Medically reviewed by: Antoinette Wozniak, MD
Lung cancer is caused by damage to the DNA of lung cells, which makes them grow out of control. Smoking is by far the biggest cause, linked to most cases — but it is not the only one. Radon gas, secondhand smoke, asbestos and other workplace exposures, air pollution, and inherited gene changes can all raise the risk, and some people develop lung cancer with no known cause at all.
Understanding what causes lung cancer does two useful things: it shows where risk can be lowered, and it explains why the disease also affects people with no smoking history. At the cellular level, lung cancer begins when the DNA inside lung cells is damaged over time. That DNA contains the instructions that tell cells when to grow, when to stop, and when to die. When enough of those instructions are disrupted — often by years of exposure to harmful substances — a cell can begin to multiply uncontrollably and form a tumor. The more risk factors a person has, and the longer the exposure, the higher the risk. But risk is not destiny, and reducing exposure helps at any age.
Is smoking the main cause of lung cancer?
Yes. Smoking is the leading cause of lung cancer. According to the Centers for Disease Control and Prevention, cigarette smoking is linked to about 80% to 90% of lung cancer deaths in the United States. Tobacco smoke contains thousands of chemicals, dozens of which are known to cause cancer, and they damage the cells lining the airways with every exposure.
Cigars and pipe tobacco also raise risk, and so does long-term exposure to second-hand smoke. The encouraging part is that quitting works: risk begins to fall after a person stops and continues to decline the longer they stay smoke-free. It is never too late to benefit, which is why every lung cancer conversation includes support to quit. Quitting also improves outcomes for people who have already been diagnosed.
In recent years the use of electronic cigarettes and vaping has become popular as a substitute for smoking. The relationship between these practices and lung cancer is unclear. Vaping been linked to other lung problems, and it is very likely an unsafe practice.
Can you get lung cancer without smoking?
Yes. As many as 20% of people diagnosed with lung cancer in the U.S. — an estimated 46,000 people each year — have never smoked. If lung cancer in people who never smoked were counted as its own category, it would still rank among the more common cancers. It is also often biologically different from smoking-related cancer: it tends to occur at younger ages and is more likely to carry specific gene changes that can be matched to targeted treatment, which makes biomarker testing especially important for these patients. The major causes in people who never smoked include radon, secondhand smoke, air pollution, workplace exposures, and inherited risk.
What is radon, and why does it matter?
Radon is a naturally occurring radioactive gas that is invisible and odorless. It forms as uranium in soil and rock breaks down, and it can seep up into homes and other buildings, where it can build up to harmful levels. According to the Environmental Protection Agency, radon is the second leading cause of lung cancer overall and the leading cause among people who have never smoked.
Because radon cannot be seen or smelled, the only way to know whether a home has elevated levels is to test for it. Test kits are inexpensive and widely available, and elevated levels can be reduced with a radon mitigation system installed by a professional. Testing your home — and fixing it if levels are high — is one of the most concrete and underused steps a person can take to lower lung cancer risk, particularly for people who don’t smoke and their families.
What other things can cause lung cancer?
Beyond smoking and radon, several other exposures and factors raise lung cancer risk. Many of these matter most for people with long-term or workplace exposure, and several are more dangerous when combined with smoking.
Secondhand smoke
Breathing in other people’s tobacco smoke causes lung cancer in people who do not smoke themselves. There is no risk-free level of secondhand smoke exposure and reducing it — especially in homes and cars — protects both adults and children.
Asbestos and workplace exposures
Asbestos is a well-established cause of lung cancer, and the risk is dramatically higher for people who were exposed to asbestos and also smoked. Other workplace exposures linked to lung cancer include diesel exhaust, silica dust, and certain metals and chemicals such as arsenic, chromium, cadmium, and nickel. People who work or worked in mining, construction, manufacturing, or transportation may have higher exposure and should follow recommended safety precautions.
Air pollution
Outdoor air pollution — including the fine particles produced by traffic, industry, and the burning of fuel — is recognized by the International Agency for Research on Cancer as a cause of lung cancer. The added risk for any single person is much smaller than the risk from smoking, but because pollution affects entire populations, its overall impact is significant. How air pollution triggers lung cancer, including in people who never smoked, is an active area of research.
Family history and inherited risk
Having a parent, sibling, or child with lung cancer raises a person’s own risk, and some inherited gene changes increase susceptibility. Most lung cancer is not directly inherited, but a strong family history is worth mentioning to your doctor, who can help you understand your risk and whether any extra vigilance is warranted.
Previous radiation and lung disease
Radiation therapy to the chest for another cancer can raise the risk of lung cancer later in life. A personal history of certain lung diseases, such as chronic obstructive pulmonary disease (COPD) or pulmonary fibrosis, is also associated with higher risk.
Does air pollution cause lung cancer in people who don’t smoke?
Yes — outdoor air pollution is classified as a cause of lung cancer by the International Agency for Research on Cancer, and it can contribute to lung cancer in people who have never smoked. Researchers are working to understand exactly how pollution damages lung cells and how it interacts with other risk factors. While individuals have limited control over the air around them, this work is important both for prevention policy and for understanding the rising share of lung cancer in people who never smoked.
Are some groups affected differently?
Lung cancer does not affect everyone equally. It is now the leading cause of cancer death among women, and a meaningful share of women diagnosed have never smoked. Black Americans have historically faced worse outcomes and lower rates of early diagnosis and screening, in part because of differences in access to care. Understanding these disparities is part of why LCRF maintains community-specific resources, including pages on women and lung cancer and risk in different communities. To understand your own risk, the Know Your Risk resources are a good starting point.
How can you lower your risk of lung cancer?
- Don’t smoke, and if you do, get help to quit — it is the single most effective step at any age.
- Avoid secondhand smoke at home, in cars, and in shared spaces.
- Test your home for radon and fix elevated levels with a mitigation system.
- Follow safety precautions for workplace exposures such as asbestos, diesel exhaust, silica, and industrial chemicals.
- If you are eligible for lung cancer screening, get a yearly low-dose CT scan.
- Talk with your doctor about your personal risk if lung cancer runs in your family.
What is a pack-year, and how does smoking history affect risk?
Doctors often describe smoking history in pack-years, a way of capturing both how much and how long a person smoked. One pack-year equals smoking an average of one pack a day for one year, so 20 pack-years could mean one pack a day for 20 years or two packs a day for 10 years. The more pack-years, the higher the risk — and pack-years are also used to decide who qualifies for lung cancer screening. Risk stays elevated for years after quitting, which is why someone with a smoking history may still be eligible for screening even after they stop.
Can doctors tell what caused a person’s lung cancer?
Usually not with certainty. Risk factors raise the odds of developing lung cancer, but they rarely point to a single, provable cause in one individual. Someone with a heavy smoking history and high radon exposure may have had several contributing factors, while a person who never smoked may have no identifiable cause at all. This matters for a practical reason: lung cancer carries a stigma that can keep people from seeking care or talking openly about their diagnosis. The disease can affect anyone with lungs, and no one deserves it — what matters most after a diagnosis is getting the right testing and treatment, not assigning blame.
Does everyone exposed to these risks get lung cancer?
No. Most people exposed to a given risk factor never develop lung cancer, and some people with no known risk factors do. Risk is about probability, not certainty, and it reflects a combination of exposures, genetics, and chance accumulating over time. That is also why lowering exposure is worthwhile even though it cannot guarantee protection: reducing smoking, radon, and other exposures shifts the odds in a person’s favor.
What causes lung cancer FAQ
What is the number one cause of lung cancer?
Smoking is the number one cause, linked to roughly 80–90% of lung cancer deaths in the U.S. Among people who have never smoked, radon is the leading cause.
Can people who don’t smoke get lung cancer?
Yes. As many as 20% of people diagnosed in the U.S. have never smoked. Radon, secondhand smoke, air pollution, asbestos, and inherited gene changes are the main causes in people without a smoking history.
Is radon really a cause of lung cancer?
Yes. Radon is the second leading cause of lung cancer overall and the leading cause among those who don’t smoke, according to the EPA. It is invisible and odorless, so testing your home is the only way to detect it.
Does vaping cause lung cancer?
The long-term cancer risk of vaping is not yet fully known because the products are relatively new. E-cigarettes are not considered safe and are not a proven way to prevent lung cancer. Anyone trying to quit smoking should ask a doctor about proven cessation methods.
Is lung cancer hereditary?
Most lung cancer is not directly inherited, but having a close relative with lung cancer raises risk, and some inherited gene changes increase susceptibility. Tell your doctor if lung cancer runs in your family.
How long does it take for lung cancer to develop?
It usually develops over many years of cellular damage, which is why risk rises with age and with the length of exposure to causes like smoking and radon. There is no single timeline.
If I quit smoking, does my risk go away?
Quitting lowers your risk substantially over time, and the benefit grows the longer you stay smoke-free. Risk does not drop to that of someone who never smoked, which is why people with a smoking history may still qualify for screening — but quitting is the most powerful step you can take.
Is lung cancer contagious?
No. Lung cancer cannot be passed from one person to another. It is caused by damage to a person’s own cells, not by an infection that spreads between people.
At what age is lung cancer usually diagnosed?
Lung cancer is most often diagnosed in older adults. According to the American Cancer Society, the average age at diagnosis is about 70, and most people diagnosed are 65 or older — though it can occur earlier, particularly in people who never smoked.
Research is uncovering why lung cancer develops — including in people who never smoked — and how to prevent and treat it. Your support funds that work.
This page is for general education and is not a substitute for professional medical advice. Talk with your own health care team about your diagnosis, symptoms, and treatment options.
Sources
- Centers for Disease Control and Prevention. What Are the Risk Factors for Lung Cancer? cdc.gov, 2024.
- U.S. Environmental Protection Agency. Health Risk of Radon. epa.gov.
- American Cancer Society. Lung Cancer Risk Factors. cancer.org, 2025.
- International Agency for Research on Cancer. Outdoor Air Pollution. IARC Monographs, Vol. 109.
- Lung Cancer Research Foundation. Lung Cancer Fact Sheet. LCRF.org.