The temperature at which small dogs overheat is when the small dogs body temperature rises over 102 F degrees.
Small dogs can overheat easily even when the outdoor temperature or indoor temperature is 80 F or above.
The dogs body temperature can rise even when the temperature is only 80 F to 90 F and they can begin to feel hotter and can easily overheat.
This is especially true when a dog is left in a car or vehicle where the outdoor temperature is only 75 F to 80 F degrees.
The inside of the car or other vehicle can reach higher temperatures than the outdoor temperatures and it can easily get to 100 F or higher inside the vehicle even though the outdoor temperature is only 80 F to 85 F degrees.
This is why even in slightly cooler temperatures that may feel just warm outside you should never leave a dog or child inside the vehicle as they can easily overheat.
Any temperature above 85 F is considered too hot or too warm for a dog to be left in longer than 4 hours.
If the temperature rises above 85 F for any length of time the dog should be in a cooler environment to avoid them overheating.
Also make sure the dog has plenty of water to drink.