Sometimes, extreme heat and humidity are too much for our bodies to handle safely — so much so that heat stress days are designated for those days when they make a dangerous, and even deadly, combination.

A heat stress day occurs when the combination of high temperatures and humidity puts significant strain on the human body, making it harder to cool down and increasing the risk of heat-related illnesses or death.

Unlike just a hot day, a heat stress day factors in both temperature and how much moisture is in the air, because when humidity is high, sweat doesn’t evaporate as easily and the body struggles to regulate its core temperature.

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Meteorologists often use tools like the heat index or wet bulb globe temperature to determine when conditions are dangerous. These measurements consider how heat, humidity, sunlight and wind combine to affect the human body.

On days when the heat index climbs above certain thresholds, outdoor activity can become hazardous, especially for vulnerable populations like children, the elderly and outdoor workers. The National Weather Service will issue heat alerts when temperatures climb dangerously high.

Heat is the No. 1 killer when it comes to weather, so make sure you pay attention to any heat-related alert that is issued.

As the climate warms, the number of heat stress days is increasing. On average, cities across the U.S. are experiencing about 10 more heat stress days compared to 1979, according to Climate Central.

However, some locations, especially across the southern U.S., are experiencing as many as 25 to 35 more heat stress days per year. What used to be rare, high-heat events are now happening more often and lasting longer.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.