If dogs are too hot and are unable to reduce their body temperature by panting, they could develop heatstroke which can kill.
If a dog is displaying any signs of heatstroke, move them to a cool, shaded area and call a vet immediately. Some types of dog are more prone to heatstroke, like very old or young dogs, dogs with thick, heavy coats or dogs with very short flat faces like pugs and bulldog types. Dogs with certain diseases or on some types of medication are also more at risk.
Warning signs of heatstroke:
Is the dog panting heavily? Is the dog drooling excessively? Does the dog appear lethargic, drowsy or uncoordinated? Is the dog collapsed or vomiting?
Emergency First Aid for dogs
For the best chance of survival, dogs suffering from heatstroke urgently need to have their body temperature lowered gradually.
Move him/her to a shaded/cool area. Immediately douse the dog with cool (not cold) water, to avoid shock. If possible, you can also use wet towels or place him/her in the breeze of a fan. Allow the dog to drink small amounts of cool water. Continue to douse the dog with cool water until his/her breathing starts to settle but never so much that he/she begins to shiver. Once the dog is cool, take him/her to the nearest vet as a matter of urgency.