Question
I see a fair number of people in my area leaving their dogs in the car while they eat at a restaurant.
This doesn't strike me as a particularly safe or healthy practice.
Aside from expected risks like criminal activities, having your car towed, etc., what are the objective risks associated with leaving a dog locked in an unattended car?
Answer
The interior of a car is capable of reaching far higher temperatures than the outside environment, becoming an oven for a dog (or child, for that matter) left inside it. This can happen quite rapidly and affect swiftly.
Dogs rely on cooling their body temperature through their paws (which is where most of their sweat glands are - have you noticed a vet's exam table get wet from a dog's paws?) and through panting; this makes them even more susceptible to overheating in an enclosed environment like a car.
Even with the windows left down, a car can heat up quickly. People have been known to resort to smashing windows to remove a dog from a locked car. In places where it is illegal to leave dogs alone in cars, the cost of the window repair is not recoverable by the owner.
It's important to note that if any dog is left in a car, the dog is at a much higher risk of developing heat stroke; which can be fatal. (Thanks to @sulfureous) This Wiki How Article gives some good advice on managing heat stroke.
The RACQ study below demonstrates temperature changes in cars over time, with windows closed and open:
Answered By - user6796