The structure of a dog's nose gives it a sense of smell that is thousands of times better than a human being's. A dog's nose has two hundred million nasal olfactory receptors. Each receptor detects and identifies the minute odor molecules that are constantly flying off different objects.
Of all a dog's senses, its sense of smell is the most highly developed. Dogs have about 25 times more olfactory (smell) receptors than humans do. These receptors occur in special sniffing cells deep in a dog's snout and are what allow a dog to "out-smell" humans.
Dogs can sense odors at concentrations nearly 100 million times lower than humans can. They can detect one drop of blood in five quarts of water! Sniffing the bare sidewalk may seem crazy, but it yields a wealth of information to your dog, whether it's the scent of the poodle next door or a whiff of the bacon sandwich someone dropped last week.
When a dog breathes normally, air doesn't pass directly over the smell receptors. But when the dog takes a deep sniff, the air travels all the way to the smell receptors, near the back of the dog's snout. So for a dog, sniffing is a big part of smelling.
Wild canines rely on smell for hunting. They also use smell to decode scent messages left by other animals -- friend or foe, predator or prey. A wild canine's sense of smell is especially important in habitats where seeing prey is difficult such as the thick underbrush of forestsre at home: sniffing dogs
A dog can sniff out all sorts of smells that human noses miss. Because of this keen sense of smell, we can train them for jobs such as tracking, rescue, or drug and bomb detection and to detect a wide variety of specific scents, such as drugs, fruits or the feces of particular animals. Dogs that make a living by sniffing are trained to alert their handlers to the presence of these things by pawing, barking, or in the case of something dangerous, sitting quietly. They are trained by the promise of a favorite toy or play time each time they successfully sniff out the target scent.
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A human can recognize 10,000 scents
"Generally" dogs have an olfactory sense approximately 100,000 to 1,000,000 times more acute than a human's. A Bloodhound, (The dog with the highest sense of smell) has a 10,000,000 to 100,000,000 higher ability than a human.