They are surrounded by hundreds of other people. They have a large menu of group activities: cards, tennis, art classes, and theater outings. How could anyone living in a vibrant senior community be lonely?

But they are, according to a new study by researchers at UC San Diego’s medical school.

The scholars assessed residents of an independent living facility in San Diego County and found 85 percent have moderate to severe levels of loneliness, a complex condition that mounting evidence links to higher risks for heart disease, dementia, elevated blood pressure, depression and premature death.

“Loneliness rivals smoking and obesity in its impact on shortening longevity,” said Dr. Dilip Jeste, a geriatric neuropsychiatrist who led the research team.

The small study, published online in the journal Aging and Mental Health, is part of a nationwide push to better understand what happens to people as they get older, and to improve their outcomes. It comes as the population of older Americans is growing. The number of those who are at least 65 went up 34 percent from 2007 to 2017, according to the Census Bureau, and now totals almost 51 million.

By 2030, it’s projected to be 73 million, more than 20 percent of the total U.S. population. Specialists refer to the group as a “silver tsunami” because it’s expected to wash over many aspects of society: health care, housing, transportation, technology.

The concern about loneliness, which some have labeled a public health epidemic, is not limited to the United States. Two years ago, the United Kingdom appointed a minister for loneliness after government research showed that millions of people often or always feel lonely. Some 200,000 older people in Britain said they had not had a conversation with a relative or a friend in more than a month.

Researchers are diving into the differences between social isolation (the objective reality of living alone) and loneliness (the subjective feeling of distress). They know that not everyone who lives alone is lonely, and not everyone who lives with others isn’t. But they don’t yet fully understand why.

They are exploring various risk factors, including genetics and the loss of loved ones. They are studying the biological and behavioral processes triggered by loneliness. And they are identifying ways to intervene when someone is lonely.

“It’s a major problem,” Jeste, 75, said, “but we can do something about it, both on an individual level and a societal one.”