Is lung cancer only caused by smoking?

Question

Accepted Answer

No. The National Cancer Institute's prevention summary states it plainly: in countries where cigarette smoking is common, about 10% to 20% of lung cancer cases occur in never smokers.

Both halves of that sentence are true at once. Smoking is still by far the biggest cause. NCI calls it the most important risk factor for lung cancer, and estimates that 90% of lung cancers in males and 78% in females are caused by tobacco smoking. NCI also puts numbers on the individual risk: in one Swiss population, the lifetime risk of lung cancer was 15% in men who smoke and 12% in women who smoke, against 2% or less in nonsmokers.

Where the other cases come from

NCI names radon and secondhand smoke as established causes of lung cancer in never smokers.

Risk is also raised in never smokers by asbestos, by ionizing radiation from sources other than radon, and by indoor air pollution from burning coal or other solid fuel.

For everyone, NCI's list of occupational causes rests on solid evidence: workplace exposure to asbestos, arsenic, beryllium, cadmium, chromium, and nickel increases lung cancer incidence and deaths, with risk rising as exposure rises. Outdoor air pollution, specifically small particles, does the same.

One warning is easy to miss. Beta-carotene supplements are listed by NCI under interventions associated with an increased risk of lung cancer in current smokers.

NCI also flags a combination effect. For radon and asbestos, exposure to cigarette smoke plus the other factor increases risk far more than adding the two risks together would suggest.

The exposure that can be measured this month

Radon is a radioactive gas released as uranium, thorium, and radium decay in rock and soil. NCI describes it as invisible and odorless, seeping up through the ground and collecting indoors through cracks in floors, walls, and foundations. Basements and first floors usually have the highest levels.

The Environmental Protection Agency estimates radon is responsible for about 21,000 lung cancer deaths a year, and that about 2,900 of those occur in people who never smoked. EPA calls radon the number one cause of lung cancer among non-smokers, and the second leading cause overall.

EPA's own risk table gives the action level. At 4 pCi/L it says "fix your home," and estimates that if 1,000 people who smoked were exposed at that level over a lifetime, about 62 could get lung cancer, against about 7 of 1,000 people who never smoked. Between 2 and 4 pCi/L, EPA says to consider fixing. It notes that getting below 2 pCi/L is difficult, and that the average indoor level is 1.3 pCi/L.

NCI's advice is to check radon levels in the home regularly, and it points to EPA's guide on reducing them. Our guide to radon testing at home covers how, and secondhand smoke covers the other established cause.

Screening is tied to smoking history

This is the practical gap for never smokers.

The U.S. Preventive Services Task Force recommends annual screening with low-dose CT for adults aged 50 to 80 who have a 20 pack-year smoking history and who currently smoke or quit within the past 15 years. That is a grade B recommendation, dated March 9, 2021.

Someone who never smoked does not meet those criteria, no matter what else they were exposed to. So symptoms carry more weight.

MedlinePlus lists the ones to act on: chest pain, a cough that does not go away, coughing up blood, fatigue, loss of appetite, unintended weight loss, shortness of breath, and wheezing. It adds that early lung cancer may cause no symptoms at all, and that later signs can include hoarseness or a changing voice, difficulty swallowing, and bone or shoulder pain.

Any of those deserves a visit and a chest image, even in someone who never touched a cigarette.

Is lung cancer only caused by smoking?

Where the other cases come from

The exposure that can be measured this month

Screening is tied to smoking history

Sources

Know someone who needs this?