(WHTM) — Everybody knows, it’s the humidity that gets you. But why is that?

Humidity, which is a measure of the air’s water vapor content, is no stranger in the Midstate. It feels sticky, heavy and — on the hottest days — suffocating.

This is because high humidity blocks an important function for the human body: sweating.

The body’s natural way of cooling itself off is, of course, sweating. When sweat evaporates, it reduces the body’s temperature.

But the National Weather Service says this cannot happen when humidity is high. When the atmospheric moisture content increases, the service said, the rate of evaporation off the body decreases.

“In other words, the human body feels warmer in humid conditions,” the service said.

Forecasters measure this effect with the heat index. It’s a blend of the air temperature and relative humidity percentage, and it shows what the temperature outside actually feels like to the human body.

So, it’s not just the humidity that gets you.

How to stay safe in extreme heat

Extreme heat is when heat and humidity with temperatures higher than 90 degrees Fahrenheit lasts for at least two to three days, according to Ready PA.

Extreme heat is responsible for the most weather-related deaths each year, the agency said. So, it recommends taking some precautions:

• Drink water to stay hydrated
• Take cool showers or baths
• Dress in loose-fitting fitting, lightweight and light-colored clothing
• Stay indoors and limit exposure to the sun