Before today, if someone said, "did you know your hair smells?", you might well have taken offence.
In fact, your hair – and your skin – are packed with olfactory receptors, the same sorts of cells found in our noses, which enable us to smell the world around us.
New research published in the journal Nature shows that these cells react to certain smells in ways that can have a fascinating impact on the body, potentially offering a new treatment for hair loss.
Humans have approximately 350 different types of olfactory receptors in their noses. When we smell something, it's the result of odour molecules floating around in the air triggering these receptors, which send a signal to our brain – we process and interpret that signal as a "smell".
These receptors have also been found in other parts of our body, including the prostate, intestine, kidneys and in keratinocytes - cells that form the outermost layer of skin.
Our skin has multiple functions. As a barrier, it protects the body against water loss and pathogens (and helps keep our insides inside). It also senses information about our environment such as temperature, humidity and touch.
Through these olfactory receptors, our skin also seems to be able to "smell" odours - this is thought to help our body to sense chemicals in our environment, a process called chemosensation.
Previous research has found that OR2AT4, an olfactory receptor found on the skin, can be stimulated by a particular odour molecule - a synthetic sandalwood smell commonly found in home products like incense sticks and air fresheners.
This stimulation, essentially by exposing the skin to the smell, has been shown to help to heal wounds more quickly. The smell triggers a calcium-dependent signal pathway, which increases the migration and proliferation of skin cells.
This same smell receptor is also found on the scalp, and this week researchers discovered that one smell in particular stimulates hair growth - a sniff of hope to those who might be thinning on top, perhaps.
Taking samples of human scalp donated by patients undergoing facelifts, the researchers exposed the tissue to different smells for six days. They found that when the tissue was exposed to the synthetic sandalwood smell its levels of keratin began to increase, and the natural death of cells involved in hair formation was delayed. They also found a 25 to 30 per cent increase in the secretion of a growth factor hormone in the scalp, a hormone which is known to play a key role in promoting hair growth.
To test the theory that this hair growth was the effect of the OR2AT4 receptor, the team also exposed the tissue to a rose-like smell known as Phenirat, known to block the OR2AT4 receptor.
They found that when the hair tissue was exposed to Phenirat, even if the sandalwood smell was still present, the growth hormone levels in the scalp tissue significantly decreased and the rate of cell death in the hair follicles increased.
The research shows for the first time that the growth of human hair might be regulated by a simple smell, which could help hair to have a longer growing phase.
Before we all rush out to buy a home's worth of aromatic diffusers though, it would be worth waiting for the results from a clinical trial currently underway to see if the results are transferable from donated scalp to live human hair.
If the results do transfer, though, the sweet smell of sandalwood might eventually replace current hair loss treatments - the combined benefit of fresh air and fresh hair.