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The sense of smell is a useful tool in crime detection

Posted in Biology on Monday, 30 April 2012

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This edited article about the sense of smell originally appeared in Look and Learn issue number 697 published on 24 May 1975.

Scent bottles, picture, image, illustration

Bottles of perfume

When anyone talks about a “keen sense of smell” we often think of dogs in general – and bloodhounds in particular.

Yet in spite of the fact that smell is the weakest of our five senses, human noses can detect very small traces of certain chemical odours, even if they are in the air with a concentration of less than one part in a million.

Our ability to smell begins with the nostrils. These lead into what is called the “nasal cavity” which is lined, at its top part, with thousands of olfactory (smell-detecting) cells. From these cells nerve fibres lead to the brain.

When air is breathed into the nasal cavity, any gases, dust particles or traces of vapour in the air irritate the sensitive olfactory cells, which then send “odour messages” to the brain.

Many substances, of course, do not affect the olfactory cells so that we cannot smell them. Water vapour and air itself are good examples.

Because these don’t cause odour messages to be sent to the brain, we simply say they are “odourless” or don’t smell.

In general, we tend to divide odours into two kinds – pleasant and unpleasant. Most of us agree that the smell of a rose is pleasant, but that the odour of decaying food is not.

But sometimes it can be a matter of opinion – some people get quite ill when they smell lavender-scented soap while others like the smell of ripe gorgonzola cheese!

Most things have a characteristic odour because of some chemical substance they contain. It is this chemical which acts upon the olfactory cells in the nasal cavity and produces the “smell” sensation.

The familiar odour of bananas, for example, is caused by a pale yellow liquid chemical called amyl acetate which is present in the fruit.

The bad smells from decaying food are caused by certain gases, like phosgene, from decaying fish, and hydrogen sulphide from rotten eggs.

Our olfactory cells are really sensitive chemical laboratories which analyse separate smells.

The most pleasant odours are often a mixture of many chemical substances. Modern perfumes can contain more than two hundred chemicals, all expertly blended together.

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