I have on numerous occasions left my dogs in the car with the A/C on, windows up and car locked. I always carry a spare key with me in case I have to run in somewhere and I have to leave them in the car. They are not in there for hours so don't think that. I put the sun shade up in the windshield to keep more sun out and my windows are tinted. I would never leave them in a car unlocked with no A/C on, but with the A/C on and the car locked, I see no problem with it. Like I said it's only so I can run a few errands and never for more than 15 minutes. They have never had a problem in there, they are laying in the front seat when I come back curled up.