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It's no secret that dogs possess an incredible sense of smell, able to detect week-old fingerprints, objects underground and even cancer. Scent is the dog's No. 1 sense, and is up to 100,000 times more powerful than a human's sense of smell. This power, along with the amount of distance a dog can smell, is up to its breed and basic physiology.
Built to Smell
Your dog's nose is divided into two different passages. These two passages allow for a continual circulation of air that help the dog smell for extended periods of time. Unlike humans, who block odors when exhaling through their nose, a dog exhales through the sides of his nose, keeping the stream of inhaled odors constant. When a dog sniffs something, scented chemicals are caught in the mucus of the nose. Lining the surface of the nasal passage are minute cilia, each loaded with scent receptors, that capture and transmit the scent information to the brain.
How Well Can They Smell?
It's no secret that a dog's sense of smell is much more powerful than ours. James Walker, the former director of The Sensory Research in Tallahassee, Florida compares its power to the power of sight in this analogy, saying that what "you and I can see at a third of a mile, a dog could see more than 3,000 miles away and still see as well."
The Power Of Reception
How far away dogs can smell depends upon the amounts of scent receptors they have; a number that is differentiated by breed. At the top of the list, with 300 million scent receptors, are the noble bloodhounds, while beagles and German shepherds both sport 225 million. Humans, in comparison, only have 5 million scent receptors. Generally, the smaller the surface area of the dog's nose, the fewer scent receptors there are.
In addition to the dog's physiology, the object being scented and wind direction also comes into play when distance is the determinate. Whale feces, an undoubtedly unique scent, has been detected by a Lab mix and a Rottweiler at a distance of 1.2 miles, while cadaver dogs have been able to identify decaying bodies 80 feet under water. Scenting at great distance takes training as well; not every dog can pinpoint a smell at those lengths. A dog trained to smell cadavers wouldn't recognize drugs or weapons, for example.