Avoid working out with your pet during hot days. Avoid hot sidewalks. Take water along for your pet on long walks. People who mean well, but don't think about how quickly, even on a mild 75 to 80 degree day, the temperature inside the car can rise to well over 100 degrees. Leaving the window partially open will not prevent intolerable heat buildup.
Dogs have a limited ability to cool themselves.
Panting and heat radiation from the skin surface are the primary means of animals regulating their body temperature. For this reason, obese dogs and short-nosed breeds, such as pugs, bulldogs, boxers, Pekingese, etc., are at increased risk of suffering from heat stroke. Signs of heat exhaustion include excessive panting, reddening of the gums and flushing of the skin inside the ears. If these signs are noted, stop exercising at once and take your dog to a cool area and give cool water to drink. If your dog doesn't seem to recover quickly, his heat exhaustion can proceed to heat stroke, which is an emergency and may be characterized by weakness, wobbling, stumbling or collapse.