Dogs have sweat glands, but almost none of them are involved in cooling. The system they rely on instead has specific limits, and knowing them changes how heat risk is judged.
Where the sweat glands are and what they do
The glands that produce watery sweat are confined largely to the paw pads, and the surface area involved is far too small to shed meaningful heat.
Their output leaves damp prints on a hard floor when a dog is anxious or hot, which is a visible sign rather than an effective cooling mechanism.
The rest of the skin carries a different type of gland producing oily secretion, which conditions the coat and plays no part in temperature regulation.
Panting as the primary system
Panting moves air rapidly across the moist surfaces of the tongue, mouth and upper airway, and evaporation from those surfaces removes heat.
The breathing pattern is shallow and rapid, which ventilates the upper airway without over-ventilating the lungs and disturbing blood chemistry.
Blood flow to the tongue increases substantially during heavy panting, which is why a hot dog's tongue broadens and reddens noticeably.
Why humidity is the hidden variable
Evaporative cooling depends on the air being able to absorb water vapour, and that capacity falls as humidity rises.
In humid conditions a dog can pant at full effort and shed very little heat, which is why dangerous days are not always the hottest ones.
Panting itself generates heat through muscular work, so a dog in humid conditions can be adding heat almost as fast as it removes it.
The other routes and their limits
Heat also moves out through the skin by direct transfer, which requires the surrounding air or surface to be cooler than the dog.
Once air temperature approaches body temperature that route reverses, and the dog begins gaining heat from its surroundings instead.
Lying on cool tiled floors or damp ground uses direct conduction, which is why dogs seek those surfaces and why cooling mats work at all.
Which dogs have less margin
Short-faced breeds have a reduced airway and therefore a reduced capacity to move air, which removes much of the safety margin the mechanism normally provides.
Overweight dogs carry insulation and a higher heat load, and thick double coats slow heat transfer at the skin.
Heatstroke is a medical emergency in which the temperature regulation system has been overwhelmed. It requires immediate veterinary care, and cooling on the way rather than instead of going.