Humid heat is blanketing the eastern U.S. this week, exacerbated by “corn sweat” in the Midwest

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.”

It’s midsummer, so heat and humidity are pretty standard in the wetter eastern half of the country. It’s unlikely this heat wave will break records, but it could still be dangerous, says Bob Oravec, lead forecaster at the National Weather Service’s office in College Park, Md.

On Monday the heat and humidity are centered over the Southeast and along the Gulf Coast. By midweek, they will move northward along the Mississippi Valley and up into the Midwest before they shift toward the mid-Atlantic and Northeast around the end of the week. Highs are expected to be around 95 to 100 degrees Fahrenheit (35 to 38 degrees Celsius) as the heat wave moves along, but the humidity means it could feel closer to 110 degrees F (43 degrees C) in the most affected areas. Large swaths of the eastern U.S. will be in the “major” HeatRisk category, a NWS classification that incorporates heat, humidity and data on when heat-related hospitalizations tend to rise in a given area. Pockets will be in the “extreme” category, the highest on the four-category scale.

Part of the reason for the oppressive humidity is that “the weather pattern has been favorable for wet weather,” Oravec says. “Everything is wet, saturated,” which means there is more evaporation from soil and transpiration from plants. This is particularly true in the Midwest, where huge fields of corn, soybeans and other crops release moisture as the temperature climbs. The process is akin to how humans perspire in the heat, hence the nickname “corn sweat.” “The Midwest is famous for high dew points from the vegetation,” Oravec says.

Plants aside, the phenomenon has serious implications for humans. High humidity and heat raise the risk of heat illness—it is harder for the body to cool itself via sweating because the air is already so full of moisture that perspiration doesn’t evaporate. Those concerns are especially high for at-risk groups such as young children, older adults, those who have various health conditions or take certain medications, people who work outdoors and unhoused people.