Dogs normally start sniffing with their right nostril, then keep it there if the smell could signal danger, but they’ll shift to the left side for something pleasant, like food or a mating partner.
It becomes easier to understand the way dogs uses their noses to understand their world by comparing it to the way we use our eyes, but, really, it’s hardly a fair comparison. A dog trained to sniff out explosives, for example, can detect a trillionth of a gram of TNT. That would be a little like you walking around with a pair of high-powered binoculars taped to your face. But even ordinary pet dogs, who have not been specially trained, have incredible olfactory powers, especially when compared with our own.
Dogs, for instance, have “stereo olfaction,” meaning that their nostrils work independently of each other; again, one way to better understand how this works is to compare it to our vision.
Just as the images from our two eyes are constructed into a three-dimensional image of the world, the differences in the strength of the smell image in each nostril help the dog locate the source of the smell in space, whether it’s to his left or right, fore or aft.