It’s been a hot summer in much of the country so far. July was the third hottest on record for the country, according to National Centers for Environmental Information. And there’s another heat wave hitting parts of the Pacific Northwest and Northern California this week.

The number of extremely hot days is expected to increase significantly across much of the U.S. over the next 30 years, according to a new analysis out this week from the nonprofit First Street Foundation, which focuses on climate risks.

What does that mean for labor productivity and the economy, as climate change makes it hotter all across the country?

“There isn’t a place that’s actually getting cooler,” said Matthew Eby, founder and CEO of First Street Foundation.

Eby said the hotter it is, the less productive people are, “because we just can’t function in the same way under those extreme conditions.”

That’s especially true for people who work outside or in places that aren’t air conditioned.

“If you’re working in a large warehouse, or if you’re a UPS truck driver, or if you’re doing construction, you’re directly exposed to really high temperatures,” said Amir Jina, an assistant professor at the University of Chicago’s Harris School of Public Policy.

Extreme heat isn’t just uncomfortable, Jina said. “It can also make you more sluggish, it can make you more prone to errors, it can affect your health.”

The threshold where heat starts to really affect how well people can focus and work, is roughly 90 degrees. “Above that, we just start to see the hours that people work being eaten away,” Jina said.

Those days are becoming increasingly common. By mid-century, the Union of Concerned Scientists projects there will be twice as many days where the heat index tops 100 degrees.