Anosmia, the inability to smell, affects some 2 million people in the United States and an undetermined number worldwide. While most people take their sense of smell for granted, the subtle olfactory cues lost to anosmia sufferers are more important than you'd think.

"It's horrible — you lose the smell of the environment," said Dr. Terence Davidson, director of the University of California-San Diego Nasal Dysfunction Clinic. "Your room, the trees, perfume ... it all becomes flat."

Perhaps worst of all for those who love good eats, anosmia means that food has very little flavor: What we think of as "taste" is mostly made up of smells, Davidson said, so when your olfactory signals disappear, "all food all turns into wet cardboard."

"I can't tell the difference between apple juice and onion juice!" said Max Christian, a 25-year-old telecommunications executive living outside of London who has been without a sense of smell since birth.

More seriously, the olfactory warning signals that protect most people from danger often go ignored, with potentially deadly consequences. "I left a pot on the stove accidentally, and went upstairs," said Christian. "The whole ground floor of my house was filled with smoke, and I didn't realize anything until the smoke started coming through the floorboards."

A sense of smell is also important in detecting natural gas leaks and spoiled food, Davidson said, noting that he commonly sees anosmia patients suffering from food poisoning when "somebody serves you a putrid six-month-old fruit salad and you eat it anyway."

The Smell Killers

Anosmia is usually caused by nasal inflammations, as "almost everybody will lose their sense of smell when they have a cold," said Dr. Daniel Kurtz of the State University of New York's Upstate Medical Center. "There's also nasal sinus disease, seasonal allergies and nasal polyps," which can cause longer lasting impairment but are usually treatable with allergy drugs, surgery and anti-inflammatory steroids.

However, there are some cases that are more stubborn: "Some people who have a cold, the infection goes away and the smell never comes back," Kurtz said. Unlike other nerves, receptor cells in the nose regularly die and regenerate every 30 or 60 days. But certain viruses "kill the cells, called progenitors, that give rise to new nerve receptors," he said, and "when the old ones die, there's nobody there to replace them."

Another common cause of anosmia is a traumatic head injury such as a concussion, which can sever the nerves that carry electric impulses from the nose to the brain. "When your brain moves within the cranium, the olfactory nerves are very thin and delicate — when the brain moves they're often torn, and you lose your ability to smell," Davidson said.

Born With It

A small percentage of people like Christian have congenital anosmia, either because of a defective gene passed down through generations or because of a single, one-time mutation. Never having known what it means to smell, anosmia sufferers say it is hard to fit in with friends and families.

"It caused a lot of problems with my childhood," wrote Dawn on Christian's anosmia Web site. "I grew up thinking that I had to learn how to smell and I was just not catching on like the other kids, so I told no one at school about it."

Anosmia may "cause profound psychological effects resulting in feelings of physical and social vulnerability and victimization," according to researchers from the University of Coventry in Great Britain. "In addition, there may be unhappiness related to the loss of the ability to detect pleasurable food smells and, as a consequence, anosmics may develop problems relating to eating."

It's common for anosmics to pretend to be able to smell. "[When] everyone was complaining about bad smells ... I'd always play along," said Joe Balfantz on Christian's Web site, "saying 'yeah, that smells bad' until I realized, 'Hey, I'm not really smelling anything.'"

Christian said his mothers and sister still put food under his nose and ask him if he can smell anything, but he's grown used to his anosmia, which is "not really a huge thing in my life ... If you were to lose a sense, most people would choose smell."

Besides, "the sense of smell is almost completely useless in modern Western society," he writes wryly on his Web site. "The logical inference from this is that smell is being evolved out of the human race, and that anosmic individuals are the most evolved members of the species."