A new report published on bmj.com suggests that smoking reduces the life expectancy by 10 years in Japan, instead of 4 years as previously thought, but the risk could be reduced by giving up the habit.
Previous studies in Japan suggested smoking reduced life expectancy by only a few years compared with about ten years in Britain and the USA. This new report, from researchers in Oxford and Japan, investigates the impact of smoking on mortality in a large group of Japanese people who were 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. 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 USA.
These differences in smoking habits are reflected in the mortality patterns. Smokers born before 1920 lost just a few years. In contrast, men born later (1920-45) who started to smoke before age 20 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.
Previous studies of the effects of smoking in Japan had been mainly of individuals born in the first few decades of the twentieth century who probably didn't start to smoke until well into adult life and smoked only a few cigarettes per day. 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.
In addition to studying the risk of smoking, the researchers were able to examine the benefits of stopping. As elsewhere, those who stopped smoking before age 35 avoided almost all the excess risk among continuing smokers, and even those who stopped around age 40 avoided most of it.
The researchers conclude that the future health risks to young smokers are likely to be just as big in Japan as in other countries although much of the risk can be avoided by stopping.