Environmental factors like secondhand smoke, air pollution and toxic gases may be increasing the incidence of lung cancer in nonsmokers, scientists said following the publication of a study that showed a surprisingly high rate of the disease in nonsmokers.

In the study, which was published on Feb. 10 in the Journal of Clinical Oncology, scientists at the Stanford Comprehensive Care Center and the Northern California Cancer Center found that the incidence of lung cancer in female “never smokers” is between 14.4 and 20.8 cases per 100,000 people annually.

Cancer specialists say that number is higher than was previously thought, but hastened to add that smoking is still the single most important cause of lung cancer.

“By far, the greatest incidence of lung cancer is in smokers,” said Beverly Mitchell, deputy director of the Stanford Comprehensive Care Center. “[Cancer epidemiology] is very important because only by studying large populations can you determine what factors in the environment or what genes underlie disease. Only then can you deal with causality.”

According to Heather Wakelee, an assistant professor of medicine at Stanford and one of the study’s lead researchers, smoking causes the vast majority of lung cancer cases — 80 percent in women and 90 percent in men.

A variety of factors are considered possible causes of the remaining cases, including a gene mutation in a growth factor called EGFR as well as inhaled substances like secondhand smoke, air pollutants and radon gas.

According to Wakelee, the problem is that researchers cannot use the data currently available to discern which causes are most important and how they interact.

“It’s known that lung cancer in nonsmokers behaves a little bit differently than cancer in smokers,” she said. “The problem is that most of our statistics come from databases that don’t include smoking information.”

Mitchell added that the nature versus nurture interaction complicates things further.

“There is a strong interaction between genetic factors and the environment,” she said. “We need to find out which group of factors predisposes people to cancer.”

Wakelee said that cancer epidemiologists have suspected that certain environmental factors like secondhand smoke are responsible for lung cancer development for years, but that in order to study those factors properly, scientific groundwork is required.

“There has been work in China looking at secondhand smoke but it’s been easier because [in China] women don’t smoke but men do,” she said. “If we are seeing an increase [in the U.S.], this study doesn’t answer that question. It provides the framework for answering that question using further studies.”

Future studies will most likely be longitudinal, she said, meaning that they will follow groups of people over time and will watch for the onset of lung cancer.

Studies that examine the nonsmoking causes of lung cancer are important to the clinicians responsible for treating the disease because there is a worrisome lack of treatment currently available, Wakelee said.

“The hardest thing is that with lung cancer today, there is nothing we know how to do to stop it from taking someone’s life,” she said. “Societally, we are moving forward but lung cancer [research] is not very well funded and part of that has to do with the stigma of smoking.”

“Lung cancer is not only a disease of smokers,” she added. “I have a lot of patients who come to me and I have to say, ‘Nice to meet you. And by the way, you’re going to die of this disease.’”