There are two things wrong here, according to clinicians who treat this killer of nearly 160,000 people a year — more than deaths from cancers of the breast, prostate and colon combined.

One is the element of blame, as if all smokers who get lung cancer began smoking and continued to smoke knowing the possible consequences.

“Lung cancer is underfunded and a major reason is the idea that it’s all related to smoking and it’s the smoker’s fault,” said Dr. Michael Thun, an epidemiologist at the American Cancer Society. “This stigma has influenced both advocacy and research dollars.”

In fact, most smokers who develop lung cancer these days were hooked on nicotine long before it was recognized as an addictive drug and before smoking was clearly linked to cancer. I recall ads for a cigarette brand “most doctors recommend,” as well as the wanton distribution of free cigarettes to college students and young military recruits. I also recall the more recent industry ploy of marketing “low tar” and “light” cigarettes with a higher nicotine content that kept people hooked.

“Regardless of how patients get lung cancer, it’s not deserved,” Dr. Joan Schiller, an oncologist at the University of Texas Southwestern Medical Center in Dallas, told me in an interview. “Nicotine addiction is not their fault. Most became hooked at a very young age. It’s a real dependence, like heroin or cocaine.”

Diagnosing Difficulties

As for nonsmokers who get lung cancer — about two-thirds of them women — Dr. Schiller said they “are a disenfranchised group that did nothing wrong, yet women with breast cancer get all the support and empathy.”

“It’s a sizable number of nonsmokers who get lung cancer, more than get leukemia or AIDS,” she went on. “If lung cancer unrelated to smoking was listed as a separate disease, it would be the sixth or seventh most common cause of cancer deaths.”

Smoking-related lung cancer typically strikes older people (the average age at diagnosis is 71), but it often afflicts nonsmokers much earlier, in the 30s and 40s or even younger. And because doctors rarely suspect lung cancer when people who never smoked develop respiratory symptoms, the disease is typically diagnosed too late for any hope of a cure.

Many nonsmokers are treated for months for conditions like pneumonia, bronchitis or asthma before the real problem is uncovered. In fact, one type of lung cancer unrelated to smoking, bronchoalveolar carcinoma, can even look like pneumonia on a chest X-ray.

At the same time, however, recent discoveries of unusual genetic abnormalities in lung cancers that afflict nonsmokers have spawned targeted treatments that can buy patients years of quality time.

“We’ve made phenomenal progress in the last decade,” Dr. Heather Wakelee, an oncologist at Stanford University, said in an interview. “In about half of nonsmokers who get lung cancer, we can identify mutations within the cancers themselves that are driving the disease. And for the 20 to 30 percent of cases that have a particular mutation in the epidermal growth factor receptor, we already have drugs that can target this mutation with a pretty dramatic response.”

She said, “Even if we can’t cure it, our goal is to make lung cancer a chronic disease, like diabetes.”

Researchers have identified a number of factors that can account for the development of lung cancer in nonsmokers. Probably best known is passive smoking — chronic exposure to smoke from other people’s cigarettes. For example, the actor Christopher Reeve’s widow, Dana, was a nonsmoker who died of lung cancer in 2006, at age 44. While the cause was not determined, some experts speculated that she was a victim of secondhand smoking, exposed when she worked in nightclubs.

On average, Dr. Thun said, nonsmokers married to smokers increase their risk of lung cancer by 20 percent. For that reason, smokers who live with others should not smoke at home or anywhere near a nonsmoking partner or child, even when outdoors.

Everyday Hazards

An estimated 21,000 cases of lung cancer each year result from chronic exposure to radon, a radioactive gas in dirt and rocks that can seep into the air inside homes, offices and schools. Radon is second only to smoking as a known cause of lung cancer. The Environmental Protection Agency recommends that all homeowners get a radon-detection kit — and, if levels are elevated (as they are in 1 out of 15 homes in this country), install a radon-reduction system.

Asbestos exposure is another potential risk, one that sometimes accounts for lung cancer in men who never smoked but who worked in Navy or civilian shipyards. Nonsmoking asbestos workers face a fivefold risk of developing lung cancer compared with other nonsmokers; the risk is greatly magnified in asbestos workers who smoke.

Air pollution from vehicles, industry and power plants is a relatively minor threat, resulting in perhaps 2,000 lung cancer deaths a year in nonsmokers, primarily those who are heavily exposed to polluted air.

The naturally occurring hormone estrogen is believed to account, at least in part, for the disproportionate number of women who develop lung cancer unrelated to smoking, and for the fact that female nonsmokers tend to get the disease at a younger age than men. As with many breast cancers, estrogen receptors can be found in some lung cancers, and Dr. Schiller says estrogen may promote their growth.

Then there is the possibility of an inherent genetic susceptibility. Dr. Ping Yang of the Mayo Clinic in Rochester, Minn., has found that significantly reduced activity of a gene called GPC5 is associated with lung cancer in people who never smoked. She reported that this gene, a member of a family of genes that regulate growth factors, is the first to be specifically linked to lung cancer in nonsmokers.