October 29, 2012 / Forbes / — Four new studies offer powerful evidence of the dangers of smoking and the health benefits of quitting or not being exposed to secondhand smoke.

Smoking in the UK– Between 1996 and 2001 the Million Women Study started following more than one million women aged 50 to 65 years of age. In a report published in the Lancet, trial investigators, including renowned epidemiologist Richard Peto, found that 12-year mortality was significantly higher in women with a history of smoking compared to women who never smoked (rate ratio 2.76, CI 2.71-2.81). Smokers, the authors calculated, lose 10 years of life. The good news is that stopping smoking before the age of 40 reduces the excess mortality by 90%.

Smoking in Japan– The Life Span Study, published in BMJ, was started in 1950 and has followed more than 65,000 men and women in Hiroshima and Nagasaki, Japan. The results were consistent with the Million Women Study in the UK: the rate ratio for mortality was more than doubled for smokers compared to nonsmokers both for men (2.21, CI 1.97-2.48) and for women (2.61, CI 1.98-3.44). The investigators also reported that stopping smoking before age 35 eliminated almost all of the risk associated with smoking.

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