Humans are exceptional at cooling. We sweat over almost our entire body surface, which is a highly efficient way of dumping heat, and it is one reason we can run long distances in conditions that stop most animals.
Dogs cannot do this, and understanding what they do instead explains most of what matters about hot weather.
Panting
The main mechanism, and it accounts for the large majority of a dog’s heat loss.
A panting dog draws air rapidly over the moist surfaces of the tongue, mouth, throat and nasal passages. Water evaporates from those surfaces, and evaporation removes heat.
Panting is rapid and shallow — typically several hundred breaths per minute in a hot dog, compared with 10 to 30 at rest. Crucially it moves air mostly through the dead space of the airways rather than deep into the lungs, so the dog does not blow off carbon dioxide and become alkalotic.
Why it fails: panting relies on evaporation, and evaporation slows dramatically as humidity rises. A hot, humid day is far more dangerous than a hot dry one, and a dog panting in a humid car has almost no cooling capacity at all.
Sweating, barely
Dogs have merocrine sweat glands in the paw pads — genuine sweat glands that produce watery secretion. You can sometimes see damp paw prints on a smooth floor from an anxious or hot dog.
The contribution to cooling is small. Paw pads are a tiny fraction of body surface area.
Dogs also have apocrine glands across the body, but these produce a scent secretion rather than watery sweat, so they do not cool.
Vasodilation
Blood vessels near the skin surface, particularly in the ears and face, dilate to bring warm blood close to the surface where heat can be lost to the air. This is why a hot dog’s ears feel warm and look flushed.
It works reasonably well when the air is cooler than the dog. It stops working when the ambient temperature approaches body temperature — which is why a dog in a hot car cannot lose heat at all.
Behaviour
The mechanism dogs rely on most in practice:
- Seeking shade
- Lying on cool surfaces — tile, stone, bare earth
- Digging a shallow hollow to reach cooler soil
- Stretching out flat to maximise surface contact
- Reducing activity
- Seeking water and wetting the belly and legs
- Drinking more
A dog who has stopped seeking these out, or who cannot, is in trouble.
Who is most at risk
Brachycephalic breeds — Bulldogs, French Bulldogs, Pugs, Boxers, Shih Tzus. Compressed airways make panting far less effective, and research has found these breeds at substantially higher risk of heat-related illness. Some studies place it at several times that of other breeds.
Overweight dogs. Fat insulates, and there is more metabolic heat to lose.
Thick and double-coated breeds, particularly northern breeds in hot climates.
Very young and very old dogs.
Dogs with laryngeal paralysis, which directly impairs airflow.
Dogs with heart or respiratory disease.
Dark-coated dogs, which absorb more solar radiation.
Any dog chasing a ball on a hot day. Exertional heatstroke is the most common form, and dogs will run themselves into it because the game overrides the signal.
Why heatstroke is so dangerous
Once a dog’s core temperature rises past about 41°C, cells begin to be damaged directly. What follows is a cascade — inflammation, clotting abnormalities, kidney injury, gut barrier breakdown — that continues even after the temperature comes down.
Mortality rates in severe cases are high even with prompt treatment, and dogs who appear to recover can deteriorate over the following hours and days.
That is why every suspected case needs a vet, even one that seems to resolve.
What to do
Signs: heavy frantic panting, drooling, bright red gums, restlessness, wobbliness, vomiting, collapse.
- Move to shade, out of any vehicle
- Pour cool water over the whole body, continuously. Current evidence supports aggressive early cooling, including immersion in cool water. The older advice to use only tepid water has been revised
- Create airflow — a fan, an open window in a moving car
- Offer small amounts of cool water if conscious
- Go to a vet immediately, cooling on the way
Do not cover the dog in wet towels and leave them — a towel that warms up traps heat.
Prevention
- Walk early and late. Use the seven-second pavement test with the back of your hand
- Skip the walk entirely on the worst days. Sniffing games indoors meet the need
- Never leave a dog in a parked car, for any length of time, in any weather above about 18°C
- Shade, airflow and water at all times
- Do not shave a double coat — the undercoat insulates against heat as well as cold, and the guard hairs block sun
- Cooling vests and mats, used with airflow
- Paddling pools and frozen food toys
- Watch the humidity, not just the temperature
And know your own dog. A Husky and a Whippet do not experience the same afternoon, and a flat-faced dog is operating with a fraction of the cooling capacity of a Labrador standing next to them.
