U.S. researchers have found that women who smoke face a much higher risk of bladder cancer than previously thought.

A study in the Journal of the American Medical Association says the newest information shows that about half of bladder cancer cases in women are caused by smoking — up from 20-30 percent in previous studies.

The researchers say the higher risk comes from the changes in cigarette ingredients since the 1960s. Today's cigarettes contain less tar and nicotine, but more carcinogenic chemicals including beta-napthylamine, which has been directly linked to bladder cancer.

The researchers also say that when taking other factors into account, including age, ex-smokers are more than twice as likely to develop bladder cancer than those who never smoked.