Odor

Odor receptors on the antennae
of a Luna moth

Odors correspond to the objective phenomenon of chemicals dissolved in air, although, as with other senses, psychological factors can play a part in perception.

Certain odors, such as perfumes and flowers, are much sought after and large prices are paid for the most elite ones. Other whole industries have developed products to remove unpleasant odors. See deodorant.

Odors that are mostly perceived as "pleasant":

• fresh breath, flowers, trees, some plants, cologne, mint, fresh fruit

• bad breath, foot odor, feces, vomit, rotting fruit

The study of odors is a growing field, but is a complex and difficult one. The human olfactory system can detect many thousands of scents based on only very minute airborne concentrations of a chemical. The sense of smell of many animals is even better. Some fragrant flowers give off odor plumes that move downwind, and are detectable by bees more than a kilometer away.

Pheromones are odors that are deliberately used for communication. A female moth may release a pheromone that can entice a male moth from several kilometers. Honeybee queens constantly are releasing pheromones that regulate the activity of the hive. Workers can release such odors to call other bees into an appropriate cavity when a swarm moves in, or to "sound" an alarm when the hive is threatened.

There are hopes that smelling machines could do everything from test perfumes to help detect cancer, but as yet artificial noses are still quite inadequate.

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