Smoking can more than double a woman's risk of developing cervical cancer, according to a new analysis published in the journal Cancer Causes and Control (Vol. 14, No. 9: 805-814). The finding was based on studies of women who are infected with human papillomavirus (HPV), the virus responsible for nearly all cases of cervical cancer.

HPV is an extremely common virus that is usually transmitted sexually. In most people, the virus never causes any symptoms and goes away on its own; people may never even know they were infected. Sometimes, though, the virus lingers in the body and can eventually cause changes in the cervix that could lead to cancer if not treated.

Doctors have been trying to find out what factors influence this process, so they can understand why only a small percentage of women who become infected with HPV actually develop cervical cancer. Smoking was a prime suspect because cervical cancer rates were observed to be higher in smokers than non-smokers. Researchers also have found cancer-causing chemicals from tobacco smoke in the cervix, and smoking is known to affect immune function, which could affect the body's ability to rid itself of the virus.

The new study confirms what some evidence had already suggested: that smoking raises the risk that an HPV infection will progress into cancer.

"The HPV exposure comes first, followed by other factors like smoking that put some women at high risk for the infection lasting and progressing into something that needs to be treated," explained Debbie Saslow, PhD, director of breast and gynecologic cancer for the American Cancer Society. "It's a multi-step process and HPV is just one step."

Although other studies have looked at smoking as a risk factor for cervical cancer, many of them did not control for HPV infection, so the results were not conclusive. Researchers from the International Agency for Research on Cancer and other institutions reanalyzed 10 studies that did take HPV into account.

The studies included 1,798 women who had cervical cancer, and 254 women who did not. All the women were infected with HPV. Smokers were more than twice as likely to develop cervical cancer as nonsmokers.

The message is clear, Saslow said: "Don't smoke. If you smoke, stop."

Quitting is important even for women who already have cervical cancer or precancerous changes, she noted. Previous research has shown that cervical abnormalities heal more quickly when women stop smoking.

The best strategy, of course, is to prevent cervical cancer in the first place, and that can be done with regular Pap tests, Saslow said. This simple procedure can find changes in the cervix before they become cancerous, or at least find cancer at an early stage when it is easier to treat.

Wider use of the Pap test is credited with significantly lowering both the incidence and death rate from cervical cancer in the US. Today, cervical cancer is not among the top 10 most common cancer sites for American women. Internationally, however, cervical cancer is more common and deadlier because Pap screening is not as widespread.