Dogs can't perspire. They don't have a cooling system. Therefore, if you're on errands and you leave your dog in a parked car, it can get overheated. It's dangerous, and if you're not convinced, watch veterinarian Dr. Ernie Ward show you how quickly temperatures rise in a parked car, within 30 minutes. It definitely makes me realize my dog should stay home, with an air conditioner or fan on, during hot summer days.