When Juan Flores steps out of his Delano, California home into the midst of a heat wave, “it’s like walking into a wall,” he says. Flores lives in the San Joaquin Valley, one of the places where summer heat regularly creeps well above 100 degrees Fahrenheit.

But it isn’t just the heat that Flores is experiencing. During extreme temperatures, the air can be so thick with ozone and fine particle pollution that breathing becomes a chore. Flores has healthy lungs but still feels a painful pressure in his chest on days like that.

Bad heat and bad air together can cause particularly dangerous conditions for vulnerable communities across California, a new study published in the Proceedings of the National Academy