"Dogs' powerful noses have 300 million sensors, compared with a human's measly 5 million. In addition, dogs have a second smelling device in the backs of their noses that we don't have, called Jacobson's organ."
That double smelling system allows trained dogs to detect cancer's unique odors, called volatile organic compounds.
It took humans thousands of years to figure this out. In 1989, doctors at King's College Hospital in London wrote in The Lancet about a woman whose dog persisted in smelling a particular mole on her leg. That mole turned out to be early-stage malignant melanoma."
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