Cigarette smokers die on average 10 years earlier than non-smokers but kicking the habit, even at 50 years old, can halve the risk, according to half a century of research reported on Tuesday.

Findings from a 50-year study into the dangers of smoking showed that if people quit by the age of 30 they can avoid nearly all of the risk of dying prematurely, China Daily reported on Tuesday. 

"Cigarette smoking reduces the expectation of life by 10 years," said 91-year-old Oxford University Professor Richard Doll who discovered the link between cancer and smoking.

"It is clear that consistent cigarette smoking doubles mortality throughout adult life -- middle and old age. It is also clear that giving up smoking can eliminate a very large part of the hazard," he told Reuters.

Doll and Bradford Hill confirmed the link between smoking and lung cancer in a landmark study published in the British Medical Journal on June 26, 1954.

Half a century later, Doll and Oxford University Professor Richard Peto report the 50-year results from the same study of 34,439 British doctors in the journal.

"This study is a remarkable achievement. Studies that last 50 years are highly unusual in medicine, and it's even more unusual for one to have an author who was there at the beginning and after 50 years," the journal's editor Dr Richard Smith told a news conference.

"It has taught us a great deal," he added.

Doll, who had smoked for 19 years before giving up, had planned only a five-year project but the initial findings were so intriguing he carried on for five decades.

"I gave up smoking at age 37 when I saw the results of our first study. They were quite convincing to me," he said in an interview.

The early results from his second study confirmed that smoking causes lung cancer and suggested that it also causes heart disease.

"We thought we'd better carry the study on for a lot longer and see if smoking causes anything else and by goodness it does," said Doll.

"By the time we did the 40-year follow-up...we found there were some 25 diseases which smoking seemed to cause and that the mortality was about double with the consistency of smoking."

Fifty-year results showed that among men born in the 1920s who became persistent smokers, about two-thirds died from their habit.

"Nobody has ever before seen a population in which two-thirds of the smokers get killed by their habit," said Peto, who has worked with Doll for 30 years.

"These men who were born in the 1920s and went into the army in the Second World War...became serious cigarette smokers from the age of 18 in a way no previous generation had ever done. Among those who continued, about two thirds got killed by it."

Since Doll began the study five decades ago, tobacco has already killed about 100 million people worldwide. During the present century there will be about one billion tobacco deaths if current smoking patterns continue, according to Peto.

"Smoking kills people and stopping works," he said.

(China Daily via Agencies June 23, 2004)