Fifty years after British researchers published the first study firmly linking cigarette smoking to lung cancer, the same scientist following the same group of subjects has reported the most detailed and long-term results ever of the health effects of smoking. His stark conclusion: A life of cigarette smoking will be, on average, 10 years shorter than a life without it.

While the lethal effects of cigarette smoking have long been known, the new study, published yesterday in the British Medical Journal, is the first to quantify the damage over the lifetime of a generation. The effects, the researchers reported, were "much larger than had previously been suspected."

In the 50-year study of a group of almost 35,000 British doctors who smoked, the pioneering epidemiologist Richard Doll, who is now 91, and his colleagues found that almost half of all persistent cigarette smokers were killed by their habit, and a quarter died before age 70.

The study also found, however, that kicking the cigarette habit has equally dramatic effects. He found, for instance, that someone who stops smoking by age 30 has the same average life expectancy as a nonsmoker, and that someone who stops at 50 will lose four, rather than 10, years of life.

"What we now know is that consistent cigarette smoking doubles mortality rates in both middle age and old age," said Richard Peto, Doll's 30-year associate in the ongoing study. "But we also know that stopping smoking will significantly limit the harm."

The consequence of the tobacco "epidemic" has been to undercut great strides in public health that would otherwise have kept millions more people alive, the researchers said.

"Over the past few decades, prevention and better treatment of disease have halved nonsmoker death rates in Britain," said Doll, lead author on both the 1954 and current tobacco studies. "But these improvements have been completely nullified by the rapidly increasing hazards of tobacco for those who continue to smoke cigarettes."

Doll began studying smoking among British doctors in 1951, and the research has continued every decade since, with the final study begun in 2001. At that time, almost 6,000 of the doctors first studied in 1951 were still alive.

The effects of smoking show up especially starkly after age 60. At 70, the study found, 88 percent of nonsmokers were still alive, compared with 71 percent of smokers. And at age 80, 65 percent of nonsmokers were alive but only 32 percent of smokers were.

To look at the data another way, a 70-year-old who never smoked has a 33 percent probability of living to 90. For a 70-year old smoker, the probability of living 20 more years is only 7 percent.

The researchers also found that a subset of British doctors, born around 1920, died of tobacco-related illnesses at a much higher rate than others -- accounting for almost two-thirds of all deaths. The researchers reported that the men, who were young soldiers during World War II, smoked more because they could buy low-cost cigarettes from the government during the war and became more addicted to tobacco.

Doll and Peto said that while the harm of smoking is dramatic, so is the benefit from quitting. According to their findings, a person who stops smoking at 60 will have a life expectancy three years longer than someone who continues; a 40-year-old will have a life expectancy nine years longer; and a 30-year-old will have a life expectancy no different from that of a nonsmoker.

Peto said that in 1951, about 77 percent of the doctors in the study smoked -- a percentage just below the national average of 80 percent of adults. Today, Peto said, the United Kingdom has the lowest cigarette smoking rate in the developed world, about 20 percent of adults. It is unknown, however, how many of the doctors still smoke.

Yesterday's results were published in the same journal exactly 50 years after the initial study appeared.

Also yesterday, the Proceedings of the National Academy of Sciences published a study done at the Boston University School of Medicine that may explain why some but not all former smokers remain at high risk for lung cancer. The researchers found that cigarette smoking changes the pattern of genes expressed in lung cells, and that the changes return to normal in some people who stop smoking, but not in others.

Avrum Spira and colleagues found that 97 bronchial cell genes were expressed differently in smokers than in people who had never smoked. Some of those changes increased the expression of genes that can lead to cancer growth, and other changes decreased the expression of suppressor genes that keep cancers at bay.

Former smokers who had stopped at least two years before the study generally had gene patterns similar to those seen in nonsmokers, while those who quit more recently had gene patterns that resembled those of current smokers.