Dogs have about 300 million olfactory receptors in their nose, while humans only have 6 million. That means that your dog could possibly smell a stinky trash can from a mile away! With their amazing ability, dogs could be smelling something to give them clues to a person’s fear. A fearful person sweats and releases certain chemicals in their bodies. A dog can certainly smell sweat, and it is possible that he can detect those chemicals, too. However, your dog cannot differentiate the smell of fear sweat from, for example, heat sweat. If dogs can detect those chemical being released, he can smell fear, but this has yet to be proven.
While it is still unsure whether or not canines can smell fear, dogs do seem to be very wise on the matter. Dogs do not have smart phones, TV, or books, so they spend a lot of their time watching their humans. They learn humans’ reaction to stress, happiness, grief, fear, etc. Therefore, when they meet someone fearful of dogs, they use their past knowledge and put two and two together. A quickening pulse, stiffness, prolonged eye contact, and heavy breathing are all clues to fear.