The ability to perceive an odor varies widely among individuals. More than a thousand-fold difference between the least and the most sensitive individuals in acuity have been observed.
Differences between individuals are, in part, attributable to age, smoking habits, gender, nasal allergies or head colds. Non-smokers over the age of 15 show greater acuity than smokers in general. Furthermore, females tend to have a keener sense of smell than males, a finding that has been substantiated in recent work at Iowa State University. Generally, the olfactory sensory nerves atrophy from the time of birth to the extent that only 82% of the acuity remains at the age of 20; 38% at the age of 60 and 28% at the age of 80. Consequently, olfactory acuity and like or dislike of an odor decrease with age. Infants appear to like all classes of odorous materials, perhaps because of the lack previous experience and because of their innate curiosity. In studies, children younger than five years old rated sweat and feces as pleasant but above that age, as unpleasant. Like and dislike of a particular odor can change with odor concentration or intensity.
Generally, humans can distinguish between more than 5,000 odors but some individuals experience anosmia (smell blindness) for one or more odors. In this situation, the individual apparently has a normal sense of smell, but is unable to detect one particular odor regardless of its intensity. For example, because methyl mercaptan has an odor recognition threshold of only 0.0021 parts per million (ppm), it is often mixed with natural gas as an indicator of leaks; however, approximately one in 1,000 people is unable to detect the strong odor of this mercaptan. An estimated 30% of the elderly have lost the ability to perceive the minute amount of this mercaptan used in natural gas.
An estimated 100 million receptor cells are present in humans. For a substance to be detected as an odor by the receptor cells, several criteria must be met:
1) the substance must be volatile enough to permeate the air near the sensory area;
2) the substance must be at least slightly water-soluble to pass through the mucous layer and to the olfactory cells;
3) the substance must be lipid-soluble because olfactory cilia are composed primarily of lipid material; and finally,
4) a minimum number of odorous particles must be in contact with the receptors for a minimum length of time.