Most of us have seen search and rescue dogs locating missing people on the TV. These dogs use any means possible to find a lost person, from sniffing for human scent in the air to being given an article of clothing and tracking that individual's unique scent.
While search and rescue dogs are capable, in principle, of being trained as both air scent and trailing dogs, most dog handlers train their dogs to perform only one of these disciplines. Therefore, the most valuable dog team, in terms of obtaining a high probability of detection, is one with a dog that can switch back and forth between air scenting and trailing as conditions dictate.
All humans, alive or dead, constantly emit microscopic particles bearing human scent. Millions of these are airborne and are carried by the wind for considerable distances.
The air scent dog is the type most frequently encountered. This dog finds lost people by picking up traces of human scent that are drifting in the air, and looks for the "cone" of scent where it is most concentrated. This dog will not normally discriminate scents, so there is the possibility of a "false alarm" if other people (searchers, citizens) are nearby. Airscent dogs work best in situations such as large parks or private lands that are closed at the time, since the dog will home in on any human scent. The success of an air scent dog will be affected by a number of factors, including wind conditions, air temperature, time of day, terrain, and presence or absence of contamination (auto exhaust, smoke, etc.). The best conditions for air scent dogs to work are early mornings or late afternoons on cool, cloudy days when there is a light wind.