With an estimated 36,000 schools across the country needing upgrades to their heating, ventilation, and air conditioning systems, heat waves hit schools hard this summer. This week, school districts in five states ended school early on Tuesday as temperatures soared, and some schools in at least two cities held school remotely.

There’s good reason for keeping kids out of classrooms during extreme heat. “Children can be affected by heat stress in multiple ways,” Lindsey Burghardt, chief science officer at Harvard University’s Center on the Developing Child, told The Boston Globe. “With extreme heat, children don’t sweat as much as adults where the adult body uses sweating to cool itself off. And if bodies can’t cool themselves, heat can bring on a variety of effects.”