Researchers also found that higher rates of smoking translate directly into higher rates of skin cancer, and current smokers are more likely to contract skin cancer than former smokers. Smoking increases risk independent of age, sex, sun exposure, and other factors related to skin cancer, according to the authors.

"When you smoke, you have a three-fold increased risk of developing squamous cell carcinoma," says co-author Maarten Bastiaens, MD, a member of the Leiden Skin Cancer Study Group at the Leiden University Medical Center (LUMC) in the Netherlands. "This type of cancer is easily treated, but also very common. The impact of smoking on squamous cell carcinoma of the skin may be substantial."

Skin cancers kill about 9,800 people in the US alone each year, according to the American Cancer Society (ACS). About 2,000 of these deaths are from squamous cell and basal cell skin cancers, with the remainder from malignant melanoma, the most serious form of skin cancer.

The Leiden group followed 966 subjects between 1985 and 1997. Of the group, 580 patients had some form of skin cancer. The other 386 were control patients who did not have skin cancer, and were matched with cancer patients for age and sex. Both groups received identical interviews regarding medical history, ethnic background and other potential risk factors for skin cancer. They were also given physical exams by dermatologists who did not know whether they were seeing a cancer patient or a control patient. The subjects knew they were in a skin cancer study, but not that the study was designed to explore links between smoking and skin cancer.

Leiden interviewers used specific questions to quantify both tobacco use and sun exposure over each subject's lifetime. They found that the smokers also tended to spend more time in the sun than nonsmokers, which made it difficult to separate the risks related to sun from risks related to tobacco smoking. Sun exposure is the primary risk factor for skin cancer.

This one study isn't enough to add skin cancer to the growing list of cancers caused by smoking, but it's a significant step in that direction, says Michael Thun, MD, vice president, epidemiology and surveillance research for the ACS. Earlier studies found higher rates of skin cancer among smokers but no direct link.

"The camel's back was broken long ago in terms of smoking and cancer," Thun says. "There have been overwhelming health reasons to avoid smoking for a long time. This is one more reason for people to stop smoking or not to start."

Just how tobacco smoke leads to skin cancer remains unclear, Bastiaens says. Compounds in tobacco smoke may act as skin carcinogens (cancer-causing agents), either by direct contact with smoke or by the action of carcinogens absorbed by the lungs into the bloodstream. Applying tobacco smoke to the skin induced squamous cell carcinoma in animal experiments. Tobacco smoke also causes cancers in the bladder, pancreas, cervix, and other sites that are protected from direct contact with smoke.

Bastiaens' team also noted that smoking may induce skin cancer by inhibiting the immune system. And patients with suppressed immune systems due to organ transplants or other causes are more susceptible to squamous cell carcinoma in many locations, including the skin.