The human sense of smell
Although the human sense of smell is feeble compared to that of many animals, it is still very acute. We can recognise thousandsof different smells, and we are able to detect odours even ininfinitesimal quantities.
Our smelling function is carried out by two small odourdetecting patches - made up of about five or six million yellowish cells - high up in the nasal passages.
For comparison, a rabbit has 100 million of these olfactory receptors, and a dog 220 million. Humans are nonethelesscapable of detecting certain substances in dilutions of less than one part in several billion parts of air. We may not be able to match the olfactory feats of bloodhounds, but we can, for example, ëtrackí a trail of invisible human footprints across clean blotting paper.
The human nose is in fact the main organ of taste as well as smell. The so-called taste-buds on our tongues can only distinguish four qualities - sweet, sour, bitter and salt -all other ëtastesí are detected by the olfactory receptors high up in our nasal passages
V a r i a t i o n s
Our smelling ability increases to reach a plateau at about the age of eight, and declines in old age. Some researchers claim that our smell-sensitivity begins to deteriorate long before old age, perhaps even from the early 20s. One experiment claims to indicate a decline in sensitivity to specific odours from the age of 15! But other scientists report that smelling ability depends on the personís state of mental and physical health, with some very healthy 80-year-olds having the same olfactory prowess as young adults.
Women consistently out-perform men on all tests of smelling ability (see Sex differences, below). Schizophrenics, depressives, migraine sufferers and very-lowweight anorexics often experience olfactory deficits or
dysfunctions. One group of researchers claims that certain psychiatric disorders are so closely linked to specific olfactory deficits that smell-tests should be part of diagnostic procedures. Zinc supplements have been shown to be successful in treating some smell and taste disorders.
Although smoking does not always affect scores on smell-tests, it is widely believed to reduce sensitivity.A recent study at the University of Pennsylvania suggests that, contrary to popular belief, blind people do not necessarily have a keener sense of smell than sighted people. In their experiments on blind and sighted people, the top performers on most tests were (sighted) employees of the Philadelphia Water Department who had been trained to serve on the Departmentís water quality evaluation panel. The researchers conclude that training is the factor most likely to enhance performance on smell tests. (University of Pennsylvania researchers are probably fairly clued-up on this subject - they designed the University of
Pennsylvania Smell Identification Test (UPSIT) which is the standard test used in almost all experiments.)
The importance of ëtrainingí in the development of smellsensitivity
is confirmed by many other studies. Indeed, this factor can sometimes be a problem for researchers, as subjects in repetitive experiments become increasingly skilled at detecting the odours involved.

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