The reports from COVID- 19 patients are disconcerting. Only a few hours before, they were enjoying a cup of pungent coffee or the fragrance of flowers in a garden. Then, as if a switch had been flipped, those smells disappeared.

All ages are affected — more than 80% to 90% of those diagnosed with the virus, according to some estimates. While most people recover in a few months, 16% take six months or more to do so. An estimated 1.6 million Americans have chronic smell problems due to COVID-19.

Seniors are especially vulnerable, experts suggest. “We know that many older adults have a compromised sense of smell to begin with. Add to that the insult of COVID, and it made these problems worse,” said Dr. Jayant Pinto, a professor of surgery and specialist in sinus and nasal diseases at the University of Chicago Medical Center.

Recent data highlights the interaction between COVID- 19, advanced age and loss of smell. When Italian researchers evaluated 101 patients who had been hospitalized for mild to moderate COVID- 19, 50 showed signs of smell impairment six months later. Those 65 or older were nearly twice as likely to be impaired; 75 or older, more than 2-1/2 times as likely.

Few are aware of the extent to which smell can diminish in later life. More than half of 65- to 80-year-olds have some degree of olfactory dysfunction. That rises to as high as 80% for those even older. Those affected often report concerns about safety, less enjoyment eating and an impaired quality of life.

But because the ability to detect, identify and discriminate among odors declines gradually, most older adults — up to 75% of those with some degree of smell loss — don’t realize they’re affected.

A host of factors are believed to contribute to age-related smell loss: a reduction in the number of olfactory sensory neurons in the nose, which are essential for detecting odors; changes in stem cells that replenish these neurons; atrophy of the processing center for smell in the brain, called the olfactory bulb; and the shrinkage of brain centers closely connected with the olfactory bulb, such as the hippocampus.

Also, environmental toxic substances such as air pollution play a part. “Olfactory neurons in your nose are basically little pieces of your brain hanging out in the outside world,” and exposure over time damages them and the tissues that support them, explained Pamela Dalton, a principal investigator at the Monell Chemical Senses Center, a smell and taste research institute in Philadelphia.

Still, the complex workings of the olfactory system have not yet been mapped in detail, said Dr. Sandeep Robert Datta, a professor of neurobiology at Harvard Medical School.

“We tend to think of our sense of smell as primarily aesthetic,” he said. “What’s very clear is that it’s far more important. The olfactory system plays a key role in maintaining our emotional well-being and connecting us with the world.”

Common consequences of smell loss include a loss of appetite, difficulty monitoring personal hygiene, depression and an inability to detect noxious fumes. In older adults this can lead to weight loss, malnutrition, frailty, inadequate personal care and accidents caused by gas leaks or fires.

Jerome Pisano, 75, of Bloomington, Ill., has been living with smell loss for five years. Before he became smell-impaired, Pisano was certified as a wine specialist. He has an 800-bottle wine cellar. “I can’t appreciate that as much as I’d like. I miss the smell of cut grass, flowers, my wife’s cooking,” he said. “It certainly does decrease my quality of life.”

Smell loss is also associated in various research studies with a higher risk of death for older adults. One study, authored by Pinto and colleagues, found these older adults were nearly three times as likely to die over a period of five years as seniors whose sense of smell remained intact.

“Our sense of smell signals how our nervous system is doing and how well our brain is doing overall,” Pinto said. According to a review published earlier this year, 90% of people with early-stage Parkinson’s and more than 80% of those with Alzheimer’s have olfactory dysfunction — a symptom that can precede other symptoms by many years.

There is no treatment for smell loss associated with neurological illness or head trauma, but if someone has persistent sinus problems or allergies that cause congestion, an over-the-counter antihistamine or nasal steroid spray can help.

For smell loss following a viral infection, the picture is less clear. It’s not known, yet, which viruses are associated with olfactory dysfunction, why they damage smell and what trajectory recovery takes. COVID-19 might help shine a light on this.

“What characteristics make people more vulnerable to a persistent loss of smell after a virus? We don’t know that, but I think we will because that research is underway and we’ve never had a cohort (of people with smell loss) this large to study,” said Dalton, of the Monell center.

Some experts recommend smell training. This involves sniffing four distinct scents (often eucalyptus, lemon, rose and cloves) twice a day for 30 seconds each, usually for four weeks. Sometimes the practice is combined with pictures of the items, a form of reinforcement.

The theory is that “practice, practice, practice” will stimulate the olfactory system, said Charles Greer, a professor of neurosurgery and neuroscience at Yale School of Medicine. Although scientific support isn’t well established, he said, he often recommends that people who think their smell is declining “get a shelf full of spices and smell them on a regular basis.”

Richard Doty, director of the University of Pennsylvania’s Smell and Taste Center, remains skeptical. He’s writing a review of smell training and notes that 20% to 30% of people with viral infections and smell loss recover in a relatively short time, with or without this therapy.

“The main thing we recommend is avoid polluted environments and get your full complement of vitamins,” since several vitamins play an important role in maintaining the olfactory system, he said.