust. That limit depends on an individual’s health, as well as the temperature and humidity outside. If the outside temps are hotter than the body, blood at the skin won’t release heat. And where humidity is high, sweating won’t cool the skin. That’s because the sweat can’t evaporate. In 2008, two scientists suggested that humans can’t cool off well if they spend extended time at a wet-bulb temperature over 35° C, or 95° F. (Wet-bulb temperatures are measurements that combine heat, humidity and other factors.)

If the body has to keep dealing with heat without a break, it gets worn out. People can experience heat exhaustion, which causes weakness, dizziness and nausea. If a person still doesn’t cool off, heat stroke may occur. This signals that the body’s ability to regulate heat has broken down. This can allow core body temperature to climb as high as 40° C (104° F). Heat stroke can trigger seizures, convulsions or a coma. Without treatment, death may follow.

Many people see heat as more of an annoyance than a threat. But climate change, extreme heat and human health are all connected. As Earth’s temperatures climb, extreme heat waves will probably become more common, endangering more people.