Dogs' sense of smell overpowers human by orders of magnitude—it's 10,000 to 100,000 times as acute, scientists say. "Let's suppose they're just 10,000 times better," says James Walker, former director of the Sensory Research Institute at Florida State University, who, with several colleagues, came up with that jaw-dropping estimate during a rigorously designed, oft-cited study. "If you make the analogy to vision, 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."
Put another way, dogs can detect some odors in parts per trillion. What does that mean? In her book Inside of a Dog, Alexandra Horowitz, a dog-cognition researcher at Barnard College, writes that while we might notice if our coffee has had a teaspoon of sugar added to it, a dog could detect a teaspoon of sugar in a million gallons of water, or two Olympic-sized pools worth. Another dog scientist likened their ability to catching a whiff of one rotten apple in two million barrels.