At least 38 people in the United States have died from heat-related conditions so far this summer. Now researchers are trying to determine how hot is too hot for humans.

The human body cannot tolerate its core temperature reaching 109-degrees Fahrenheit, according to an NBC News article. In fact, 99% of people who reach that core temperature would die, according to health experts.

The body normally cools down its core temperature by sweating. However, climate scientist Scott Denning at Colorado State University said if it’s too humid, your sweat can’t evaporate and your skin can’t cool down as fast as your insides. This can increase the risk of high fever and heat stroke.

Everyone’s survival limit will vary depending on many factors like age and underlying health issues, according to doctors.

It’s important to keep your core temperature down during a heat wave by drinking water and staying in an air-conditioned environment.