Dogs have more than 100 million sensory receptor sites in the nasal cavity compared to 6 million in people, and the canine brain devoted to analyzing odors is about 40 times larger than human brain. Dogs can smell odors at concentrations of 1 part per trillion, while human sometimes can't even smell 1 part per million. Smells travel through the air by diffusion; odor particles within the air move freely in ALL directions. Under perfect conditions, dogs can smell objects or people as far as 20km away.