Just to clarify one point, K-9's are left in cars during the shift. We check on them constantly. Our cars have heat alarms, temperature sensors and will page or call our cell phone if the interior temp goes off 85 degrees. The windows come down, a large fan kicks on and the lights and alarm go off. The system we use costs about $1500. Also our cars have radiator coolers and are designed to run all day. When I go to work, I never turn my car off. My car runs or idles for about 10 hours a day.
I have travelled extensively across the US with dogs in my car, in all kinds of weather. If I stop at a store I leave my personal car running with the doors locked. I do have fans running on batteries as a back up, temperature monitors in my personal car as well. I will also pop the hood or leave the hood up to vent heat from the engine. The biggest thing that I would worry about would be my car overheating. My Honda Odyssey has proven to be very reliable and barely burns any gas idling for hours. Again, I check on my dogs or bring them where I go.
Some one breaking into your car is a concern, most dogs can be easily stolen. Few dogs will put up a real fight. My female wouldn't bite anyone, my male is a completely different story. If you got him out of the car you would be wearing him like a fur coat and a trip to the hospital would be the next stop.
I would never leave a dog in a car with the windows up or even cracked unless it was very cool outside, like well under 60 degrees. Even then, I would have the windows down a few inches. I have vent visors on my windows so I can leave them down a few inches and rain will not come in. It also prevents some one from reaching in and trying to pet the dog and getting bit.
“Beware of false knowledge; it is more dangerous than ignorance”. George Bernard Shaw