• Tracking or trailing dogs work with their nose to the ground. They follow a trail of human scent -- typically heavy skin particles that fall quickly to the ground or onto bushes -- through any type of terrain. These dogs are not searching, they're following: Tracking dogs need a last seen starting point, an article with the person's scent on it to work from and an uncontaminated trail. For this type of work, time is an issue. The sooner a dog can start the better.

• Air-scent dogs work with their nose in the air. They pick up human scent anywhere in the vicinity -- they don't need a last seen starting point, an article to work from or a scent trail, and time is not an issue. Whereas tracking dogs follow a particular scent trail, air-scent dogs pick up a scent carried in air currents and seek out its origin, the point of greatest concentration.

• Air-scenters might specialize in a particular type of search, such as:

o Cadaver - specifically search for the scent of human remains, detecting the smell of human decomposition gasses in addition to skin rafts. Cadaver dogs can find something as small as a human tooth or a single drop of blood.

o Water - search for drowning victims by boat. When a body is under water, skin particles and gases rise to the surface, so dogs can smell a body even when it's completely immersed. Typically, more than one SAR team searches the area of interest, and divers use each dog's alert point, along with water-current analysis, to estimate the most likely location of the body.

o Urban disaster - The most difficult SAR specialty, urban disaster dogs search for human survivors in collapsed buildings. They must navigate dangerous, unstable terrain.

o Wilderness - search for human scent in a wilderness setting.