Causes and Risk Factors of Lung Cancer

Lung cancer is the most common type of cancer worldwide. It begins when mutations, or changes, happen in the cells of the lungs. These changes cause the cells to grow and spread in an abnormal and often very rapid way.

Lung cancer has a number of possible causes. The most common is smoking or exposure to secondhand smoke. There can be other causes, however, and sometimes there is no known cause.

This article looks at the most common causes of lung cancer. It also helps you to identify lung cancer risks so that you can make lifestyle choices to reduce those risks.

lung cancer causes
Verywell / Catherine Song

Common Causes and Risk Factors

A cause is something that directly contributes to the development of a disease. For example, a person may have lung cancer that was caused by smoking. By contrast, a risk factor is something that might cause the development of a disease, but has not yet. If you are a smoker but you do not have lung cancer, then smoking is simply a risk factor for lung cancer.

Lung cancer is linked with a range of possible causes and risk factors. A person may develop lung cancer due to exposure to chemicals or because of genetics. Sometimes there is no clear cause. People who never smoked and have no known long-term exposure to chemicals, for example, may still develop lung cancer. The most common cause, however, is smoking.

Smoking

Smoking is responsible for around 80% to 90% of lung cancer deaths in the United States. The risk of someone who smokes developing lung cancer is 15 to 30 times greater than that of a nonsmoker.

The risk of heart disease drops when you stop smoking. Unfortunately, though, the risk of lung cancer may persist for years or even decades after you've quit. In fact, most people who develop lung cancer today are not smokers but former smokers.

Approximately 10% to 20% of lung cancer deaths in the United States occur in nonsmokers. Worldwide, an estimated 25% of lung cancer patients are never-smokers.

Cigar smoking is less dangerous than cigarette smoking. Still, cigar smokers are 11 times more likely to develop lung cancer than nonsmokers. There is debate over whether smoking marijuana raises lung cancer risk. There is, however, good evidence that hookah smoking (smoking a waterpipe) raises risk.

Lung cancer isn't the only kind of cancer that's smoking-related. Many cancers of the digestive system, as well as a type of leukemia, are linked with smoking. It's important to quit even if you already have cancer, because quitting smoking may improve survival.

Age

Age is an important risk factor for lung cancer. The disease becomes more common with increasing age. This is why the U.S. Preventive Services Task Force recommends lung cancer screenings for people over age 50 who have a smoking history.

That said, young adults and sometimes even children may develop lung cancer.

Screening Guidelines

Lung cancer screening is recommended for people between 50 and 80 who:

  • Have at least a 20 pack-year history of smoking
  • Continue to smoke or quit smoking in the past 15 years

To calculate pack-years, multiply the number of packs smoked per day times the number of years smoked. For example: 2 packs per day times 10 years equals 20 pack-years.

Depending on your other risk factors, you and your healthcare provider may consider screening outside of these guidelines.

Radon

Exposure to radon in the home is the second leading cause of lung cancer. It is also the top cause of lung cancer in nonsmokers. 

Radon is an invisible gas. It can enter a home through cracks in the foundation or walls and through gaps around service pipes and other routes where gas seeps.

Radon is found in homes in all 50 states and across the world. It has no color or odor so you may not know if you're exposed. The only way to know if you are at risk is to have your home tested. If radon is found, there are ways to lower the levels.

The U.S. Environmental Protection Agency (EPA) estimates that there are 21,000 lung cancer deaths each year related to radon.

Secondhand Smoke

In 2013, a large prospective cohort study of more than 76,000 women confirmed a strong association between cigarette smoking and lung cancer, but found no link between the disease and secondhand smoke.

Since then, there's been a vast body of research that finds secondhand smoke does raise the risk of lung cancer for nonsmokers by 20% to 30%. Secondhand smoke is now thought to be responsible for roughly 7,000 cases of lung cancer each year in the United States.

Lung cancer isn't the only risk attached to secondhand smoke. The American Cancer Society says there's evidence suggesting it's linked to cancers of the larynx, the nose, and even breast cancer.

Air Pollution

In 2013, outdoor pollution was classified as a carcinogen by the International Agency for Research on Cancer. A carcinogen is a cancer-causing substance. Air pollution contains tiny particles that are linked to burning and to chemical by-products from industrial and other sources.

Indoor pollution is a problem too. Coal is used for cooking and heating in China and other parts of the world, for example. Wood stoves used in developing countries also create risk. Poorly ventilated modern gas stoves can emit carbon monoxide, nitrogen dioxide, and formaldehyde into the home.

Chemical Exposure

Exposure to chemicals and substances is another important risk factor for lung cancer, especially when combined with smoking.

You could be exposed to some of these at home, but on-the-job exposure may be more likely. Some chemicals associated with lung cancer include:

  • Formaldehyde
  • Asbestos
  • Silica
  • Chromium
  • Arsenic
  • Nickel compounds
  • PAHs (polycyclic aromatic hydrocarbons)
  • Vinyl chloride
  • Wood dust

Some occupations associated with an increased lung risk include:

  • Truck driving
  • Sandblasting
  • Metal working
  • Printing
  • Ceramic making
  • Uranium mining
  • Glass manufacturing

Check the Material Safety Data Sheet (MSDS) for any chemicals you may be exposed to at work. U.S. employers are required to provide these, so if you don't have them, ask.

Radiation

Exposure to high-energy radiation is a risk factor for lung cancer. This kind of exposure could come from X-rays and other radiation in health care, or from high levels of a more general radiation in the environment.

People with cancer who undergo radiation therapy to the chest have an increased risk for lung cancer. This type of therapy may be done after a breast cancer mastectomy or for someone with Hodgkin's disease. The risk is higher when radiation is received at a younger age. It can also vary depending upon the dose of radiation.

Lung Diseases 

Chronic obstructive pulmonary disease (COPD) and lung cancer are both caused by smoking. COPD is also itself a risk factor for lung cancer. This means that someone with COPD is much more likely to develop lung cancer than someone who smoked the same amount. The same is true if both individuals never smoked.

Overall, the chance that someone who has COPD will develop lung cancer is two to four times greater than someone who does not have COPD. The risk is even greater for heavy smokers.

Asthma and tuberculosis appear to be risk factors too. Idiopathic pulmonary fibrosis may increase the risk of lung cancer by up to 20%.

Medical Conditions 

People with certain cancers and other health conditions appear to have an increased risk of lung cancer. This may be due to genetics or other causes. These include:

  • Hodgkin disease
  • Non-Hodgkin lymphoma
  • Testicular cancer
  • Head and neck cancers
  • Esophageal cancer
  • Bladder and kidney cancers
  • Cervical cancer

Organ transplant recipients are also at a greater risk for lung cancer.

Infections

10% of cancers in the United States and about 25% worldwide are related to infectious diseases.

Studies have found an association between human papillomavirus (HPV) infection and lung cancer. It's not yet known if this is simply a correlation or if HPV is an actual cause.

Correlation vs. Causation

When two things are correlated, it does not mean that one causes the other. Here's an example often used to illustrate this: There are more drownings in the summer, the same time of year when more people eat ice cream. This does not mean that eating ice cream causes drownings.

Likewise, there may be a correlation between HPV and lung cancer, but there is no evidence to date proving that the infection causes the disease.

Genetics

Overall, approximately 8% of lung cancer cases are considered hereditary. Genetic factors are more likely to be at work when lung cancer develops in nonsmokers, women, and people under 60.

Having a first-degree relative with lung cancer doubles the risk of developing lung cancer yourself. First degree relatives include parents, siblings, and children. Having a second-degree relative with lung cancer raises your risk by around 30%. Second degree relatives include aunts, uncles, nephews, and nieces.

Cardiovascular

Angiotensin-converting enzyme inhibitors (ACE inhibitors) are used to treat high blood pressure. They have raised concerns as lung cancer risk factors for a few reasons. These drugs increase bradykinin, a peptide in the lungs that has been known to stimulate the growth of lung cancer. They also cause the accumulation of substance P, a compound that helps transmit nerve signals. Substance P has been associated with the growth of cancer.

A 2018 study of more than 300,000 people found that those who used ACE inhibitors were 14% more likely to develop lung cancer. The risk came with longer-term use and did not become clear until at least five years of use. The greatest risk was linked to more than 10 years of use. Drugs used in the study included:

  • Altace (ramipril)
  • Zestril or Prinivil (lisinopril)
  • Coversyl (perindopril)

An elevated platelet count may also be a risk factor. A 2019 study comparing close to 30,000 people with lung cancer to over 56,000 people without the disease found that an elevated platelet count was associated with an increased risk. People with high counts were 62% more likely to develop non-small cell lung cancer and 200% more likely to get small cell lung cancer.

The researchers believe the high platelet count could play a direct role in the development of the disease.

Lifestyle Risk Factors

Smoking and exposure to secondhand smoke are two major risk factors you can modify. If you're struggling to quit, speak with your healthcare provider about support options, such as cessation aids.

Beyond that, examine your overall habits to see if you can make additional changes that can decrease your risk of lung cancer.

Diet and Food Supplements

The American Lung Association (ALA) recommends eating a diet high in fruits and vegetables and low in red and processed meats and processed sugar. There is no direct link between sugar and lung cancer, or any food and lung cancer. Cured meats like sausage and deep-fried cooking methods, however, may be associated with an increased lung cancer risk.

The ALA also cautions that smokers should avoid taking beta-carotene supplements. Beta-carotene is a carotenoid, which is an antioxidant found in yellow, orange, and red plants and their fruits. Some studies suggest carotenoids decrease lung cancer risk but results are mixed. Other studies have found that high-dose supplements of vitamin A can be harmful.

Alcohol

Some studies have found drinking alcohol is linked to lung cancer, but more research is needed. One earlier analysis of 3,137 lung cancer cases found that people who consumed at least 30 grams (0.6 ounces) of pure alcohol a day had a slightly greater risk of lung cancer.

Summary

Smoking and radon exposure are the top causes of lung cancer in the United States, but they're not the only ones. Your genetic makeup, your other medical conditions and treatments, your job, lifestyle factors, or even the air pollution in your community may contribute to your lung cancer risk.

A Word From Verywell

Knowing the causes and risk factors of lung cancer is important for making lifestyle choices. Avoiding or quitting smoking may be at the top of the list, but smoking is not the only possible cause of lung cancer.

Some people never truly know the cause of their lung cancer. It is possible, however, to reduce those risks that are clear.

If you have concerns about your lung health, it's important to tell your healthcare provider right away. Roughly 40% of people are diagnosed when lung cancer has already progressed to stage 4. In this stage, curative surgery is not possible and the five-year survival rate is just 4%. In contrast, the survival rates are much higher when the cancer is diagnosed in earlier stages.

Be sure to ask your healthcare provider if lung cancer screening is appropriate for you.

Frequently Asked Questions

  • What are the signs and symptoms of lung cancer?

    Signs of lung cancer include a persistent cough and coughing up blood or brown mucus. Other signs are hoarseness, loss of appetite, shortness of breath, fatigue, unexplained weight loss, wheezing, and repeated bouts of bronchitis or pneumonia.

  • What is the survival rate of lung cancer?

    For non-small cell lung cancer, the five-year relative survival rates are:

    • Localized: 63%
    • Regional spread: 35%
    • Distantly metastasized: 7%

    For small cell lung cancer, the survival rates are:

    • Localized: 27%
    • Regional spread: 16%
    • Distantly metastasized: 3%
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