There is no set temperature that has been determined to be safe for dogs to remain outside. In fact, there are many factors that play a role in the ability of animals to tolerate hot weather.
Physical characteristics that predispose an animal to developing heatstroke include obesity, brachycephalic conformation (short-nosed dogs like Bulldogs and Pugs), very old or very young animals, and animals with certain medical conditions (e.g., upper respiratory disease, heart disease, lung disease or central nervous disease).
Environmental factors that contribute to overheating include high temperatures (one study showed working dogs were more at risk when temperatures reached 104 degrees Fahrenheit or higher), high humidity, poor ventilation and limited access to water.
When the temperature exceeds about 90 degrees Fahrenheit, heat must be dissipated by evaporation. This means sweating in humans, horses and cows, and panting in dogs. Cats are relatively resistant to heatstroke. All pets should have unlimited access to cool water and access to a shaded and well ventilated area to reduce the risk of overheating. Sometimes, the safest thing is to bring a pet inside an air conditioned building during the heat of the day. Check out VetDepot's infographic for more tips on how to help your pets beat the heat and stay safe.