Like humans, dogs are at risk of heat stroke during the summer months. It can only take a few minutes for a pet left in a vehicle on a warm day to succumb to heatstroke. Unlike people, our canine friends have very limited ability to cool off by sweating. Dogs have sweat glands on their foot pads only and their main mechanism for cooling down is panting which can be insufficient to lower their body temperature on hot days.
Just a few minutes in a car can be enough for an animal's body temperature to climb to deadly levels that will damage the nervous and cardiovascular systems, often leaving the animal comatose, dehydrated and at risk of permanent impairment or death. Even on a moderate summer day, the sun can turn a car into an oven very quickly. On a mild 70° day the temperature inside a car parked in the shade – even with the window “cracked”- can reach 89° in a few minutes; in direct sunlight it can reach more than 100°.
The law in Maryland: MD Code, Transportation § 21-1004.1
It is illegal for a cat or dog to be in a “standing or parked motor vehicle in a manner that endangers the health or safety of the cat or dog.”