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Washington - Up to 20 percent of women who develop lung cancer have never smoked, US researchers found in a study that suggests secondhand smoke may be to blame.

A survey of a million people in the United States and Sweden shows that just 8 percent of men who get lung cancer are nonsmokers.

"I have a lot of patients who have never smoked," said Dr. Heather Wakelee of Stanford University in California, who led the study.

"And because of the stigma, people are embarrassed to speak out about their disease. They feel like as soon as they say they have lung cancer, everyone judges them."

She said it is not clear why women may be more likely to get lung cancer even if they have never smoked.

"There is a lot of controversy over whether women are more susceptible to smoking at all, whether direct or secondhand smoke," Wakelee said in a telephone interview.

Writing in Friday's issue of the Journal of Clinical Oncology, Wakelee and epidemiologist Ellen Chang said they tracked the incidence of lung cancer in more than 1 million people aged 40 to 79. All had taken part in various studies of diet, lifestyle and disease, some going back into the early 1970s.

Some groups were mostly white women, such as an ongoing nurse's study, while others included ethnically diverse groups, Wakelee said.

Among women who never smoked, the lung cancer incidence rate ranged from 14.4 per 100,000 women per year to 20.8 cases per 100,000. In men, it ranged from 4.8 to 13.7 per 100,000. Rates were about 10 to 30 times higher in smokers.

This would translate to about 20 percent of female lung cancer patients having been nonsmokers and 8 percent of males, they said. That compares with American Cancer Society estimates of about 10 percent to 15 percent for all lung cancer patients.

"That estimate has been thrown about without any hard data to support it. This data sort of supports it," Wakelee said.

Chang said that because more men smoke than women, women may be more likely to be exposed to secondhand smoke, even when they are classified as never-smokers.

"We know that secondhand smoke does increase the risk of lung cancer so it's likely that a lot of these cases we observe are attributable to that," she said in a statement.

Smoking is by far the leading cause of lung cancer, but radon, asbestos, chromium and arsenic are also associated with lung cancer.

The American Cancer Society projects that lung cancer will be diagnosed in 213,000 Americans in 2007 and kill 160,000.

Weill Cornell Medical College last week said it was starting a lung cancer study of 5,000 people working in industries with a high degree of secondhand smoke exposure, such as flight attendants, restaurant workers and entertainers.