People sweat over the whole body. Dogs cannot: their sweat glands are limited to the paw pads, and they cool themselves mainly by panting. Every summer rule follows from this, and without it they look like fussing.
Who overheats faster
Flat-faced dogs — pugs, bulldogs, boxers, pekingese: short nasal passages make panting far less effective. Puppies and old dogs. Overweight animals. Dogs with a thick undercoat and dark fur. Any dog with heart or airway disease.
If your dog is on that list, summer rules are not advice for you, they are obligations.
What to change in summer
Walk early in the morning and late in the evening, not at midday. Test the tarmac with your hand: if you cannot hold your palm there for five seconds, it is too hot for paws — walk on grass or in shade.
Water must be available at all times and must be fresh. Many dogs drink noticeably more from running water than from a standing bowl, and in summer that is the difference between drinking enough and drinking a little.
Never leave a dog in a car. Not in the shade, not with a window cracked, not "for five minutes". The interior reaches dangerous temperatures faster than you can get back.
Signs of overheating
Rapid noisy breathing with a wide-open mouth and the tongue hanging out. Thick, ropy drool. Bright red gums and tongue. Unsteadiness; the dog lies down and will not get up. Vomiting. In severe cases, seizures and collapse.
This develops within minutes and is life-threatening. Do not wait to see whether it passes.
What to do
Move the dog into shade or a cool room. Wet it with cool, not ice-cold, water: belly, inner thighs, paw pads, neck. Ice water and ice packs constrict the skin vessels and trap heat inside — the effect is the opposite of what you want.
Use a fan. Offer water but do not pour it in. At the same time, call the clinic and take the dog there.
Heatstroke is not a condition where home care replaces a vet. Even if the dog looks much better, it needs to be seen: the effects on the kidneys and on blood clotting show up later, when everything appears normal from the outside.