Danny Miner, a 66-year-old retired chemical plant supervisor, spends most days alone in his Tooele, Utah, apartment, with “Gunsmoke” reruns to keep him company and a phone that rarely rings.

Old age wasn’t supposed to feel this lonely. Mr. Miner married five times, each bride bringing the promise of lifelong companionship. Three unions ended in divorce. Two wives died. Now his legs ache and his balance is faulty, and he’s stopped going to church or meeting friends at the Marine Corps League, a group for former Marines. “I get a little depressed from time to time,” he says.

Baby boomers are aging alone more than any generation in U.S. history, and the resulting loneliness is a looming public health threat. About one in 11 Americans age 50 and older lacks a spouse, partner or living child, census figures and other research show. That amounts to about eight million people in the U.S. without close kin, the main source of companionship in old age, and their share of the population is projected to grow.

Policy makers are concerned this will strain the federal budget and undermine baby boomers’ health. Researchers have found that loneliness takes a physical toll, and is as closely linked to early mortality as smoking up to 15 cigarettes a day or consuming more than six alcoholic drinks a day. Loneliness is even worse for longevity than being obese or physically inactive.

Along with financial issues including high debt and declining pensions, social factors such as loneliness are another reason boomers are experiencing more difficult retirement years than previous generations.

The lack of social contacts among older adults costs Medicare $6.7 billion a year, mostly from spending on nursing facilities and hospitalization for those who have less of a network to help out, according to a study last year by Harvard University, Stanford University and AARP.

“The effect of isolation is extraordinarily powerful,” says Donald Berwick, former administrator of the Centers for Medicare and Medicaid Services. “If we want to achieve health for our population, especially vulnerable people, we have to address loneliness.”

The Trump administration is looking at expanding faith-based partnerships to combat isolation among seniors, says U.S. Assistant Secretary for Aging Lance Robertson. Earlier this year, the British government appointed its first minister of loneliness to tackle the issue.

The baby boomers prized individuality and generally had fewer children and ended marriages in greater numbers than previous generations. More than one in four boomers is divorced or never married, census figures show. About one in six lives alone.

The University of Chicago’s General Social Survey, which has tracked American attitudes since 1972, asked respondents four years ago how often they lacked companionship, felt left out and felt isolated from others. Baby boomers said they experienced these feelings with greater frequency than any other generation, including the older “silent generation.”

Fewer friends

Karen Schneider, a 69-year-old in East San Jose, Calif., went through an acrimonious split from her husband in the mid-1990s that left her estranged from her two daughters and without anywhere to live. Friends let her sleep on couches and a garage as she scraped by on jobs as a home health aide and Walmart greeter. Sometimes she slept in her car.

Over the years, that support network shriveled as people moved away or died, she says. When Ms. Schneider landed in the hospital with a heart attack six years ago, she had no one to call for help. “When you get older you don’t have as many friends,” she says. “Everything changes.”

Among the most likely to lack close kin are college-educated women and people with little money, says Ashton Verdery, an assistant professor of sociology and demography at Pennsylvania State University. More senior women than men are kinless because women’s life expectancies are nearly five years longer, at 81 years. Of Americans age 50 and over in 2016, 27% of women were widowed or never married, compared with 16% of men. Women are also less likely to cohabitate and date later in life, research shows.

Paula Lettice of Alexandria, Va., got divorced at age 39, remarried at 42 and was a widow by 44. Now age 69, the former senior executive says she’s struggled to find a new partner.