01:00 AM EST on Monday, November 22, 2004

There are few sensations available to mere mortals that can surpass the spontaneous pleasures aroused by the aroma of freshly brewed morning coffee, the ozone in the atmosphere during the initial phases of a thunderstorm, or an attar of roses in the early evening. Brief encounters with certain smells give punctuation and rapture to human existence. Yet, of the handful of senses available to man, smelling is now considered the least important; and indeed, in the evolution of animal life, the capacity to smell has diminished to the point where, in man, its relative absence is no longer life-threatening.

Physiologists tell us that the human receptors for smell can detect only about a dozen primary odors, and that the panoply of aromas that man can distinguish therefore represents combinations of these primary aromas, augmented by such other senses as vision and taste, while aided by one's imagination.

The phenomenon of smelling, in man, represents little more that some volatile molecules leaving their source, carried passively in the atmosphere to arrive eventually upon a small zone, about a square inch, in the upper nasal passages, where they react chemically with a network of neural receptors. Any hindrance in this olfactory zone -- such as when a human develops a "stuffy nose" accompanying an upper-respiratory infection -- renders the person temporarily anosmic; that is, incapable of smelling. This may sometimes be disagreeable, since the flavor of many foods depends upon its accompanying aroma. But anosmia is certainly not a mortal impairment. And given the amount of smog and other disagreeable components in the air, anosmia might not constitute a material loss for some people.

Smelling is one of the first receptive capabilities to develop in primitive one-cell creatures. Certain molecules, dissolved in the surrounding waters, "inform" these organisms that danger lurks or, alternatively, that there is a nearby source of food. Thus there are some molecules that provoke a reflex retreat, and some that prompt attraction. Organisms such as the amoeba will then swim toward or away from these sources, their direction determined by the relative aqueous concentrations of the inciting molecules.

Certain fish -- sharks, for example -- possess an astonishing capacity to detect particular organic molecules dissolved in the surrounding sea water. A shark can detect sources of food many miles distant. The brain of the shark is little more than a huge neural center devoted largely to the reception, integration and translation of chemical sensations impinging upon the shark's nose. In essence, a shark's nervous system consists of a group of highly sensitive nerve receptors transducing the impulses into electrical messages to the motor nerves that control the creature's motion toward its prey. There is no intermediate "thinking brain" to determine the merit of such an attack.

There is, incidentally, little distinction in fish between smelling and tasting. Each sense represents a specific neural response to a concentration of molecules, either volatilized into the atmosphere or dissolved in the ambient waters. The separation of the two senses becomes more apparent in land-dwelling creatures, although overlaps persist.

To humans who can barely distinguish between two wines, the nasal abilities of lower animals seem truly amazing. The salmon, for example, can "remember" the path from the deep oceans, over thousands of miles, back to the stream where it was born; the pig can locate and dig out the forest truffles; the bloodhound can follow a days-old circuitous path of a specific human; the male Chinese silkworm moth (Bombyx mori) is drawn by the scent of a female moth over six miles away; and many of the animals of the forest can detect the distant odor of humans, prompting the animals to flee.

For humans, especially city dwellers, smelling the atmosphere is now a voluntary undertaking; it is no longer an essential, life-sustaining activity. Indeed, Western society now regards most natural smells as unpleasant, and accordingly makes every effort to neutralize them or replace them with perfumes. Say, "I detect an odor," and most people will anticipate a dead animal, human sweat, or the residue of some unseemly bodily function, rather than the essence of lilies or the aroma of an infant.

Air-conditioning machines have vigilant filters and many a home boasts electronic air purifiers. Body deodorants, breath fresheners, and mouthwashes are commonplace in the lavatories of most Americans. Odor, per se, is now considered bad. Roy Bedichek, in his text The Sense of Smell, wryly observes that we may compliment a woman by declaring that she is a lady of taste -- but never a lady of smell.

Despite the campaign to regard virtually all smells as deserving to be masked, the appreciation of life's aromas is still acknowledged by two groups: poets and neurologists.

Poets frequently invoke the sensations of smell in their literary labors. For example, a casual reading of the Bible's Song of Songs emphasizes how much smells were integrated in the less martial of the human passions. Hardly a stanza passes without some smell's being mentioned as a metaphor for or an accompaniment to human love.

And then there are the smells that do not exist in the real world. Neurologists may encounter distortions in the detection of smells -- sometimes called olfactory hallucinations -- in a small number of people with epilepsy. These curious episodes are sometimes preludes to a motor seizure. The patient will frequently describe the smell as intensely disagreeable. Such smells, however, remain in the imagination of the patient, representing an abnormal stimulation of those parts of the inner brain where odors are perceived.

In the distant past, man survived by depending heavily upon his capacity to detect, by smelling, hints of danger carried by the winds. The modern urban dweller has gradually discarded these primitive skills in odor detection, relying increasingly on his vision, hearing, extracorporeal electronic devices and the newspaper to inform him about his environment.

Stanley M. Aronson, M.D., a weekly contributor, is dean of medicine emeritus at Brown University.