Dogs first taught to track or trail may later be trained in air scent. However, dogs first taught to air scent can rarely be taught to track or trail because air scenting is the easiest and most natural for the dog. Often air scent dogs naturally combine all three methods to locate a subject. Dogs instinctively use their noses to locate things in everyday life, so the real training is to teach them (1) to locate the lost person or article and (2) to notify the handler that the subject or article has been found. This notification is called the alert..
Dogs can locate people because all humans have an individual human scent left behind by the 40,000 skin cells dropped per minute. Air currents carry these cells that dogs can detect with their sensitive nose.