Smoking is the single biggest risk factor for lung cancer, linked to 80% to 90% of lung cancer deaths in the United States. But it’s far from the only one. Radon exposure, workplace chemicals, family history, and even outdoor air pollution all play a role, and some of these factors multiply each other’s effects in ways that matter.
Smoking and Tobacco Use
People who smoke cigarettes are 15 to 30 times more likely to develop lung cancer or die from it compared to people who have never smoked. That enormous range depends on how much you smoke, how long you’ve smoked, and how deeply you inhale. The metric doctors use is “pack-years”: one pack-year equals smoking one pack (20 cigarettes) per day for one year. Someone who smoked two packs a day for 10 years has a 20 pack-year history, the same as someone who smoked one pack a day for 20 years.
Quitting reduces your risk over time, but it doesn’t reset the clock to zero. Former smokers carry elevated risk for years, which is why screening guidelines still apply to people who quit recently. The U.S. Preventive Services Task Force recommends annual low-dose CT scans for adults aged 50 to 80 who have at least a 20 pack-year history and either still smoke or quit within the past 15 years.
Secondhand smoke also raises risk, though far less dramatically than smoking directly. There is no safe level of exposure, but the risk is proportional to the amount and duration of contact.
Radon: The Leading Risk for Non-Smokers
Radon is a naturally occurring radioactive gas that seeps into homes through cracks in foundations, floors, and walls. It’s colorless and odorless, so you won’t know it’s there without testing. Radon is responsible for roughly 21,000 lung cancer deaths in the U.S. each year. About 2,900 of those deaths occur in people who have never smoked, making it the top cause of lung cancer in non-smokers.
The EPA recommends taking action when indoor radon levels reach 4 pCi/L (picocuries per liter) or higher. Testing kits are inexpensive and available at most hardware stores. If your levels come back high, a radon mitigation system (essentially a ventilation setup that redirects the gas outside) can reduce concentrations by up to 99%. It’s one of the most straightforward ways to lower your lung cancer risk at home.
Workplace Chemical Exposures
Certain industrial substances are confirmed lung carcinogens. The International Agency for Research on Cancer identifies five major occupational lung carcinogens: asbestos, respirable crystalline silica, nickel, chromium(VI), and polycyclic aromatic hydrocarbons (PAHs). Workers in construction, mining, metalworking, welding, and manufacturing face the highest exposure.
Asbestos deserves special attention because of how it interacts with smoking. When someone is exposed to both asbestos and cigarette smoke, the two don’t simply add together. Roughly one-third of lung cancer cases among smokers exposed to asbestos are attributed to the synergistic behavior of the two carcinogens working together, beyond what either would cause on its own. If you’ve worked around asbestos at any point in your life, that history matters even decades later, and it matters significantly more if you also smoke.
Exposure to multiple workplace carcinogens simultaneously also compounds risk. Researchers studying combinations of these five chemicals found that paired exposures raised lung cancer risk beyond what each substance would contribute individually.
Family History and Genetics
If a parent, sibling, or child has had lung cancer, your risk is about 50% higher than someone with no family history of the disease. That increased risk exists even after accounting for shared environmental exposures like secondhand smoke in the household. Some of this appears to be driven by inherited genetic variations that affect how your body repairs damaged DNA or processes carcinogens.
Family history is especially important to consider if you have other risk factors. A 50% increase on top of an already elevated baseline (from smoking or occupational exposure, for example) translates to a meaningfully higher absolute risk.
Pre-Existing Lung Conditions
Chronic obstructive pulmonary disease (COPD) is one of the strongest independent risk factors for lung cancer beyond smoking itself. Smokers who also have COPD face a six-fold risk of developing lung cancer compared to never-smokers without COPD. Even after adjusting for smoking history, the presence of COPD roughly doubles lung cancer incidence. Data from the National Lung Screening Trial confirmed this: participants with COPD had twice the rate of lung cancer compared to those without it.
The combination of COPD, heavy smoking history (35 or more pack-years), emphysema, and age over 70 creates an especially high-risk profile. Pulmonary fibrosis, a condition where lung tissue becomes scarred and stiff, also increases lung cancer risk, though the exact magnitude is less well-defined. The chronic inflammation and repeated tissue damage these conditions cause appear to create an environment where cancerous changes are more likely to take hold.
Air Pollution
Outdoor air pollution is classified as a Group 1 human carcinogen, the same category as cigarettes and asbestos. Fine particulate matter (PM2.5), the tiny particles produced by vehicle exhaust, power plants, and industrial emissions, is the primary concern. These particles are small enough to penetrate deep into lung tissue, where they can cause the kind of sustained inflammation and cellular damage that promotes cancer over years of exposure.
The risk from air pollution is much smaller per person than from smoking, but because billions of people are exposed, it contributes to a large number of lung cancer cases globally. People who live near major highways or in cities with poor air quality carry higher risk, especially over decades of exposure.
Beta-Carotene Supplements in Smokers
This one surprises most people. Beta-carotene is an antioxidant found naturally in carrots, sweet potatoes, and other orange and green vegetables. Eating these foods is associated with lower cancer risk. But taking high-dose beta-carotene supplements has the opposite effect if you smoke or have been exposed to asbestos.
Two major clinical trials demonstrated this clearly. The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study found an 18% increase in lung cancer incidence among male smokers taking beta-carotene supplements. The Beta Carotene and Retinol Efficacy Trial, which studied people at high risk due to smoking or asbestos exposure, found even worse results: a 28% rise in lung cancer, a 17% increase in deaths, and higher cardiovascular mortality. Both trials were stopped early because the supplements were causing harm. If you smoke or have a history of asbestos exposure, avoid high-dose beta-carotene supplements entirely.
E-Cigarettes and Vaping
The long-term lung cancer risk from e-cigarettes is still being studied, but early signals are concerning. A 2025 systematic review of observational studies found a potential association between e-cigarette use and increased lung cancer risk, particularly among people who use both e-cigarettes and conventional cigarettes. Dual users showed a higher likelihood of lung cancer compared to those using neither product.
The challenge is that vaping hasn’t been around long enough for the kind of decades-long data that established smoking’s risks. Lung cancer typically develops over 15 to 30 years of exposure, and widespread e-cigarette use only dates back about a decade. What researchers can say now is that e-cigarette aerosol contains known carcinogens, and using e-cigarettes alongside regular cigarettes does not appear to reduce lung cancer risk in any meaningful way.
How These Risks Stack Up
Lung cancer risk isn’t a single-factor equation. The most dangerous situations involve multiple risk factors layered on top of each other. A smoker with COPD, a family history of lung cancer, and workplace asbestos exposure carries a profoundly different risk profile than a non-smoker with none of those factors. Some combinations, like smoking plus asbestos, create synergistic effects where the total risk exceeds what you’d expect from simply adding the individual risks together.
The most actionable steps are quitting smoking (or never starting), testing your home for radon, following workplace safety protocols around known carcinogens, and getting screened if you meet the criteria. Screening with low-dose CT scans catches lung cancer at earlier, more treatable stages, and it’s recommended for the highest-risk group: current or recent smokers aged 50 to 80 with at least a 20 pack-year history.