Heat and humidity will once again smother the eastern half of the country this week, pushing the heat index to dangerous levels for tens of millions of people. In the Midwest, the humidity will be boosted by a phenomenon called “corn sweat.”

Dangerous temperatures will persist across parts of the U.S. through July, and in some areas, there's an unexpected contributor.

Corn sweat is an agricultural phenomenon that makes humidity during a heat wave even worse, especially in states like Illinois where it's a major cash crop.