All humans are constantly shedding microscopic flakes of skin. These flakes are carried by air currents and deposited on the ground and in vegetation. A dog detects these human scents and is able to follow them.
Air scenting dogs use their noses to detect any human scent present in a particular area.
The dog is trained to pick up and follow indiscriminate human scent. The handler's job is to get the dog downwind of any place a lost or missing person could be. Once the dog picks up human scent he begins to follow it, trying to find the strongest areas. Each human being has a unique scent, and these dogs learn how to identify it from a piece of clothing or other item with which the individual has had contact. The dog then proceeds to track only that particular scent by following the trail of skin flakes left behind by the lost person.