The heat wave baking the central United States this week may feel worse than those earlier in the summer, and corn is partly to blame, forecasters say.

July has already been more humid than average, and now another, less obvious factor will enhance humidity this week: corn sweat. Like a person breathing, corn exhales water vapor through its leaves — a process called evapotranspiration — which exacerbates humidity in agricultural regions during the summer.

Corn sweat, a midwestern phenomenon, could increase humidity levels in already-sweltering states from Ohio to Illinois through Thursday.