Cigarette smoking causes the vast majority of lung cancer, linked to 80% to 90% of lung cancer deaths in the United States. But smoking is far from the only cause. Between 20,000 and 40,000 Americans are diagnosed with lung cancer each year despite never having smoked or having smoked fewer than 100 cigarettes in their lifetime. Understanding the full range of causes helps explain why lung cancer remains one of the most common cancers worldwide.
Smoking and Tobacco Use
Tobacco smoke contains dozens of chemicals that directly damage the DNA inside lung cells. When you inhale cigarette smoke, those chemicals are metabolically activated and bind to DNA, creating what scientists call DNA lesions. If the cell copies its DNA before repairing the damage, the error becomes a permanent mutation. Mutations in genes that control cell growth can cause cells to divide uncontrollably, which is the beginning of cancer.
This process doesn’t happen overnight. Carcinogenesis unfolds in stages: initiation (when the first DNA damage occurs), promotion (when damaged cells are stimulated to multiply), and progression (when those cells accumulate enough mutations to become fully malignant). Chronic exposure to tobacco smoke drives all three stages over years or decades. The effect of each exposure is irreversible and additive, meaning every cigarette contributes cumulative damage that the body cannot fully undo.
Cigars and pipes also increase lung cancer risk, though cigarettes remain the dominant contributor because of how deeply and frequently smokers inhale.
Secondhand Smoke
You don’t have to smoke to be harmed by tobacco. Non-smokers who are regularly exposed to secondhand smoke have a 20% to 30% higher risk of developing lung cancer compared to those who aren’t exposed. The same carcinogens that damage a smoker’s lungs are present in sidestream smoke, the smoke that drifts off the burning end of a cigarette, and in the exhaled smoke of people nearby.
Radon Gas
Radon is the leading cause of lung cancer among non-smokers and the second leading cause overall. This naturally occurring radioactive gas seeps up from soil and rock and can accumulate inside homes, particularly in basements and ground-floor rooms with poor ventilation. It’s colorless and odorless, so you can’t detect it without a test kit.
Radon is responsible for roughly 21,000 lung cancer deaths each year in the United States. The EPA recommends taking action to reduce radon levels when testing shows concentrations at or above 4 pCi/L (picocuries per liter). Home testing kits are inexpensive and widely available, and mitigation systems can reduce indoor radon by up to 99%.
Air Pollution and Fine Particulate Matter
Long-term exposure to polluted air, specifically fine particulate matter known as PM2.5, is an established cause of lung cancer. These particles are small enough to travel deep into the lungs, where they trigger a chain reaction. Immune cells called macrophages respond by releasing reactive oxygen species, molecules that create oxidative stress. That stress damages DNA and inflames lung tissue, pushing cells toward abnormal gene expression and, eventually, cancerous transformation.
PM2.5 exposure has also been shown to drive specific cancer-promoting mutations in lung cells, particularly in the same gene (involved in cell growth signaling) that is frequently mutated in lung cancers among non-smokers. This means air pollution doesn’t just cause generic damage. It can flip the same molecular switches that other, better-known carcinogens do.
Workplace Chemical Exposures
Certain jobs carry a meaningfully higher lung cancer risk because of chronic exposure to industrial carcinogens. The most well-established occupational cause is asbestos, which was widely used in construction, shipbuilding, and insulation manufacturing. Workers in asbestos processing plants face the highest documented risk, but construction workers exposed to asbestos-containing materials are also affected.
Other workplace carcinogens linked to lung cancer include:
- Crystalline silica: found in mining, pottery factories, and construction
- Diesel engine exhaust: common in trucking, mining, and construction
- Arsenic: encountered in uranium mining and smelting
- Nickel and chromium compounds: used in electroplating and metal finishing
- Polycyclic aromatic hydrocarbons (PAHs): present in carbon steel production and other high-heat industrial processes
Workers exposed to combinations of these substances, or who also smoke, face compounded risk. Asbestos exposure combined with smoking, for example, multiplies risk far beyond what either factor would produce alone.
Genetic Mutations and Family History
Some lung cancers are driven by specific genetic mutations that can occur regardless of smoking history. The most common of these affects a gene involved in cell growth signaling, found in about 15% of lung cancer cases in white and Black patients and up to 40% in patients of Asian descent. Another mutation, involving a gene that regulates cell development, appears in 5% to 6% of cases and is more common in people with little to no smoking history. Women and never-smokers are disproportionately affected by these mutation-driven cancers.
Family history also plays a role. Having a first-degree relative (parent, sibling, or child) diagnosed with lung cancer roughly doubles your risk, with an odds ratio of about 2.08. When both you and your relative were diagnosed before age 60, that risk jumps nearly fivefold. These patterns suggest inherited genetic susceptibility that makes some people’s lung cells more vulnerable to damage from any carcinogen they encounter.
Vaping Combined With Smoking
The long-term cancer risk of e-cigarettes alone is still being studied, but early evidence is concerning, particularly for people who both vape and smoke. A 2025 study found that young people who vaped and smoked had nearly 14 times the odds of early-onset lung cancer compared to non-users, a risk 2.8 times higher than smoking alone. For one specific type of lung cancer (adenocarcinoma, the most common form), the combined risk was 3.7 times higher than smoking by itself.
Animal studies have shown that exposure to nicotine-containing vape aerosol can produce lung tumors in mice. Research in humans has found that exclusive vapers show levels of DNA damage in mouth cells comparable to smokers, with damage increasing based on how often they vape. A 2025 systematic review concluded that vaping is consistently associated with markers of DNA damage, oxidative stress, and tumor growth. While definitive long-term data will take years to accumulate, the biological signals point in a worrying direction.
How These Causes Work Together
Lung cancer rarely results from a single event. It develops through the accumulation of DNA damage over time, and multiple causes can stack on top of each other. A non-smoker living in a home with elevated radon and high outdoor air pollution faces a different risk profile than someone in clean air, even though neither has touched a cigarette. A smoker exposed to asbestos at work faces dramatically higher risk than either factor alone would predict.
The 10% to 20% of lung cancers that occur in never-smokers are driven by some combination of radon, air pollution, occupational exposures, genetic susceptibility, and secondhand smoke. For the remaining 80% to 90%, tobacco is the primary driver, but co-exposures often accelerate the process. The common thread across all these causes is the same: something damages DNA in lung cells, the damage accumulates faster than the body can repair it, and eventually a cell acquires enough mutations to grow without restraint.