The vocabulary of odour is rich with names of substances that elicit a great variety of olfactory qualities. One of the best-known published psychological attempts at classification was in 1916 on the basis of more than 400 different scents on human subjects. On the basis of the apparent similarities of perceived odour quality or confusions in naming, it was concluded that there were six main odour qualities: fruity, flowery, resinous, spicy, foul, and burned.

Electrical activity can be detected with fine insulated wires inserted into the olfactory bulb. Portions of the olfactory bulb toward the anterior or oral region in the rabbit are found to be more sensitive to water-soluble substances, whereas the more posterior parts of the olfactory bulb are more sensitive to fat-soluble substances. In addition, when very fine electrodes are used, individual cells (mitral cells) are sensitive to different groups of chemicals. Evidence for the existence of only a few primary receptors, however, does not emerge from such studies; a variety of different combinations of sensitivity has been found. Similarly, recordings from the primary receptor nerve fibres reveal different patterns of sensitivity. Electrical recording of this type also shows that olfactory sensitivity can be enhanced by a painful stimulus, such as a pinch on the foot. This appears to be a reflex that serves to enhance the detection of dangerous stimuli in the environment. Different parts of the olfactory neural pathways seem to be selectively tuned to discriminate different classes of olfactory information. For example, the third- and fourth-order olfactory neurons found beyond the olfactory bulb of the rat seem particularly concerned with distinguishing the odour of sexually receptive females. These neurons appear to be especially important in the preference the male rat shows for the smell of urine from the female in heat.

Odourous substances

To be odorous, a substance must be sufficiently volatile for its molecules to be given off and carried into the nostrils by air currents. The solubility of the substance also seems to play a role; chemicals that are soluble in water or fat tend to be strong odorants. No unique chemical or physical property that can be said to elicit the experience of odour has yet been discovered.

Only seven of the chemical elements are odorous: fluorine, chlorine, bromine, iodine, oxygen (as ozone), phosphorus, and arsenic. Most odorous substances are organic (carbon-containing) compounds in which both the arrangement of atoms within the molecule as well as the particular chemical groups that comprise the molecule influence odour. Stereoisomers (i.e., different spatial arrangements of the same molecular components) may have different odours. On the other hand, a series of different molecules that derive from benzene all have a similar odour. It is of historic interest that the first benzene derivatives studied by chemists were found in pleasant-smelling substances from plants (such as oil of wintergreen or oil of anise), and so the entire class of compounds was labelled aromatic. Subsequently, other so-called aromatic compounds were identified that have less-attractive odours.

The scent of flowers and roots (such as ginger) depends upon the presence of minute quantities of highly odorous essential oils. Although the major odour constituents can be identified by chemical analysis, some botanical essences are so complex that their odours can be duplicated only by adding them in small amounts to synthetic formulations.

Odour sensitivity

In spite of the relative inaccessibility of the olfactory receptor cells, odour stimuli can be detected at extremely low concentrations. Olfaction is said to be 10,000 times more sensitive than taste. A threshold value for the odorant ethyl mercaptan (found in rotten meat) has been cited in the range of 1/400,000,000th of a milligram per litre of air. A just-noticeable difference in odour intensity may be apparent when there is a 20 percent increase in odorant strength, but at low concentrations as much as a 100 percent increase in concentration may be required. Temperature influences the strength of an odour by affecting the volatility and therefore the emission of odorous particles from the source; humidity also affects odour for the same reasons. Hunting dogs can follow a spoor (odour trail) most easily when high humidity retards evaporation and dissipation of the odour. Perfumes contain chemicals called fixatives, added to retard evaporation of the more volatile constituents. The temporary anosmia (absence of sense of smell) following colds may be complete or partial; in the latter case, only the odours of certain substances are affected. Paranosmia (change in perceived odour quality) also may occur during respiratory infections. Changes in sensitivity are reported to occur in women during the menstrual cycle, particularly in regard to certain odorants (steroids) related to sex hormones. Olfactory sensitivity also is said to become more acute during hunger.

Adaptation to odours is so striking that the stench of a junkyard or chemical laboratory ceases to be a nuisance after a few minutes have passed. Olfactory adaptation, as measured by a rise in threshold, is especially pronounced for stronger odours. Cross adaptation (between different odours) may take place; thus, eucalyptus oil may be difficult to detect after one becomes adapted to the smell of camphor. Adaptation was long regarded solely as the result of changes in the olfactory receptor; however, the receptor cells in the nose seem to adapt only partially. Rhythmic discharges continue in the olfactory bulb long after one ceases to detect an odour. Apparently, some olfactory adaptation may occur in the brain as well as in the sense organ.

Effects on behaviour

Mammals in the wild state appear to utilize their odour glands for sexual attraction. Rats show a preference for the branch of a maze that has been scented with the odour of a sexually receptive female. It is likely that some rudiments of these effects operate in humans. The most sexually provocative perfumes have a high proportion of musk or a musklike odour. Genuine musk is derived from the sexual glands of the musk deer and is chemically related to human sex hormones; odour sensitivity in humans varies with the menstrual cycle.

Among laboratory animals the secretion of reproductive hormones can be markedly influenced by odour stimulation. This seems to be an innate physiological process rather than the result of learning. When the odour of a strange male is presented to a recently mated female, pregnancy block occurs. The normal hormonal changes following copulation are blocked under these conditions, and the fertilized egg fails to survive. A related study of the periodicity and length of the menstrual cycle in women exposed to the normal odours of men suggests that there may be similar effects among humans. Human behaviour, though it is molded and shaped by custom and culture, has many of its roots in basic sensual appetites.