Exercise, excitement, or infection can cause a dog’s temperature to rise normally. But when a dog’s body temperature rises only a few degrees over 102 degrees Fahrenheit / 39 degree Celsius, a dog becomes overheated, and it’s important to take action before heat exhaustion occurs.
When humans sweat, heat is transferred from the moisture over our entire body to drier, cooler air around it. Evaporation of this sweat cools us, and a fan makes it happen faster.

Dogs don’t sweat. At least, not how we experience sweating.

Dogs pant because they have few sweat glands. Dogs inhale cooler, drier air through the nose and upper airways, exchanging that in the lungs for warm, moist air as evaporative cooling takes place (the dog’s equivalent of our sweating mechanism).

The breathing rate increases from 40 to 400 breaths per minute, with an occasional deep breath.
After the dog sucks cooler air into the lungs, they exhale hot, moist air through the mouth over the dripping tongue, expelling excess body heat.
Panting animals, then, need enough cool water to drink on warm days to keep those airways moist, maintaining a delicate temperature balance. But if the outside environment is also very moist, less evaporation and cooling occur. This may result in a reduced tolerance for hot, humid weather and a greater danger of the dog overheating.

As a dog’s temperature rises, blood rushes to the surfaces of the tongue, gums, and membranes to help transfer excess heat. Clear warning signs of overheating in dogs include frantic panting, extreme salivation, bright-red membranes, and labored breathing. The condition may quickly progress to a metabolic meltdown as their temperature rises to over 106 degrees Fahrenheit and dogs can no longer cool themselves.

An overheated dog may gasp for air, and the entire mouth will become grayish to purple because of the unmet oxygen demand. As the dog becomes dehydrated, their saliva thickens, and the dog may vomit and have diarrhea. Unable to stand, the dog might even have a seizure, become comatose, and die. Even if you can cool down the dog and they’re acting normally, rush them to an emergency clinic because their organs may have already been damaged and death could follow.

Keep a close eye on double-coated dogs when summer temperatures rise, when traveling, or when there’s an unusual heat wave in your area. Although a dog’s coat can provide insulation from both heat and cold, double coats can make a dog more vulnerable to overheating. Black and other dark-colored coats also absorb heat faster in the sun.

Brachycephalic dogs — dogs with shorter muzzles and flatter faces — tend to be more heat-sensitive. Their airways may be less efficient at moving air in and out of the lungs, making the dog more vulnerable to overheating.

All dogs are vulnerable to heatstroke, and every owner needs to be acutely aware of their dogs’ limits. Some brachycephalic dogs, like Pugs and Bulldogs, may be more sensitive to high heat and humidity, so owners should pay close attention to their breathing. Also, even dogs accustomed to exercise may overheat easily when exercising on hot days.

Grebe warns, “Anything that causes brachycephalic dogs to breathe harder places additional stress on their airways, whether it is overheating on a hot day, excitement that causes panting, or exercise that increases the oxygen demand of their muscles, requiring more strenuous breathing.”

Chronic respiratory challenges also tend to cause irritation with more salivation and swelling in the throat. This further restricts air movement as passages narrow. Eventually, the respiratory system loses elasticity with repeated challenges, and with an increased effort to breathe, the walls of the airway may be drawn inward. Grebe explains, “Try pinching your nose shut, close your mouth, and inhale. Feel the walls of your larynx and trachea being sucked in.”

Over time, the increased effort to breathe causes a progressive inward collapse of the walls of the larynx, which may be fatal. “Careful breeding of brachycephalic dogs should make good airways a priority in breeding stock,” Grebe says. Carrie Forsyth, who breeds and shows Pekingese, avoids the problem of overheating by selecting against pinched nostrils and other upper-respiratory abnormalities in her breeding program.

With some dog breeds or similar mix of dog types it is advisable to bring a towel, ice, cool packs for dogs and spray bottles in any weather.
Hereby we are thinking of pugs, a bull type of dog, pekingese.
Usually those breeds or mixes thereof with a flat snout.
You never know when it can become too warm for your dog.

If you think your dog is overheating, you quickly move them to a cool place and immediately spray cool (not cold or icy) water on the coat, ensuring it reaches the skin.
Cold water tends to constrict the surface blood vessels in the skin and this reduces, instead of increases, heat loss. Here are other ways to help prevent overheating in dogs:

What is the right action if you are faced with an overheated dog? Jumping into the (too hot) car and driving to the vet is not always the best choice. It is often better to at least start cooling the dog.

These are some options:

• Cool the dog with running water
  It is best to cool your dog with running water. A garden hose or tap is a very good way to soak the entire coat and essentially rinse away the heat. Sponging generously or pouring over with a bucket or watering can is also possible.
  Exposure to cold water all at once is often a shock to the already unfit dog and can cause cardiac arrest.
  Therefore, take a few minutes to let the dog get used to the water temperature, and do not use very cold water.
• Place your dog near the fan/air conditioner
  If there is a fan nearby, use it immediately. The car's fan or air conditioning can also be used to cool the dog.
  Start by aiming it at the head so that the dog can breathe in fresh air and the warm, moist exhaled air is immediately removed. Make sure that the eyes do not dry out too much.
• Most drastic option, shaving the dog
  Taking off the fur coat is usually not possible. If you have clippers, shave the abdomen. Here the coat is usually thinner and the muscles are not as massive. Below the abdominal muscles lies the abdominal cavity, which is highly vascularized.
  Blood vessels can release heat relatively easily there. Once the coat has been shaved, it can be wetted and the fan can be aimed at the belly. The evaporation contributes to lowering the temperature of the skin.

A combination of these three works fastest, but is often difficult to achieve yourself.
Sometimes proper cooling can only take place once the dog has arrived at the vet.
Do not refrigerate for too long. As soon as you notice that the dog is becoming more alert, you can reduce the amount and stop cooling the dog shortly afterwards. Dry the animal if it is very wet; Hypothermia is lurking, especially in small dogs.

• Shaving the dog
  Pay close attention when temperatures increase. You could also choose to shave your dog as a preventative measure.
• Do not leave in a warm car
  Prevention is better than cure. Don't leave a dog in the car on a sunny day. Not even with the windows open and not in spring or autumn. Even if the outside temperature is only 20 degrees, it will be 32 degrees in the car within 10 minutes, and 40 degrees after 30 minutes. See the image below about temperatures in a closed car.
• Fan, air conditioning or cooling mat.
  In addition to using a fan or air conditioner to cool your dog, you can also consider purchasing a cooling mat.
  With a cooling mat for dogs, the dog always has a place where conduction can take place.

If you manage to cool the dog in time, the animal usually recovers quickly. However, if the overheating has lasted too long, organ damage may have occurred.
The first indication of this is persistent lethargy, which indicates reduced brain function. The intestine is also very sensitive to overheating and shock, resulting in bloody-black diarrhea.

Kidneys, liver and blood clotting are often the next organ systems to fail.
The more organs are damaged, the greater the chance that the dog will not survive the overheating.

That's why our advice is:
Always take a dog that is or has been overheated to a vet.
The vet will not only treat the overheating, shock and any residual symptoms, but will also pay attention to the underlying cause. Vocal cord paralysis, for example, can be diagnosed under very light anesthesia.