While previous studies showed that only 20 to 30 percent of bladder cancer cases in women were caused by smoking, these new data indicate that smoking is responsible for about half of female bladder cancer cases - similar to the proportion found in men in current and previous studies.

The increase in the proportion of smoking-attributable bladder cancer cases among women may be a result of the increased prevalence of smoking by women, so that men and women are about equally likely to smoke, as observed in the current study and in the US population overall, according to surveillance by the CDC.

The majority of the earlier studies were conducted at time periods or in geographic regions where smoking was much less common among women.

Study author Neal Freedman said: “Current smokers in our study had a fourfold excess risk of developing bladder cancer, compared to a threefold excess risk in previous studies. The stronger association between smoking and bladder cancer is possibly due to changes in cigarette composition or smoking habits over the years.

“Incidence rates of bladder cancer in the US have been relatively stable over the past 30 years, despite the fact that smoking rates have decreased overall. The higher risk, as compared to studies reported in the mid-to-late 1990s, may explain why bladder cancer rates haven’t declined.”

Although there have been reductions in the concentrations of tar and nicotine in cigarette smoke, there have been apparent increases in the concentrations of certain carcinogens associated with bladder cancer.

A 2009 NCI/DCEG study was the first to suggest a higher risk for smoking-induced bladder cancer than previously reported.

In the current study, former smokers were twice as likely to develop bladder cancer as never smokers, and current smokers were four times more likely than those who never smoked. As with many other smoking-related cancers, smoking cessation was associated with reduced bladder cancer risk.

Participants who had been smoke-free for at least 10 years had a lower incidence of bladder cancer compared to those who quit for shorter periods of time or who still smoked.

Senior author Christian Abnet said: “Our findings provide additional evidence of the importance of preventing smoking initiation and promoting cessation for both men and women. Although the prevalence of cigarette smoking has declined, about 20 percent of the US adult population continues to smoke.”

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