You shouldn’t ever leave a young child unattended in the car. It’s illegal in some states—but it can be deadly on a hot day. Children under the age of 4 are especially in danger of developing vehicular hyperthermia, which can damage the body’s organs and the brain, but there have been fatal cases in children as old as 14. “People really need to understand that cars are potential ovens for both kids and pets,” says Dr. Corey Slovis, chairman of the department of emergency medicine at Vanderbilt University Medical Center. “Even 10 minutes in the hot sun can elevate the temperature inside a car by up to 20 degrees.” It doesn’t even have to be a sweltering day to be dangerous. If it’s 83 degrees outside, the temperature inside an un-air-conditioned car can hit 109 in about 15 minutes, even with the window rolled down a couple of inches.