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.
Air-scenting search dogs are trained to scan the air currents for human scent, any human scent. The dog locates the source of the scent and indicates the "find" to the handler. Air-scent search dogs do not use scent articles and can be deployed immediately in hasty searches, running paths, and trails, or days or in conditions that a track/trail dog can not pick up the scent.
Track/trailing search dog is trained to pursue specific individuals by following the scent left from the particles that have drifted from the body to the ground and surrounding vegetation. Track/Trailing dogs are particularly effective when they have an item of the missing person to be used for scent identification. It can be an uncontaminated article of clothing, or an object such as a car seat. The dogs can pursue the trail and indicate the direction of travel.