Smell is the dominant sense for canines, unlike humans who are predominantly visual. Depending on the conditions, the scent of a passing human will linger in an area for days. That means that a dog can not only smell recent activity but can perceive a recent history of activity at a particular place.
Finding things by smell is as natural to a dog as finding things by vision is to us. Imagine if each person left a unique color trail where ever they went; we could then follow the color trails to find the person we sought. While people don’t leave color trails, they do leave scent trails, and dogs can use those unique scent trails to find people.
SAR dogs are primarily focused on two types of scent: live people and human remains. Many SAR dogs are trained to focus on only one of these odors, but others are trained on both.
Live scent
People continuously shed dead skin cells, exhale volatile compounds, and disperse other smells. It is thought that a dog identifies an individual person’s scent by integrating all the smells associated with that individual — from their cells and exhalations, the bacteria that grow on shed cells, the products they wear (shampoo, deodorant), etc. Some of this scent settles in a person’s tracks but much of it will be carried some distance in the air. Where it ends up will depend on air currents and barriers. For instance, scent often collects on the edges of side walks or trails and along buildings. Cool, moist conditions make scent detection easier than hot, dry conditions. Scent will also last a long time in cold temperatures but won’t volatilize as much, so dogs need to get closer to the scent source to detect it.
Live scent dogs may be trained to find human scent in general, or they made be trained as scent-specific, meaning that they can smell a scent article (e.g., piece of clothing) from a specific person and then distinguish it from the smell of other people.
Human remains scent
When humans die and start to decompose, they release a new mixture of scents. At first, some of the live scent will remain, but it will be rapidly swamped by the odors of decomposition. All human remains share a suite of scents that a dog can be trained to find. A trained dog can distinguish the odor of a decomposing human body from that of a different animal, although in death, some animals smell more similar to humans than others.
Human remains detection dogs are trained to find the scent of human remains in general. This also allows them to find items contaminated with human blood or other tissue.