Causes and Risk Factors of Lung Cancer

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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.

There are a number of possible causes behind a lung cancer diagnosis. The most common one is smoking, or secondhand exposure to tobacco smoke. Smoking is the leading cause of lung cancer in the United States. It is followed by radon gas exposure, which is the most common cause among nonsmokers.

Other causes of lung cancer include air pollution exposure, lung diseases like tuberculosis, and a person's own genetic makeup. This article looks a little more closely at these causes. It also helps you to identify lung cancer risks so that you can make lifestyle choices to reduce those risks.

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Common Causes

Lung cancer is linked with a range of possible causes. They may come from within your own family history if close loved ones have had lung cancer, to the possibility of work exposure if you use a lot of chemicals on the job. Some of these causes may be overlooked or less common.

Smoking, though, always leads the list—as well it should.

Smoking

Smoking is the top cause of lung cancer and 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.

And unlike the risk of heart disease, which drops by quite a bit when someone kicks the habit, the risk of lung cancer may persist for years or even decades after someone quits. In fact, most of the people who develop lung cancer today are not smokers but former smokers.

Lung cancer in nonsmokers is a leading cause of cancer deaths. It accounts for approximately 10% to 20% of lung cancer cases in the United States, according to the Centers for Disease Control and Prevention. Worldwide, an estimated 25% of lung cancer patients are never-smokers.

Although cigar smoking is less dangerous than cigarette smoking, those who inhale cigar smoke are 11 times more likely than nonsmokers to develop lung cancer. There is debate over whether smoking marijuana raises lung cancer risk; some studies suggest the opposite. There is, however, good evidence that hookah smoking raises risk.

Lung cancer isn't the only kind of cancer that's smoking-related, either. 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, as the disease becomes more common with increasing age. It's one reason why the U.S. Preventative 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 too. Not all cases of lung cancer are readily traced to an obvious cause like smoking, or a toxic exposure at work.

Lung cancer screening is recommended for people between 50 and 80 who have at least a 20 pack-year history of smoking, and continue to smoke or quit smoking in the past 15 years. Depending on the presence of other risk factors, you and your doctor may consider lung cancer screening outside of these guidelines.

Radon

Exposure to radon in the home is the second leading cause of lung cancer and the top cause in nonsmokers. Radon is an invisible gas that enters homes through cracks in the foundation or walls that allow the gas in, as well as gaps around service pipes and other routes where gas seeps.

Radon is found in homes in all 50 states and across the world, and it's a serious health issue. But 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.

To get an idea of the impact of radon, the U.S. Environmental Protection Agency (EPA) estimates that there are 21,000 related lung cancer deaths each year. The EPA and other agencies continue to raise awareness about this preventable cause of death.

Secondhand Smoke

In 2013, a large prospective cohort study of more than 76,000 women, published in the Journal of the National Cancer Institute, 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 raises the risk of lung cancer for nonsmokers by 20% to 30%. It is 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

Outdoor pollution may seem like an obvious cause, and it was classified as a carcinogen (a cancer-causing substance) by the International Agency for Research on Cancer in 2013. In later years, there's been more emphasis on the particulate matter (PM) found in air pollution. These tiny particles are linked to burning and the chemical by-products from industrial and other sources.

But indoor pollution is a problem too. Coal is used for cooking and heating in China and other parts of the world, and wood stoves used in developing countries create a risk. Modern gas stoves are getting a critical look too, because indoor cooking with poor ventilation is an important cause of lung cancer.

Chemical Exposure

Exposure to chemicals and substances, such as formaldehyde, asbestos, silica, and chromium, 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 other industrial chemicals associated with lung cancer include:

  • 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

Make sure to check the Material Safety Data Sheet (MSDS) on 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—such as X-rays and other radiation in health care—or to high levels of a more general radiation in the environment—is a risk factor for lung cancer.

People with cancer who undergo radiation therapy to the chest, as in Hodgkin disease or after a breast cancer mastectomy, have an increased risk for lung cancer. The risk is higher when radiation is received at a younger age and can vary depending upon the dose of radiation.

Lung Diseases 

Even though chronic obstructive pulmonary disease (COPD) and lung cancer are both caused by smoking, COPD is itself a risk factor for lung cancer. This means that someone with COPD is much more likely to develop lung cancer than a peer 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 among heavy smokers.

Asthma and tuberculosis appear to be risk factors too, while pulmonary fibrosis may increase the risk of lung cancer by 40%.

Medical Conditions 

People with certain cancers and other health conditions appear to have an increased risk of lung cancer, whether 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

Many people don't often think of infections as a cause of cancer, but 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, though it's not yet known if this simply means there is only a correlation or if, instead, HPV is an actual cause.

Correlation vs. Causation

When two things are correlated, it does not mean that one causes the other. 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 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 (a mother, father, sibling, or child) with lung cancer doubles the risk of developing lung cancer. Having a second-degree relative (aunt, uncle, nephew, or niece) with lung cancer raises your risk by around 30%.

Some of these causes and risk factors become known as science expands what we know about genetics. For example, the tumor suppressor gene called BRCA2 is well-known for its link with breast cancer. Any inherited BRCA2 mutations may increase lung cancer risk, especially in women who smoke.

Cardiovascular

Angiotensin-converting enzyme inhibitors (ACE inhibitors) used to treat high blood pressure have raised concern as a lung cancer risk factor 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, which 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, with the greatest risk linked to more than 10 years of use. Drugs used in the study included Altace (ramipril), Zestril or Prinivil (lisinopril), and 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 it could be a causal relationship with the high platelet count playing a role in the development of the disease.

Lifestyle Risk Factors

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

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

Diet and Food Supplements

The American Lung Association (ALA) recommends eating a diet that's 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. But cured meats like sausage, and deep-fried cooking methods, may be associated with an increased lung cancer risk.

The ALA also cautions that smokers should avoid taking take beta-carotene. 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

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

Recap

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, or even the air pollution in your community may contribute to your lung cancer risk.

Summary

Lung cancer in general is known for its poor outcomes and high death rates, but people often know less about the reasons for a diagnosis. There are many. Some can be changed even if there is little you can do about your own genes or a family history that increases your risk.

The key is to act quickly if you have concerns, and share them with your doctor. Roughly 40% of people are initially diagnosed when lung cancer has already progressed to stage 4, a point at which curative surgery is not possible and the five-year survival rate is 4%. In contrast, the survival rates are much higher for earlier stages of the disease that may be detected by lung cancer screening.

A Word From Verywell

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

In some cases, you may never even truly know the cause if diagnosed, but you can act now to reduce those risks that are clear. If you see any early symptoms of lung cancer, get them checked out as soon as possible. Be sure to ask your doctor 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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