An odorant is a substance capable of eliciting an olfactory response whereas odor is the sensation resulting from stimulation of the olfactory organs. Odors play an important part in our everyday life, from appetite stimulation to serving as warning signals for disease detection. A number of diseases have characteristic odors including gangrene, diabetes, leukemia and schizophrenia. Odors have been implicated in depression and nausea as well.
Detectable odors can have a significant impact on people by affecting moods, as well as having physiological impacts on the olfactory system. People associate odors with past experiences and, from those experiences, involuntarily assess the odor as likable, dislikable or indifferent. Effects on individuals, however, vary from one person to another.
Odor threshold is a term used to identify the concentration at which animals respond 50% of the time to repeated presentations of an odorant. This term is reserved, primarily, for use in research with animals. Most often, however, odor threshold is used to mean detection threshold, which identifies the concentration at which 50% of a human panel can identify the presence of an odor or odorant without characterizing the stimulus. Detection threshold is the term most frequently used when discussing odor research results associated with livestock operations.
The recognition threshold is the concentration at which 50% of the human panel can identify the odorant or odor, such as the smell of ammonia or peppermint.
Although the detection threshold concentrations of substances that evoke a smell are slight, a concentration only 10 to 50 times above the detection threshold value often is the maximum intensity that can be detected by humans. This, however, is in contrast to other sensory systems where maximum intensities are many more multiples of threshold intensities. The maximum intensity of sight, for instance, is about 500,000 times that of the threshold intensity and a factor of 1 trillion is observed for hearing. For this reason, smell often identifies the presence or absence of odor rather than quantifies its intensity or concentration.
Odor adaptation is the process by which one becomes accustomed to an odor. The adaptation time needed is greater when more than one odor is present. When adaptation occurs, the detection threshold increases. The detection threshold limits change faster when an odor of high, rather than low, intensity is presented. Besides, adaptation occurs differently for each odor. Odor fatigue occurs when total adaptation to a particular odor has occurred through prolonged exposure. This situation would apply to milkers or dairy managers who are exposed to the smell of dairy manure on a daily basis and appear virtually unaware of the odor.