London: Smoking cuts life expectancy by 10 years, not four as previously thought, which can be avoided if one gives up the habit, preferably before 35 years, warn experts.

This new report, from researchers in Oxford University and Japan, investigates the impact of smoking on mortality in a large group of Japanese living in Hiroshima or Nagasaki in 1950.

The findings are, however, nothing to do with radiation exposure from the bombs.

The Life Span Study (LSS) was initiated in 1950 to investigate the effects of radiation, tracking over 100,000 people. However, most received minimal radiation exposure, and can therefore provide useful information about other risk factors, reported the journal bmj.com

Previous studies in Japan suggested smoking reduced life expectancy by only a few years compared with about 10 years in UK and the U.S, according to an Oxford statement.

Surveys carried out later obtained smoking information for 68,000 men and women, who have now been followed for an average of 23 years to relate smoking habits to survival.

The younger a person was when they started smoking the higher the risk in later life. Older generations did not usually start to smoke until well into adult life, and usually smoked only a few cigarettes per day.

In contrast, Japanese born more recently (1920-45) usually started to smoke in early adult life, much as smokers in Britain and the US.

Smokers born before 1920 lost just a few years. Conversely, men born later (1920-45) who started to smoke before 20 years lost nearly a decade of life expectancy, and had more than double the death rate of lifelong non-smokers, suggesting that more than half of these smokers will eventually die from their habit.

Results on the few women who had smoked since before age 20 were similar. This explains why the risks of smoking seemed low.

Nowadays, however, young Japanese smokers tend to smoke more cigarettes per day and to start at a younger age, so their risks will be higher.