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An odor, odour or fragrance is the sensation experienced by humans and animals from the sense of olfaction, caused by one or more volatilized chemical compounds, generally at a very low concentration. The terms fragrance and aroma are often used to describe a pleasant odor, while malodor, reek, stench and stink are used specifically to describe unpleasant odors.
The strength of an odor is given by the sensitivity of the olfactory nerve. Volatile acids and halogens can also be detected at relative low concentrations, while some substances like amines and thiols can be detected at even lower concentrations.
Not everybody perceives odors the same way or even at all. People displaying anosmia (inability to perceive odor or a lack of functioning olfaction) have problems detecting certain odors, such as hydrogen cyanide and other cyanide compounds (1 in 10 in the US), or even certain thiols like butyl mercaptan (1 in 1000 in the US), component of the skunk odor. Reasons for this condition range from genetic, to infection or olfactory nerve damage.
Dealing with odors
In general, an adsorbent is the best way to remove unwanted odors. For some, like ammonia/amines or thiols, chemical neutralization may be required.
Use of ventilation, like a fume hood, is necessary to remove harmful odors. Opening the window also helps a lot, provided the source of odor may be removed by air currents.
The ability to sense certain odors is very important when working with certain chemical compounds. A classical case is hydrogen sulfide. At low concentrations, the compound has a very unpleasant odor of rotten eggs, and usually inhaling the gas at such low concentrations for short periods of time is not harmful. However, at high concentrations or continuous exposure, the gas is perceived as odorless, and at high concentrations the gas is lethal. If anyone working with the gas, suddenly stops sensing it, that person must leave the chamber or area immediately and seek medical help, as it's impossible to determine the amount of H2S that he/she was exposed to.