The film Perfume portrays the world of a man who has an unparalleled, acute sense of smell. But what is life like for the millions of people who have lost it?

Imagine burning the toast unawares, every day. Mowing the lawn without a breath of fresh-cut grass. Biting into an apple - or is that a potato? - as the flavours remain indistinguishable on the palate.

That is day to day life for the thousands of people with anosmia, who lack a sense of smell.

Perfume, the film released this week adapted from the novel by Patrick Suskind, follows a murderer, Jean-Baptiste Grenouille, who has no odour but possesses an unparalleled, acute sense of smell which he uses in his pursuit of power.

It is a story of 18th Century life in France, told in all its stinking glory, through repeated reference to smells - people's odours, street filth, country air, food and musky scents. A time and a world away from life for the 2% of people that researchers say lack the olfactory sense.

The causes range from severe head injuries, to viruses and nasal congestion conditions, to degenerative diseases including Alzheimer's and Parkinson's. Some suffer from anosmia, which means no sense of smell; hyposmia is a decreased ability; and dysosmia is when things smell differently than they should.

Sense of danger

Our sense of smell is an underestimated life sense (albeit the basis of a hugely profitable perfume industry).

We use it to detect chemicals in the air, and its receptors contribute to our ability to distinguish flavour - those unable to smell have very little sense of taste. It plays a role in mental and physical health, emotion, sex and relationships. It helps people guard against bad or off food, and helps warn against dangerous situations.

Petrol was spilling out across the garage where Robert Tambling was working at home in East Sussex. But he was oblivious to the risk until his other half came in and asked: "What's that smell?"

When sight or hearing go, it is immediately obvious. With smell, its absence can take longer to sink in. Robert's went because of nasal polyps - protrusions that blocked the inside of his nose - cutting him off from the kind of world Suskind's anti-hero occupies.

Apart from the most serious practical effect - exposing him to danger - eating has lost its sensual pleasure. And, a keen gardener, he can no longer smell the roses.

"It's a loss that needs to be mourned," he says. "In terms of my quality of life, eating is diminished. I cannot taste herbs, other than mint. Coffee or tea even are not the same experience.

"Of the flowers come out in spring, my favourite, the daphnes, are wonderful, but I haven't been able to smell them for three years."

Others talk of feeling alienated and frustrated, as demonstrated by these contributors to a website for people with the condition:

• "I couldn't smell my children, my husband, or the food I loved to cook - words failed to describe my loss" • "I got this sense that everything around me was fake, that I was on a film set" • "When I was a kid, when people said 'those cookies in the oven smell good', I thought it was a joke that everyone was in on, and I'd say it too"

Where Jean-Baptiste Grenouille is in turn revolted, thrilled and mesmerised by the scents he detects, people without the sense are cut off from their emotions, says Professor Tim Jacob, from the University of Cardiff, who researches smell.

Smell can evoke powerful and comforting memories of childhood, of home or of a loved one. When it goes, so does the trigger that recalls them.

People can become clinically depressed - up to 17% of subjects in one study. "And that's just the tip of the iceberg," he says. "You have lost a whole dimension of your life, which generally you are not aware of, but we constantly sample the atmosphere for smells. For example you go home and if it smells familiar, it is relaxing, but if it's unfamiliar it tells you that something's changed."

The effects can penetrate some of the most important parts of human existence. Weight loss or gain are side effects as people lose the anticipation of and satisfaction from food. As it goes with sex lives - if smell deteriorates completely or changes, so can a partner's attraction and the relationship.

Grenouille in Perfume is scentless, allowing him to move among others unnoticed. Conversely, people without a sense of smell may fear going out and become withdrawn because they cannot tell if they smell bad.

The likelihood of recovery from anosmia depends on what has caused it, as well as any treatment. There is no known cure for Robert, although, as he is taking steroids, his sense of smell does return on occasion.

"Sometimes, in the afternoon I can smell, it might be just for an hour and then it will go again," he says.

And what does he use his re-discovered sense to smell in that time? "Ah," he says wistfully, "I just smell the world."