QUESTION: I have completely lost my sense of smell and can taste only a few things. I have seen doctors and taken tests, but no answers. I know I’m not the only one with this problem. Any ideas?

ANSWER: Humans are able to perceive an astounding 1 trillion odors. But our sense of smell is fragile.

About a quarter of adults, and more than half of those older than 80, have some degree of olfactory impairment. The sense of taste is often affected at the same time, as the neural pathways of smell and taste commingle in the brain.

Having an impaired sense of smell may be more than a nuisance. Studies have linked a decreased sense of smell to a heightened risk for Parkinson’s disease, Alzheimer’s disease, multiple sclerosis and premature death.

Common causes of a decreased sense of smell include nasal problems, such as deviated septum and nasal polyps; viruses, such as rhinovirus and Epstein-Barr virus; chronic sinusitis; head injury; and certain cancers.

Environmental exposure to cigarette smoke, alcohol, air pollution and toxins further increase the risk. Yet, in about 16 percent of people, no cause can be identified.

Eating nuts and fish has been associated with protection against smell impairment, as have exercise and use of cholesterol-lowering drugs and oral steroids. It is unknown, however, whether changing one’s dietary or exercise habits will improve the sense of smell.

Medical evaluation typically begins with an otolaryngologist, an ear, nose and throat doctor who will use a standardized scratch-and-sniff test to assess any olfactory deficits. Laboratory tests of blood and nasal mucus and imaging studies, such as CT or MRI scans, are often needed.

In some cases, endoscopic surgery, a flexible camera inserted into the nose, may aid in diagnosis and provide therapeutic benefits.

Treatment, of course, depends on the cause. When the cause is viral, the sense of smell usually returns with time, but steroid medications may hasten recovery. Patients with chronic sinusitis and nasal polyps may get relief from steroids and surgery. In select cases, monoclonal antibody medications may help.

If you are among the 16 percent of patients in whom no cause for impaired sense of smell can be identified, you might find help at a clinic that specializes in taste and smell problems. Your otolaryngologist can refer you to such a clinic, which is usually found at large academic medical centers.