Some people have no sense of smell whatsoever, and they're not a happy lot. (Most famously, Stevie Wonder lost his sense of smell in a 1973 car accident.) But as bad as having no sense of smell is, nasal hallucinations are much, much worse.
Imagine if your nose decided to invent smells, often at random. This is called phantosmia — olfactory hallucinations — and it can make your life a living Hell.
Most people, if asked which sense they would most be willing to jettison, would pick their sense of smell. Although it certainly does help people know when something's burning on the stove, and though it does add richness to your sense of taste, it's just not as prevalent as any other sense. It's possible to go for days without consciously even noticing a smell.
One of the reasons it's so easy to not notice smells, is they come from the outside and are factored through a weak sense. Olfactory hallucinations, on the other hand, are not from an outside source. They originate inside the brain, and the brain is a very powerful organ. People with no sense of smell report a drop in quality of life — but it's nothing compared to people who are parosmic or phantosmic. Parosmia is a distorted sense of smell. Whenever someone hands you a flower you smell rotten fish. Baking bread smells caustic. Phantosmia is when smells hit you out of the blue, with no possible trigger.
Although phantom or distorted odors can be signs of brain injury — they're one of the warning signals for a stroke, a seizure, or a tumor — they're also the result of malfunctioning noses. Plenty of cases are brought on by serious infections that damage the lining of the nose. In some cases, only one nostril picks them up, and anesthetizing it can cure the problem. In almost all others, the less a nostril can process real odors, the more likely it is to make them up.
Unlike real smells, hallucinations of smells don't go away. People can't turn away from them or open a window to dispel them. Some don't even have the ability to get used to the smells. One woman smelled dirt for a year, no matter what else she smelled, it was accompanied by the aroma of dirt. After her husband burned chili one night, that smell replaced the dirt. A few years later, after a trip to France, she noticed that the scent of lavender had followed her back, and nothing she smeared under her nose took the scent away. Everyday activities became frightening, because she was never able to control what smell would take over her nose. One day she was terrified that manure had replaced lavender, only to find that gardeners had spread it on soil outside, and it did fade when she left the house. Another sufferer had to deal with on-and-off week-long hallucinations of cigarette smoke.
Parosmia is no picnic either. One woman spent seventeen years with intermittent bouts of parosmia. During active periods, if she was exposed to any smell her brain interpreted it as overwhelmingly putrid. During these times it was hard to do anything and nearly impossible to eat anything. Many patients lose weight copiously, since it's impossible to enjoy a meal that smells like sulfur, ammonia, or rotting food.
Top Image: Jebulon
Via Oxford Journals, NY Times, Ancient Reflection, and the New Zealand Herald.