The Smell Survey, comprised of a questionnaire and a set of six microencapsulated odorants, was distributed worldwide to 10.7 million members of the National Geographic Society. Of the 1.5 million surveys which were returned, the results of nearly 1.2 million United States respondents between the ages of 10 and 90 provide the basis for the conclusions herein discussed. Respondents characterized the odors, namely, androstenone (sweat), isomyl acetate (banana), galaxolide (musk), eugenol (cloves), mercaptans (natural gas warning agent), and a synthetic rose scent, according to a set of category descriptors. The huge sample size and broad range provided data which demonstrates continuous trends according to age measured in decades of life (Gilbert and Wysocki, 1987).
Wysocki and Gilbert (1989) discuss in great detail the results of the survey, but for our purpose, let it be noted that an analysis of the data showed a decline of odor perception with age for each of the six odors, though the loss of the sensibility was unequal across the odors. Generally, the intensity of the odors was perceived to decline with age, supporting previously-documented studies of suprathreshold odors involving elderly participants (Van Toller and Dodd, 1987). In addition to declining smell perception, aged participants demonstrated declining smell identification capability. These differences may not be solely due to sensory perception, as Wysocki and Pelchat (1993) point out, but to cognitive perceptions as well, including skills such as active encoding, retrieval and use of labels (Schemper, 1981).
Insert graphs WYC and GIB p 17, 19
It is interesting to note that younger participants rated themselves as having better smell sensibility than did older participants. However, those participants who performed very poorly on the survey, indicating poor olfaction, considered themselves to have average to better olfaction.
Insert graphs WYC and GIB p 16
If there does exist a general decline of smell sensibility with age, it may be logical to investigate an anatomic or physiological deterioration or change which may occur in the olfactory system over time. As has been studied by Liss and Gomez (1958), senile changes may be observed to correspond with degenerative changes in the central nervous system, including extensive degeneration of the olfactory epithelium with subsequent degeneration of the olfactory bulb. Correlations have been made between the similarities of amyloid bodies seen in the olfactory bulb and nerve fiber degeneration in the optic nerve. As the causes of the morphological changes in aging olfactory bulbs are unknown, the evidence is only circumstantial that regressive bulbs may be involved in inducing diminished smell perception (Doty et al., 1984). With age there is noted, in addition to atropy of olfactory receptors, increased exposure to olfactory-unfriendly chemicals in the environment, and increased probability of complications arising from other age-related diseases and from their medications (Bhatnagar et al., 1987).
Unlike studies dealing with the other senses, olfaction studies cannot directly measure smell perception, but must rely instead on indirect methods such as magnitude estimation ("How strongly did you smell the odor?"), threshold determinations (i.e.: at what strength was the odor perceptible), and association judgements ("What did the odor smell like?"). As there exist various and sundry other factors which may take a role in the perception of smells, including psychological, physiological, and experiential factors, it is impossible to correlate a direct relationship between any one factor, such as age, and smell sensibility. Factors which may contribute to age-related sensory decline are presumed to be not singular, and therefore potentially interacting, and probably complex. That is to say, there does not exist evidence to support the contention that age-related deficiency in olfactory sensation occurs uniformly with age, across odors and concentrations. The effects of aging with regard to the sense of olfaction are heterogeneous given the evidence which has heretofore been collected, albeit a general decline of sensory perception and identification is noteworthy with age, as the results of the Smell Survey clearly indicate.
Doty, R.L. , P. Shaman, S.L. Applebaum, R. Giberson, L. Sikorski & L. Rosenberg, Science 226: 1441-1443, 1984.
Gibbons, B. 1986. The Intimate Sense of Smell/National Geographic Smell Survey, National Geographic 170:324-361.
Gilbert, A.N. and C.J. Wysocki. 1987. The Smell Survey Results, National Geographic 122:514- 525.
Liss, L. and Gomez, F. 1958. The nature of senile changes of the human olfactory bulb and tract, Arch. Otolaryngol. (Stockh) 67:167.
Schemper, T., Voss, S. and Cain, W.S. 1981. Odor identification in young and elderly persons: sensory and cognitive limitations, J. Gerontol., 36:446, 1981.
Van Toller, S. and G.H. Dodd. 1987. Presbyosmia and olfactory compensation for the elderly, Br. J. Clin. Pract. 41:725.
Wysocki, C.J. and A.N. Gilbert. 1989. The National Geographic Smell Survey: Effects of age are heterogenous. Annals of the New York Academy of Sciences, 561:12-28.
Wysocki, C.J., and Pelchat, M.L.. 1993. The Effects of Aging on the Human Sense of Smell and Its Relationship to Food Choice, in Critical Reviews in Food Science and Nutrition, 33(1):63-82.