Missy had to take a trip to the vet the other day. Her allergies are acting up and making her eyes gooey in the mornings. Plus she has a skin fold rash that won't go away. (common in wrinkled face dogs) So off we went. She isn't much into car rides so she gets a little stressed by them. Once there she pants alot while waiting and she sounds just like a little freight train. You can see from the photo she is sitting right in front of the A/C so it isn't because she is hot.
Panting can be a very common sign of stress. Unfortunately in pushed in face breeds of dogs (also known as brachycephalic) it can be something to really watch out for. If they pant excessively it can cause them to go into respiratory distress. This can kill a dog by causing them to not get enough oxygen, in effect overheating their bodies and/or suffocating them. So it isn't always the outside temperature you have to worry about with these guys.
Does that mean all panting is stress? No. A relaxed open mouth on a dog and panting is normally fine. It is the frantic panting that can be accompanied by bulging eyes and a loud sound that is the stress pant or overheating panting to watch. Keep an eye on the color of the dogs tongue. If it is purple (and it isn't a chow or shar-pei) that is a big danger sign. Cool them off and get them to a vet. A staggering dog that acts drunk is also a danger sign.
Do NOT trust that your dog outside will seek out shade if they get hot. Some dogs do not and it can have disastrous consequences. I had an older dog that overheated once standing in the sun with available shade within easy reach in my fenced yard. It happens.
To cool off a dog quickly wet their belly, armpits (leg pits) and chest area.
Do not use ice cold water as this could put them into shock. Use cool water instead. Do not try to get them to drink if they aren't interested. If you are near a lake you can lay them in the water. Get them out of direct sunlight.
Dogs don't sweat like people do. They pant to help cool them off and they sweat through the pads of their feet. If you see wet paw prints that is either sweat or from stress. (Dogs will pant and sweat when highly stressed.)
So you can see that panting can mean a few different things. Knowing your dog is important to help you be able to read the subtle differences better.
This is Wolfie. He is a Pomeranian that belongs to one of my spinning friends. (yarn spinning, not the exercise) He is about 18 months old now. This is his "Hey are you gonna share that food with me?" face.