Few have looked closely at exactly what happens in a sniff. But recently some researchers have used a specialized photographic method that shows air flow in order to detect when, and how, dogs are sniffing… The sniff begins with muscles in the nostrils straining to draw a current of air into them — this allows a large amount of any air-based odorant to enter the nose. At the same time, the air already in the nose has to be displaced. Again, the nostrils quiver slightly to push the present air deeper into the nose, or off through slits in the side of the nose and backward, out the nose and out of the way. In this way, inhaled odors don’t need to jostle with the air already in the nose for access to the lining of the nose. Here’s why this is particularly special: the photography also reveals that the slight wind generated by the exhale in fact helps to pull more of the new scent in, by creating a current of air over it.
This action is markedly different from human sniffing, with our clumsy “in through one nostril hole, out through the same hole” method. If we want to get a good smell of something, we have to sniff-hyperventilate, inhaling repeatedly without strongly exhaling. Dogs naturally create tiny wind currents in exhalations that hurry the inhalations in. So for dogs, the sniff includes an exhaled component that helps the sniffer smell. This is visible: watch for a small puff of dust rising up from the ground as a dog investigates it with his nose…
We might notice if our coffee’s been sweetened with a teaspoon of sugar; a dog can detect a teaspoon of sugar diluted in a million gallons of water: two Olympic-sized pools full.”
~ Alexandra Horowitz, Inside of a Dog. What Dogs, See, Smell and Know