Although the human sense of smell is feeble compared to that of many animals, it is still very acute. We can recognise thousands of different smells, and we are able to detect odours even in infinitesimal quantities.

Our smelling function is carried out by two small odour-detecting patches – made up of about five or six million yellowish cells – high up in the nasal passages.

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 ‘tastes’ are detected by the olfactory receptors high up in our nasal passages.

Variations

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 person’s 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

Although smoking does not always affect scores on smell-tests, it is widely believed to reduce sensitivity.

The importance of ‘training’ in the development of smell-sensitivity 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.

Smell-sensitivity researchers have to be very careful about the odours they use in experiments, because a smell is not always a smell. Many odorous substances activate not only the olfactory system but also the ‘somatosensory’ system -the nerve endings in our noses which are sensitive to temperature, pain etc. This is why ‘anosmics’ – patients who have completely lost their sense of smell – can still detect menthol, phenylethyl alcohol and many other substances. In a study testing anosmics’ ability to perceive odorous substances, it was found that many so-called odours are in fact affecting the pain- and temperature-sensitive nerve-endings, rather than the olfactory receptors. Out of 47 ‘odorous’ substances, anosmics could detect 45. (Only two substances could not be detected by the anosmic patients: these were decanoic acid and vanillin, which affect only the olfactory receptors, and can thus safely be classified as ‘pure’ odours.) Some unpleasant ‘smells’ do more than just annoy or disgust us, they actually cause us pain.

As quoted from http://www.sirc.org/publik/smell_human.html