How lung cancer develops

Lung cancer starts when cells in the lungs grow abnormally and multiply out of control. These cells can form a tumor, and over time the cancer can spread to other parts of the body. Most lung cancers begin in the cells that line the airways—the tubes that carry air in and out of your lungs.

The disease develops when something damages the DNA inside lung cells. DNA is the instruction manual that tells cells how to grow and behave. When that damage occurs, cells may start dividing rapidly and ignoring the body's normal signals to stop. This uncontrolled growth is what makes the cells cancerous.

Not everyone exposed to a risk factor will develop lung cancer, and some people who get lung cancer have no known risk factors. The disease usually takes years to develop—often 10 to 20 years or more from the time of first exposure to a risk factor.

Key Takeaways

  • Smoking cigarettes is the leading cause of lung cancer, accounting for most cases in both smokers and people exposed to secondhand smoke.
  • Radon gas, which seeps into homes from the ground, is the second leading cause of lung cancer and is the top cause among people who have never smoked.
  • Asbestos, air pollution, family history, and certain medical conditions can all increase the risk of developing lung cancer.
  • Lung cancer can develop in people with no known risk factors, and having a risk factor does not mean a person will definitely develop the disease.

Smoking and secondhand smoke

Smoking cigarettes is the most common cause of lung cancer. Tobacco smoke contains more than 7,000 chemicals, at least 70 of which are known to cause cancer. When you inhale smoke, these chemicals damage the cells lining your airways and lungs.

The longer someone smokes and the more cigarettes they smoke per day, the higher their risk. However, lung cancer can develop in people who smoked for only a few years, and it can also develop decades after someone quit smoking.

Secondhand smoke—smoke from other people's cigarettes, cigars, or pipes—also causes lung cancer. People who live or work around smokers inhale many of the same cancer-causing chemicals as smokers do. Secondhand smoke exposure increases lung cancer risk even in people who have never smoked themselves.

Radon gas in homes

Radon is a colorless, odorless radioactive gas that comes from uranium in soil and rock. It seeps into buildings through cracks in foundations, gaps around pipes, and other openings. Radon is the second leading cause of lung cancer overall and the leading cause among people who have never smoked.

Radon levels vary widely depending on where you live. Some areas have naturally higher radon in the soil, and some homes have higher levels than others even in the same neighborhood. The only way to know if your home has elevated radon is to test for it—radon testing kits are inexpensive and available at hardware stores.

Long-term exposure to high radon levels increases lung cancer risk. The risk is higher for people who smoke, because smoking and radon exposure together create a greater danger than either one alone.

Asbestos and workplace exposures

Asbestos is a mineral fiber that was widely used in building materials, insulation, and other products before its dangers were fully understood. When asbestos fibers are inhaled, they can lodge in the lungs and cause scarring and inflammation over many years.

People who worked in construction, shipbuilding, military service, or manufacturing before asbestos use was restricted may have been exposed. Exposure can also happen during renovation or demolition of older buildings that contain asbestos. The risk of lung cancer from asbestos increases significantly for people who also smoke.

Other workplace exposures that can increase lung cancer risk include diesel exhaust, silica dust, and certain metals like chromium and nickel. The length and intensity of exposure matter—brief, low-level exposure carries less risk than years of heavy exposure.

Air pollution and environmental factors

Long-term exposure to outdoor air pollution increases lung cancer risk. Air pollution contains particles and gases that can damage lung cells over time. People who live in areas with high levels of air pollution face a higher risk than those in cleaner areas.

Indoor air pollution from cooking fires and heating sources—particularly in homes that rely on coal or biomass for fuel—also raises risk. This is especially true in parts of the world where such fuels are common and ventilation is poor.

Secondhand smoke, which was discussed earlier, is another form of indoor air pollution that increases lung cancer risk.

Family history and genetic factors

Some people inherit a higher risk of developing lung cancer. If multiple family members have had lung cancer, your risk may be higher than average, even if you have never smoked. Researchers have identified certain genetic variations that make some people more susceptible to lung cancer when exposed to risk factors.

Having a family history of lung cancer does not mean you will definitely develop the disease. It means your cells may be more vulnerable to damage from smoking, radon, or other exposures. Genetic testing is not routinely done for lung cancer risk, but your doctor can discuss your family history and what it means for your health.

Medical conditions and previous lung disease

Certain medical conditions increase the risk of developing lung cancer. Chronic obstructive pulmonary disease (COPD), which includes emphysema and chronic bronchitis, damages the lungs and increases cancer risk. People with a history of tuberculosis also have a higher risk.

Previous lung cancer is another risk factor—people who have already had lung cancer face a higher chance of developing it again, either in the same lung or the other lung. This can happen even if the first cancer was treated successfully.

Some research suggests that certain viral infections and inflammation of the lungs over many years may also play a role in lung cancer development, though this area is still being studied.

Frequently Asked Questions

Can you get lung cancer if you have never smoked?

Yes. About 10 to 15 percent of lung cancer cases occur in people who have never smoked. Radon exposure is the most common cause in non-smokers, followed by secondhand smoke, air pollution, and occupational exposures. Family history and genetic factors also play a role.

How long does it take to develop lung cancer?

Lung cancer typically develops over many years—often 10 to 20 years or longer from the time of first exposure to a risk factor. Some people develop it faster, and some slower. This is why screening can be important for people at high risk, because the disease may not cause symptoms until it is advanced.

Does quitting smoking reduce lung cancer risk?

Quitting smoking does reduce lung cancer risk over time, though the risk never returns to zero. The longer someone has smoked, the longer it takes for risk to decrease. However, former smokers still have a higher risk than people who never smoked, which is why screening may be recommended for long-term former smokers.

Is lung cancer hereditary?

Lung cancer itself is not directly inherited, but some people inherit genetic variations that make them more susceptible to developing it when exposed to risk factors like smoking or radon. If multiple family members have had lung cancer, talk with your doctor about your personal risk and whether screening is appropriate for you.

Can air pollution alone cause lung cancer?

Long-term exposure to air pollution increases lung cancer risk, but it is usually not the only factor. Most cases involve a combination of exposures—for example, someone exposed to both air pollution and radon, or someone with a family history living in a polluted area. The more risk factors present, the higher the overall risk.