Carol of the most excellent Frogdog Blog has written the best article on heatstroke I've ever seen. Go. Read.
Smooshy-faced dogs — Frenchies, Bostons, pugs, bulldogs, Pekes and others -— are especially at risk of heatstroke, but I've seen a good border collie in top working condition keel over as well, at a trial in the baking-hot Central Valley. From Frogdogz:
You must realize that ambient air temperature is not the only factor to consider when deciding it’s it hot enough for your French Bulldog to be at risk. Think about walking across sand, or pavement, in the cool of the evening after a hot day, and how hot those surfaces remain. Your dog, being close to the ground, is absorbing all of that ground heat. Remember that dogs do not sweat, and can only cool themselves by panting, which is made more difficult in humid weather, or when they are a flat faced breed with a shorter airway system.The Frogdog post has informative links, vet comments and a list of items for a heatstroke safety/emergency kit.
In short, never, ever assume that just because you think it isn’t ‘too hot’, your dog will agree. Your dog’s life depends on your being careful, and on your being prepared to deal with heat stroke if it happens.
This information is for ALL dogs. Flat-faced breeds are more at risk, but any dog can fall victim to heatstroke, even the soundest, best-conditioned dogs you can think of. [These are the days border collie owners work their dogs early in the morning, if possible, with frequent breaks for soaking in the water trough.] Heatstroke can kill. Thanks to Carol for a timely and potentially life-saving post.