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We have smell receptors all over our bodies, not only in our noses, but nobody is quite sure why. By Emma Young.

Rub sandalwood oil onto your hands, and your skin will do more than soften – it will ‘sniff’ the sandalwood scent.

While we all know there are smell receptors in our noses, recent research has revealed that exactly the same receptors are present in all kinds of organs, including the heart, lungs and testes, as well as skin. It might seem astounding, but to the researchers it makes sense: the job of smell receptors is to detect chemicals in the environment. We want to be able to sniff out the chemicals given off by potential food or even mates, and potential threats, like rotten meat, and our nose is perfectly physically positioned to do this. But the organs inside us can also use the same receptor system to detect a variety of compounds. These so-called extra-nasal olfactory receptors aren’t wired up to the brain, unlike those in the nose. They act locally, sparking a response inside the tissue when they’re triggered.

The sandalwood research was led by Hanns Hatt at Ruhr University in Bochum, Germany, and published in 2014. Hatt’s team reported that a synthetic version of sandalwood oil (called Sandalore) can activate odour receptors in human skin, triggering the production of new cells and speeding the healing of minor skin wounds.

Back in 2003, they produced some of the first good evidence that smell receptors outside the nose actually do anything useful: they showed that sperm use an odour receptor to follow a chemical trail, an ability that presumably helps them to locate a fertilised egg.

In 2009, Jennifer Pluznick at Johns Hopkins Medicine, in Baltimore, found smell receptors in the kidneys of mice. It seems signals detected by these receptors help the kidney to regulate levels of key compounds inside the body. The kidney is “sniffing the urine as it goes by”, says Pluznick.

Then, in 2014, a team based at Washington University in St Louis found odour receptors on cells inside the human lung. These receptors can detect noxious chemicals in the air we inhale, causing our airways to narrow, minimising any damage.

A little later that year, the sandalwood research secured headlines around the world. This study is intriguing, says Joel Mainland, a scientist at the Monell Chemical Senses Center in Philadelphia. But he has a lot of questions. The work showed that only a few types of sandalwood could activate these receptors, so it seems to be a very specific response. Yet it’s highly unlikely that the receptors exist in the skin to detect sandalwood. So what do they normally detect? And what might stimulating them do, aside from perhaps help wound healing?

While the German team focuses on the potential benefits, Mainland is more cautious: “The paper shows an increased proliferation of keratinocytes. But it’s a double-edged sword to have increased proliferation: it’s great for wound healing, but terrible for cancer. The fact these receptors are expressed in skin and seem to be doing something is fascinating – but where does it go from there?” 

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