Olfactory acuity in the population follows a normal bell curve distribution of sensitivity, ranging from hypersensitive to insensitive and anosmic (unable to smell). Individual odor threshold scores can be distributed around the mean value to several orders of magnitude.
Olfactory studies have revealed interesting information regarding odor perception. Two sensory channels are responsible for human detection of inhaled chemical substances in the environment. These are odor perception and the common chemical sense (CCS). Odor perception is a function of cranial nerve I, whereas the CCS is a function of cranial nerve V. The CCS is described as pungency and associated sensations such as irritation, prickliness, burning, tingling, and stinging. Unlike the olfactory structure, the CCS lacks morphological receptor structures and is not restricted to the nose or oral cavity (Cometto-Muniz and Cain 1991).
Human responses to odor perception follow patterns associated with both the olfactory and the CCS functions. These are discussed next to facilitate the understanding of what prompts odor complaints and the difficulties associated with odor identification and measurement.
SENSITIZATION, DESENSITIZATION, AND TOLERANCE OF ODORS
Repeated exposure to an odor can result in either an enhanced reaction described as sensitization or a diminishing reaction defined as tolerance. When people become sensitized to an odor, the complaints regarding the odor can increase. On the other hand, tolerance of an odor can attribute to a person's unawareness of the continuous exposure to a potentially harmful substance. As an illustration of the complexity of odor perception, pungency stimulation shows an increased response to odors from a continuous or quickly repetitive stimulus during a short term exposure. Also, a person can be more sensitive to one odorant than another. Such differences are often caused by repeated exposure to an odor. For example, tolerance is not uncommon for chemists or manufacturers who are exposed to an odorant daily over a period of years. Fatigue also affects odor perception; repeated exposure to an odorant can result in a desensitization to the odorant, where an observer can no longer detect an odor although it is strongly detectable by another.
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