Flat faced dogs are at risk in heat and when flying. Pugs, Bulldogs, English Toy Spaniel, Shi Tzu are some examples of dogs that face breathing difficulties during heat.  I never leave these dogs outside over about 95 degrees. Some dogs I cut off at 90 degrees. It is not possible for them to get enough air when breathing. In Austin, Texas heat over 90 degrees i s the norm all summer long.  I walk/exercise these dogs early in   the morning or in the evening when temps have dropped. Most airlines now do not allow a dog to fly cargo over 80 degrees. This can make flying out of Austin very difficult - some summer days don't see temps below 80 at night.  For more information see the article in the Austin American Statesman Newspaper.
http://www.statesman.com/news/nation/bulldogs-pugs-face-special-risks-flying-807636.html