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Bonnie Blodgett never stopped to appreciate her sense of smell – until she lost it.

When she did, unintentionally wiping out her olfactory receptors with a now discontinued homeopathic cold remedy, it turned her world upside down. Suddenly, the St. Paul, Minn., gardening expert and writer could no longer enjoy the loamy scent of soil, the mouth-watering aroma of freshly baked cookies, and the invigorating fragrance of a walk in the woods. Nor could she detect unpleasant odours, like spoiled milk or a pan left burning on the stove.

With an inability to smell, a rare condition called anosmia, she felt cut off from other people, as though she was "in prison, emotionally," Blodgett explains in an interview.

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"When you can't smell those things, you feel lost, you feel disconnected, you feel sad," she says. "But also the fact that you can't share those with other people makes it even worse."

Smell is arguably the most underrated of the senses. Most of us don't think much about our ability to smell until we catch a nose-clogging cold or are hit with overwhelming odours. And even then, we rarely reflect on its impact on our lives. But lately, scientists have been charting new territory in the realm of olfaction, exploring how our ability to process scents can affect our mental health, our ability to communicate emotions, and the way we perceive the world around us. Smell is proving more important than we thought.

In recent years, researchers have associated olfactory dysfunction with a number of neurological disorders, including Alzheimer's, Parkinson's and schizophrenia. A new study from Australia's Macquarie University, released this fall, also found a link between olfactory impairment and psychopathology. Participants with strong psychopathic traits had difficulty identifying and distinguishing the differences between smells.

What's more, scientists in the Netherlands believe humans can actually smell certain emotions. In a study published earlier this month, they found participants who inhaled sweat samples taken while donors were in states of fear or disgust also made corresponding facial expressions of fear or disgust, even though the scents were barely noticeable.

One of the authors, professor Gun Semin of Utrecht University, says the findings suggest smell plays a role in the way we communicate emotions. He explains his research may have practical applications, such as reducing anxiety in patients undergoing magnetic resonance imaging (MRI) tests by getting rid of the chemosignals of fear produced by previous patients. "If what we're saying is correct," he says, "then the odour [others] leave behind is actually going to amplify the fear they're going to have."

Yet much about olfaction remains a mystery. Blodgett learned this firsthand while struggling to understand her anosmia, which affects an estimated 5 per cent of people. She wrote a book about her ordeal, titled Remembering Smell: A Memoir of Losing – and Discovering – the Primal Sense.

Instead of immediately losing her perception of smell, Blodgett initially suffered odour hallucinations, or phantosmia, a condition similar to phantom limb pain described in celebrated neurologist Oliver Sacks's new book, Hallucinations. Blodgett's phantom smells were horrible, noxious odours. "Really the best description is everything smelled of death … rotting smells of all kinds," she says.

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While doctors were unable to explain the phenomenon, Blodgett wonders whether her phantosmia was some kind of distress signal. "Maybe … my brain was trying to protect me from things that might kill me, so [it decided,] 'Let's just bombard her with all the stinky things that might kill her.' "

But just as inexplicably, the phantosmia eventually disappeared, and for nearly a year, Blodgett was unable to detect any odours at all, until mercifully, in 2006, she gradually began regaining her sense of smell. She now is able to smell everything she once missed.

Debra Fadool, professor of biology and neuroscience at Florida State University, explains that neurons in the olfactory system, unlike any other part of the brain, continually regenerate. "So the nose that we have when we're three years old and we open that jar of Play-Doh and we smell it, is completely different when we open that jar of Play-Doh when we're 30 years old," she says, noting that people may lose their sense of smell as they age, partly due to changes in the rate their neurons regenerate.

But if losing one's sense of smell can be a curse, so too can smelling too much. Just ask Bree Davies.

Although she has never been diagnosed as such, Davies describes herself as a super-smeller. Even the faintest whiff of certain odours, like baby powder, can turn her stomach, and she can identify smells, like food in the fridge going bad, before any of her roommates notice.

Her perception of odours is so strong that it limits where she travels, which restaurants she visits and even whom she befriends.

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"I won't get close to certain people because they smell weird," says Davies, a Denver, Colo.-based freelance writer. "It definitely rules my life in a strange way."

But while there is plenty of anecdotal evidence that certain people are super-smellers, general hyperosmia, or having a heightened sense of smell, is difficult to prove, says Jelena Djordjevic, a neuropsychologist at the Montreal Neurological Institute and Hospital. Even perfumers and wine tasters typically do not possess a different threshold for detecting odours, she says. They are merely better at recognizing and picking out different smells.

However, Djordjevic notes, there are certain conditions, like migraines and pregnancy, that are linked with olfactory hypersensitivity, which is characterized by a heightened responsiveness to odours, rather than actually having extraordinary smelling abilities. People with olfactory hypersensitivity often perceive distress when exposed to odours at concentrations that others do not seem to mind.

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