With potentially deadly high temperatures expected to bake as much as two-thirds of the United States through this weekend, the National Highway Traffic Safety Administration (NHTSA) is reminding Americans of the dangers of leaving children unattended in vehicles. Already this year, 21 deaths have been reported nationwide.

NHTSA further cautions owners to keep their vehicles locked at all times when parked to prevent a child climbing in and becoming trapped. And never leave a child in a vehicle when running errands, even for a minute.

It doesn’t take long for the inside of a vehicle to reach potentially lethal temperatures. In 10 minutes, a car can heat up by 20 degrees. Rolling down a window does little to keep a vehicle cool, and heatstroke deaths have occurred even in vehicles parked in shaded areas.

Bystanders can also play an important role in saving a life – if you see a child alone in a vehicle, call 911 and get help immediately.