Humans can detect around 10,000 different odors. In fact, it would be more accurate to compare the olfactory ability not with different odors, but with the type of receptor sensitive to odors and chemical molecules that cause odor. According to this comparison, there are 350 different types of functional olfactory receptors in humans, 1300 different types in mice, and 125-220 million different types of olfactory receptors in dogs. In greyhounds, this number goes up to 300 million. In addition, dogs are 100 million times more sensitive to odor concentrations (odor molecules per cubic centimeter) than humans. In other words, in order for a person to be able to distinguish a particular odor in a cubic centimeter area, there must be 100 million molecules of that odor, whereas a dog can distinguish the same odor (if the relevant receptor is present) when only 1 molecule is present. Sharks likewise have highly developed senses of smell. A shark has a sensitive sense of smell that can distinguish 1 drop of odor molecule in 100 liters of water. According to another source, sharks can distinguish a single molecule of fish that hunts from among 10 billion molecules. It’s like being able to distinguish a teaspoon of molecules in a pool. According to the claims of some researchers, sharks can distinguish the smell of a drop of blood from about 10 kilometers away; but this has not been proven. It is known that within the proven limits, sharks can distinguish the smell of 1 drop of blood from a distance of up to 400 meters. In fact, 70% of sharks’ brains are specialized for smelling.