MANCHESTER, Ky. » When smoking first swept the U.S. in the early decades of the 20th century, it took hold among the well-to-do. Cigarettes were high-society symbols of elegance and class, puffed by doctors and movie stars. By the 1960s, smoking had exploded, helped by the distribution of cigarettes to soldiers in World War II. Half of all men and a third of women smoked.

But as evidence of smoking’s deadly consequences has accumulated, the broad patterns of use by class have shifted: Smoking, the leading cause of preventable death in the country, is now increasingly a habit of the poor and the working class.

While previous data established that pattern, a new analysis of federal smoking data released Monday shows that the disparity is increasing. The national smoking rate has declined steadily, but there is a deep geographic divide. In the affluent suburbs of Washington, D.C., only about 1 in 10 people smoke, according to the analysis, by the Institute for Health Metrics and Evaluation. But in impoverished places like this — Clay County, in eastern Kentucky — nearly 4 in 10 do.

"It’s just what we do here," said Ed Smith Jr., 51, holding up his cigarette in a hand calloused from his job clearing trees away from power lines. Several of his friends have died of lung cancer, he said, and he has tried to quit, but so far has not succeeded. "I want to see my grandson grow up."

The new study, which evaluated federal survey data from 1996 to 2012 to produce smoking rates by county, offered a rare glimpse beneath the surface of state-level data. It found that affluent counties across the nation have experienced the biggest, and fastest, declines in smoking rates, while progress in the poorest ones has stagnated. The findings are particularly stark for women: About half of all high-income counties showed significant declines in the smoking rate for women, but only 4 percent of poor counties did, the analysis found.

This growing gap in smoking rates between rich and poor is helping drive rising inequality in health outcomes, experts say, with, for example, white women on the lowest rungs of the economic ladder now living shorter lives.

"Smoking is leaving these fancy places, these big urban areas," said Ali H. Mokdad, a researcher at the Institute for Health Metrics and Evaluation and an author of the study. "But it has remained in these poor and rural areas. They are getting left behind."

Americans with a high school education or less represent 40 percent of the population, but they account for 55 percent of the nation’s 42 million smokers, according to a analysis of health survey data obtained from the Minnesota Population Center, at the University of Minnesota. Since 1997, the smoking rate for adults has fallen 27 percent, but among the poor it has declined just 15 percent, according to the Times analysis. And among adults living in deep poverty in the South and Midwest, the smoking rate has not changed at all.

Health experts say this finer-grained understanding of who still smokes shows that public health officials need to refocus anti-smoking efforts on the poor and working class. Michael P. Eriksen, dean of the school of public health at Georgia State University, who ran the federal Office on Smoking and Health under President Bill Clinton, said public programs aimed at struggling Americans were patchy, even as tobacco companies successfully targeted them with advertising.

With the national smoking rate stabilizing in recent years, experts say that reaching poor and working-class smokers, whose problems often include alcohol and drug abuse and mental illness, is crucial to further declines and to reduce the heavy financial burden that smoking puts on the health care system.

"The real conclusion here is we need to figure out clever ways to reach these groups," Eriksen said. "The effort has been pitiful so far compared to the potential benefit to society from getting these people to stop smoking."

Clay County, where just 7 percent of the population has a college degree and the poverty rate is double the nation’s, is trying. Manchester, the county seat, a small cluster of gas stations, municipal buildings and fast-food restaurants, banned smoking in restaurants, stores and bars in 2012. The hospital runs a smoking cessation program that offers nicotine patches and gum free in an effort to reach low-income smokers.

Still, progress has been slow. By the Institute’s analysis, Clay’s smoking rate in 2012, at 36.7 percent, was the highest of any U.S. county with a population of at least 15,000, and had not changed much since 1996.

"Smoking cessation is our biggest uphill battle," said Jeremy Hacker, the hospital’s community outreach coordinator. While smoking is no longer normal in big cities, he said, in Clay, "it’s not viewed as a problem. Smokers aren’t seen as the minority."

When Mitzi Jackson, a mother of four who works at a loan company, went on a trip to New York recently, the only person she saw smoking was the friend she was traveling with. He was hustled off sidewalks and away from doors. "I thought he would be arrested," she said.

In contrast, visiting Clay County is like traveling back in time. Its smoking rate was last seen in the country as a whole in 1970. A pack of cigarettes costs about $5 here, about half the cost in New York, where state and city taxes raise the price. Few people interviewed said they were glad to be smoking, but many said it was one of the few pleasures they could afford.

"I’d love to quit but I just can’t, I’m too addicted," said Becky Johnson, 60, as she screwed the gas cap on her aging station wagon outside Bowling’s Quick Stop, a convenience store whose electronic slot machines draw a crowd of smokers most afternoons. Asked why she smoked, she said, "Bored, I guess."

Until the mid-1960s, when the dangers of smoking became clear, high school and college graduates smoked at about the same rates. But since then, the rate has declined much more rapidly among college graduates. In the last 15 years, the decline has been twice as large among adults with a college degree, according to the Times analysis. Gaps have also widened between the employed and the unemployed, and between those with health insurance and those without.

Clay County has some of the worst health outcomes in the nation, and many said smoking seemed almost safe compared with the raging drug epidemic in the area.

"Smoking is one of the last things to worry about in this county," said Barbara Bowling, an owner of Mike’s Discount Tobacco, which has drive-through windows under a billboard that says, "Let’s Talk God." She said she was angry when she found out her daughter was smoking, but at least she was not using drugs.

"Just sit and watch the parking lot for a day," Bowling said. "If smoking is the worst thing that’s happening, praise the Lord."

Others expressed a fatalism common among poor Americans, who often control little of the circumstances of their lives.

"My dad’s attitude was, ‘Well, I’m going to die of something,’" said Lynn Bargo, 35, a part-time cosmetology student who works at a Best Western hotel. Her father, a smoker, died of lung cancer at 71. She said smoking was dwarfed by drug use: At the salon where she works, some clients come in high and occasionally lose consciousness while she does their hair.

Still, government interventions can make a difference, researchers say. While the smoking rate has largely been stuck for Americans living in deep poverty in the South, it has fallen significantly among that income group in the Northeast, where strong anti-smoking measures such as those enacted under former Mayor Michael R. Bloomberg in New York have been taken.

But, even in Clay County, change could be coming. When Manchester’s ban on smoking in public places went into effect, it was hard to tell who at the Huddle House restaurant hated it more: the clientele or the staff. Two years later, Mike Feltner, a cook, was puffing stealthily on an electronic cigarette (Marlboro flavor) while cooking eggs. He said all four of the smokers on the staff now used the devices, which he said were considerably cheaper than his old two-pack-a-day habit. He put the savings toward a down payment on a house.

"This is a new phase in this town," he said. "Everybody’s doing it. Young people, old people, everyone."

Sabrina Tavernise and Robert Gebeloff, New York Times