How lung cancer starts

Lung cancer begins when cells in the lungs grow abnormally and multiply out of control. The lungs are made of millions of cells, and normally they divide, age, and die in an orderly way. When that process breaks down, damaged cells can accumulate and form a tumor. Most lung cancers start in the cells that line the airways—the tubes that carry air in and out of the lungs.

The damage that leads to cancer usually happens over years or decades. A single exposure to a harmful substance rarely causes lung cancer. Instead, repeated or long-term exposure to something that damages lung cells makes cancer more likely. The body's repair systems can fix some damage, but if the damage outpaces repair, cells can become cancerous.

There are two main types of lung cancer based on how the cells look under a microscope: small cell lung cancer and non-small cell lung cancer. Non-small cell lung cancer is more common, accounting for about 80 percent of cases. The type matters because it affects treatment options and how the cancer is likely to behave.

Key Takeaways

  • Lung cancer develops when cells in the lungs grow abnormally and multiply uncontrollably, usually over many years.
  • Smoking is the leading cause, but lung cancer can develop in people who have never smoked.
  • Radon gas, asbestos, secondhand smoke, air pollution, and family history all increase the risk.
  • Most people with lung cancer have no symptoms in early stages, which is why screening is recommended for high-risk groups.

Smoking and lung cancer risk

Smoking is the leading cause of lung cancer. Tobacco smoke contains more than 7,000 chemicals, and at least 70 of them are known to cause cancer. When you inhale smoke, these chemicals reach the lungs and can damage the cells that line the airways. The longer and more heavily someone smokes, the greater the damage accumulates.

The risk does not disappear immediately after quitting. Former smokers still have a higher risk of lung cancer than people who never smoked, though the risk decreases over time. Someone who quit 10 years ago has lower risk than someone who quit 2 years ago. Even so, the damage from years of smoking can take decades to fully resolve.

Secondhand smoke also increases lung cancer risk in people who do not smoke themselves. When someone else's smoke enters your lungs, you are exposed to many of the same cancer-causing chemicals as the smoker. The risk is lower than for active smokers, but it is real and measurable.

Radon, asbestos, and occupational exposure

Radon is a colorless, odorless radioactive gas that comes from uranium in soil and rock. It seeps into buildings through cracks in foundations and can accumulate in basements and lower levels. Long-term exposure to high levels of radon increases lung cancer risk. Radon is the second leading cause of lung cancer after smoking, though the risk varies greatly depending on radon levels in your area and how much time you spend in affected spaces.

Asbestos is a mineral fiber that was widely used in insulation, roofing, brake pads, and other products before its dangers became clear. When asbestos fibers are inhaled, they can lodge in the lungs and cause scarring and inflammation. People who worked in construction, shipbuilding, military service, or manufacturing before asbestos was heavily regulated face higher risk. The disease can develop 20 to 50 years after exposure.

Other occupational exposures that increase lung cancer risk include silica dust, diesel exhaust, and certain metals like chromium and nickel. People in mining, welding, foundry work, and some manufacturing jobs may face these exposures. The risk is higher when exposure is heavy or prolonged and when protective equipment is not used.

Air pollution and family history

Long-term exposure to outdoor air pollution increases lung cancer risk, particularly pollution from vehicle exhaust and industrial sources. The effect is smaller than smoking or radon, but it is measurable across large populations. People who live in areas with high air pollution levels have a higher risk than those in cleaner areas.

Family history also plays a role. If a parent, sibling, or child had lung cancer, your risk is higher than average, even if you have never smoked. This may be partly genetic—some people inherit variations in genes that affect how their body processes toxins or repairs DNA damage. It may also reflect shared exposures, such as living in the same house with radon or a smoking family member.

Why some people get lung cancer and others do not

Lung cancer is not purely a matter of exposure. Two people with identical smoking histories may have very different outcomes. Some people smoke heavily for decades and never develop lung cancer. Others develop it after lighter exposure. This variation reflects differences in genetics, immune function, and how individual bodies handle damage to cells.

Certain genetic variations make some people more susceptible to cancer. These are not mutations that may provide cancer—they are variations in normal genes that affect how efficiently your body repairs DNA damage or processes carcinogens. Someone with these variations who also smokes faces higher risk than a smoker without them.

Age matters too. Lung cancer is rare in people under 40, even among smokers. The risk rises sharply after age 65. This reflects the time it takes for enough cellular damage to accumulate and for the body's repair and immune systems to become less effective.

How screening works for high-risk people

Most people with early-stage lung cancer have no symptoms. By the time symptoms appear—cough, chest pain, shortness of breath—the cancer is often advanced. This is why screening is important for people at high risk.

Low-dose CT scanning can detect lung cancers earlier than chest X-rays. A CT scan takes detailed images of the lungs and can find small nodules that might be cancer. Screening is recommended for people aged 50 to 80 who have a 20 pack-year smoking history (meaning 20 cigarettes a day for 20 years, or equivalent) and currently smoke or quit within the past 15 years. Some doctors also recommend screening for people with other risk factors like radon exposure or family history, though guidelines vary.

Screening does not prevent cancer—it finds it earlier, when treatment is more likely to work. Not every nodule found on a CT scan is cancer, and not every cancer found needs immediate treatment. A doctor who specializes in lung cancer can help interpret results and decide next steps.

Frequently Asked Questions

Can you get lung cancer if you have never smoked?

Yes. About 10 to 15 percent of lung cancers occur in people who have never smoked. Radon, asbestos, air pollution, secondhand smoke, and family history can all cause lung cancer in non-smokers. Certain genetic variations may also increase risk.

How long does it take to develop lung cancer?

Lung cancer typically develops over many years or decades. Damage to cells accumulates slowly, and a tumor usually takes years to grow large enough to cause symptoms or show up on imaging. Some cancers grow faster than others, and growth rate varies by type.

Does quitting smoking lower lung cancer risk?

Yes, but not immediately. Risk decreases gradually over years after quitting. Someone who quit 10 years ago has lower risk than someone who quit 2 years ago. However, former smokers still have higher risk than people who never smoked.

What is a pack-year, and why does it matter?

A pack-year is a measure of smoking exposure: one pack per day for one year equals one pack-year. Twenty pack-years might mean one pack daily for 20 years, or two packs daily for 10 years. Pack-year history helps doctors estimate cancer risk and decide who should be screened.

Should I get screened for lung cancer?

Screening is recommended for people aged 50 to 80 with a 20 pack-year smoking history who currently smoke or quit within the past 15 years. Talk with your doctor about whether screening makes sense for you based on your age, smoking history, and other risk factors like radon exposure or family history.