How does smell work?
If you’ve ever stopped to smell the roses, a cocktail of hundreds of different odour molecules will have wafted through the air and up your nostrils. At the top of your nose, the molecules bind to special smell receptors on the surface of nerve cells, which send a signal to the brain’s olfactory bulb, located just behind the bridge of the nose. Humans have around 400 different smell receptor types, and one odour molecule may bind to many of them. Together, the odour molecules create a pattern of activation in the nerve cells that our brain interprets as ‘a smell’.
Why do we like certain smells but not others?
Smells can warn us about danger – we’re repelled by the smell of sewage and rotting food, for example. But according to smell expert Dr Andreas Keller from Rockefeller University, it’s difficult to tell whether we’re born with these preferences or whether they’re learned. Context plays a big role. Butyric acid is a chemical that contributes to the smell of both Parmesan cheese and vomit, so it may smell repulsive or appetising, depending on the situation. The answer is also partly down to our DNA – the genes that code for our smell receptors can vary between people, so we don’t all respond to odour molecules in the same way.
What happens when we lose our sense of smell?
We all have at least one genetic ‘blind spot’ in our sense of smell, which means we simply can’t detect certain odour molecules. One famous example is how only some people notice a strong odour in their pee after eating asparagus. Anosmia – a complete loss of smell – can occur after a cold, sinus infection or even a bump to the head. Anosmia impacts the flavour of food, but goes far beyond that – one anosmiac has described it as like “living behind a pane of glass”. Luckily, anosmia isn’t always permanent, and may recover naturally or through exercises like ‘smell training’, which uses distinctive smells such as essential oils to re-stimulate the olfactory system.
Anand Jagatia is the presenter of Do You Smell What I Smell?– an episode of CrowdScience that can be streamed at http://www.bbc.co.uk/programmes/w3cswvxk