I always like to think you should not treat anyone or any animal in a way that you wouldn't treat yourself. Some long time ago (ok I'm old) when first owning a car I was waiting for Carol with the car windows fully wound down ( pre electric windows) I lasted 5 mins before it was uncomfortable enough to make me leave the car and seek shade, it was not pleasant, though I don't know how bad it would have got or whether death would ensue, I do know that I would not intentionally repeat the experiment and would not leave a dog in the same situation, I was able to leave when the comfort levels decreased, a dog hasn't got that luxury.