The Science of Smell

03/02/2015 09:22 GMT | Updated 31/03/2015 10:59 BST

For thousands of years, we have used scents to mark special occasions, to commiserate the dead, to commune with spirits, to make us laugh and cry and more recently, discover cancers lurking inside us.

Our sense of smell is more closely linked to memory and our past than any other, and can provoke strong emotional reactions that bring us closer together, but can also push us apart. Welcome to the science of smell.

While the animal kingdom may have a long list of creatures equipped with better noses than us, our olfactory organ is still a powerful thing. In a recent study carried out in New York, it was discovered that people are capable of identifying roughly 1 trillion scents.

Each day, we breathe around 23,040 times and inhale and exhale about 438 cubic feet of air, explains Diane Ackerman in her book, 'A Natural History of the Senses'. "It takes approximately five seconds for us to complete one breath, but in that time, molecules of odour pour into and out of our bodies."

When we hold something smelly up to our noses, odour molecules float to the back of the nasal cavity behind the bridge of the nose, where they are absorbed by the mucosa containing receptor cells bearing microscopic hairs called cilia.

Five million of these cells fire impulses to the brain's olfactory bulb or smell centre. There, we break down smells in into ten distinct categories, says Professor Jason Castro, of Bates College in America who has developed a computerised technique to whittle down smells to their most basic essence.

They are, fragrant, woody/resinous, fruity (non-citrus), chemical, minty/peppermint, sweet, popcorn, lemon, pungent, and decayed.

While our brains are very good at distinguishing between smells, perhaps most extraordinary thing about our sense of smell is its connection to memory. Our olfactory nerve is located very close to the amygdala, the area of the brain connected to the experience of emotion. The olfactory nerve's other neighbour meanwhile, is the hippocampus, which is associated with memory and experience. Scientists have found shared pathways between these three centres, which allow us to recall past memories and emotions when we detect a distinct odour.

But not all smells create a sense of nostalgia, and our ability to detect odours can be misleading. According to Thomas, people with body odour cannot register how bad they smell. It's the same principle of why we can't smell our own house, says Pamela Dalton, a cognitive psychologist at Monell Chemical Senses Center in Philadelphia, who has been studying this idea for more than two decades.

When we smell something new for the first time, our odour perceptors start firing signals to our brain, where our brains then decide what to do with the information. But very quickly "the receptors in your nose sort of switch off," Dalton says, and the intensity of the smell starts to fade. That's because your brain has perceived the scent to be nonthreatening, which means there's little need to pay close attention to it.

Rise of the (smelling machines)

But while we're beginning to unpack our own sense of smell, there are other species that are helping to save lives. In 2010, Japanese researchers showed that dogs can detect colorectal cancer from a breath sample, while in 2012 the European Respiratory Journal published research that found dogs could identify lung cancer in breath samples.

Dogs, whose sense of smell can be ten million times more sensitive than our own, are being used in the UK to help doctors detect cancers without the need for expensive screening.

Scientists are so interested in a dog's ability to smell, they are now developing electronic systems (e-noses) that mimic the way dogs detect the smell of cancer. The NaNose, developed by a team of British, American and Israeli scientists uses a breathalyser to detect early stage lung cancer, a disease that is responsible for almost a third of all cancer related deaths.

But machines may sneak in and steal the prize as top sniffer. According to IBM's annual "5 in 5" forecast - a list of technologies it believes will hit the mainstream in five years - machine smelling is being pursued by several big companies.

The technology company's Watson computer has already started crafting its own barbecue sauce using information it has learned about taste and texture. Are machines marching into smell, too?

It's nothing to be sniffed at.