What causes lung cancer?

Smoking — the biggest cause, but not the whole story

There’s no getting around it: smoking is by far the leading cause of lung cancer, responsible for roughly 85% of cases.¹ But here’s a nuance I always share, because it surprises people: most smokers never develop lung cancer — and yet most people who develop lung cancer are, or were, smokers. Both are true. Smoking raises your risk enormously compared with a non-smoker, but it’s a matter of probability, not certainty. It’s also why quitting is worth it at any age: your risk begins to fall from the moment you stop.

Asbestos

Asbestos is a proven cause of lung cancer (and of mesothelioma, a cancer of the lung’s lining). What many people don’t realise is how close to home it can be. For much of the last century asbestos was used as building insulation, so older homes — especially those built or renovated in the decades when it was common — can still contain it in walls, ceilings and around pipes. It’s generally only dangerous once it’s disturbed and the fibres become airborne, so if you’re renovating an older property, have it assessed first.

A striking example of mass exposure is the World Trade Center. The towers contained asbestos, and when they collapsed on September 11th, 2001, an enormous cloud of dust — laced with asbestos and other carcinogens — settled over lower Manhattan. Long-term follow-up of those exposed has since shown a real rise in lung cancer: a 2025 study found responders with the heaviest dust exposure had close to a three-fold higher rate of lung cancer.²

Radon — the one I most want you to check

If there’s a single cause I push my patients to act on, it’s radon. Radon is an invisible, odourless radioactive gas produced as uranium in the soil and rock beneath us slowly breaks down. It seeps up into buildings and accumulates indoors — especially in basements and lower levels. In Canada, radon is the second leading cause of lung cancer after smoking, and the number-one cause among people who have never smoked.³

This one is personal for me: I keep a radon detector in my own basement, and I can watch the level rise and fall through the day. That’s exactly the point — radon isn’t constant, and you can’t see or smell it, so the only way to know is to measure it. I tell every patient to test their home, particularly the basement. If the level runs high, better ventilation — and, if needed, a radon mitigation system — can bring it down. Health Canada recommends taking action at 200 becquerels per cubic metre. Testing is cheap and genuinely worth doing.

Lung cancer in people who never smoked

One of the most important shifts I’ve seen is the rise of lung cancer in people who never smoked — and it’s disproportionately affecting women.⁴ In never-smokers it behaves in some ways like a different disease: more often a type called adenocarcinoma, and more often driven by specific gene changes within the tumour. We don’t fully understand every reason for the increase, but radon, second-hand smoke, air pollution and genetics all likely contribute. The practical message is simple: you don’t have to be a smoker to get lung cancer — which is exactly why symptoms should never be brushed off just because someone never smoked.

Genetics and family history

Patients often ask whether lung cancer runs in families. To a degree, it can. Having a first-degree relative (a parent, sibling or child) with lung cancer raises your own risk by roughly 1.5 to 2 times, even after allowing for shared smoking habits.⁵ Most of that comes from many small inherited variations that each nudge risk up a little.

Less commonly, there are rare inherited mutations that cause a genuine hereditary lung-cancer syndrome. The best-described is a germline mutation in a gene called EGFR (a change known as T790M), passed down through families, which tends to cause lung cancer in people who never smoked.⁶ It’s uncommon, but it’s a real example of lung cancer that is inherited rather than caused by the environment. If several close relatives have had lung cancer — particularly non-smokers, or at a young age — that’s worth raising with your doctor.

What you can actually do

You can’t change your genes or undo a past exposure, but a lot of this is within your control:

• Don’t smoke — and if you do, getting help to quit is the single most powerful step you can take.
• Test your home for radon, especially the basement, and fix it if it’s high.
• Be careful with asbestos in older homes — have it assessed before you renovate.
• Know your family history and share it with your doctor.
• If you’re at higher risk, ask about screening — catching lung cancer early is what changes the outcome.

Dr. Kashif Irshad, MD, MSc, FRCSCThoracic & benign esophageal surgeon. This article is general information, medically reviewed September 2026. About Dr. Irshad →