Causes and Risk Factors of Lung Cancer

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When you think about the risk factors for lung cancer, it's likely that smoking is the first one you think of. But there are many additional factors that are known to cause or possibly contribute to the development of lung cancer. Radon is the second leading cause and the most common one in non-smokers. Other possible risk factors include occupational exposures, radiation, air pollution, lung diseases (e.g., asthma, COPD, and tuberculosis), some dietary supplements, and genetics.

Simply put, anyone who has lungs can get lung cancer. Since many factors work together to cause the disease, all of them should be considered when it comes to understanding your personal risk. It is also worth noting that many people who develop lung cancer don't have any obvious risk factors, and lung cancer is actually increasing in young women who have never smoked.

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Verywell / Catherine Song

Common Causes

There are many common risk factors for lung cancer. While smoking is a very significant one, it is unfortunate that many give it so much attention that other significant risk factors are sometimes overshadowed.

Smoking

Smoking is the number one cause of lung cancer and is responsible for around 80% of lung cancers in the United States. The risk of someone who smokes developing lung cancer is 13 to 23 times greater than that of a non-smoker. And unlike the risk of heart disease, which drops dramatically when someone kicks the habit, the risk of lung cancer may persist for years or even decades after someone quits. In fact, the majority of people who develop lung cancer today are not smokers but former smokers.

Smoking appears to play a bigger role in lung cancer for men than women. In the United States, 20% of women who develop lung cancer are lifelong non-smokers; worldwide, only 50% of women who develop the disease have smoked.

Although cigar smoking is less dangerous than cigarette smoking, those who inhale cigar smoke are 11 times more likely than non-smokers to develop lung cancer. There is debate over smoking marijuana raising lung cancer risk, with some studies suggesting the opposite. There is, however, good evidence that hookah smoking raises risk.

There are several smoking-related cancers in addition to lung cancer, and for those who already have cancer, quitting smoking improves survival.

Age

Age is an important risk factor for lung cancer, as the disease becomes more common with increasing age. That said, young adults and sometimes even children may develop lung cancer.

Radon

Exposure to radon in the home is the second leading cause of lung cancer and the leading cause in non-smokers. Radon is an odorless colorless gas that enters homes through cracks in solid foundations, construction joints, cracks in walls, gaps in suspended floors, gaps around service pipes, cavities inside walls, and the water supply. As such, exposure to radon is a serious health threat to children and nonsmoking men and women and may occur in their own homes.

Found in homes in all 50 states and worldwide, 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 the impact of radon, the EPA estimates that there are 21,000 deaths each year due to radon-induced lung cancer. Considering there are 40,000 deaths each year due to breast cancer, it's surprising the public isn't more familiar with this preventable cause of death.

Secondhand Smoke

A vast body of research has found that secondhand smoke raises the risk of lung cancer for nearby nonsmokers by 20% to 30% and is responsible for roughly 7,000 cases of lung cancer each year in the United States.

On the other hand, 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.

Air Pollution

Outdoor pollution may seem like an obvious cause, but indoor pollution from the use of coal for cooking and heating is also an important risk factor for lung cancer. Though more of a problem in developing countries, smoke from wood stoves and from indoor cooking with poor ventilation are important causes of lung cancer worldwide.

Chemical Exposure

Exposure to chemicals and substances, such as formaldehyde and asbestos, silica, chromium, is an 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 exposures to chemicals and substances is considered a significant cause of lung cancer.

Some industrial chemicals associated with lung cancer include:

  • Asbestos
  • Arsenic
  • Chromium compounds
  • Nickel compounds
  • PAHs (polycyclic aromatic hydrocarbons)
  • Vinyl chloride
  • Wood dust
  • Crystalline silica (silica 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 Data Safety sheets that employers are required to provide on any chemicals you may be exposed to at work.

It's estimated that in the United States, occupational exposures are a contributing factor in between 13% and 29% of lung cancers in men; that number changes to roughly 5% for women.

Radiation

Radiation, primary X-radiation and gamma radiation in the form of radiotherapy, diagnostic radiation, and environmental background radiation, is a risk factor for lung cancer.

People who have radiation therapy to the chest for cancers such as Hodgkin's disease (a type of lymphoma) or following a mastectomy for breast cancer have an increased risk of developing lung cancer. Radiation therapy after a lumpectomy for breast cancer does not appear to increase risk.

The risk is higher when radiation is received at a younger age and can vary depending upon the dose of radiation received.

Lung Diseases 

Even though chronic obstructive pulmonary disease (COPD) and lung cancer are both caused by smoking, COPD is an independent risk factor for lung cancer. This means that if two people smoked the same amount, or if neither smoked, the person who had COPD would be much more likely to develop lung cancer.

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

Asthma appears to be a risk factor as well. It's thought that pulmonary fibrosis increases the risk of lung cancer by 40%. Tuberculosis raises risk as well.

Medical Conditions 

People with certain cancers and other health conditions appear to have an increased risk of lung cancer (whether due to genetics, common exposures, or treatments such as radiation). These include:

  • Hodgkin's disease
  • Non-Hodgkin's lymphoma
  • Testicular cancer
  • Uterine sarcoma
  • Head and neck cancers
  • Esophageal cancer
  • Bladder cancer
  • Chronic lymphocytic leukemia
  • Cervical cancer
  • Kidney cancer
  • HIV
  • Autoimmune diseases, such as rheumatoid arthritis

Organ transplant recipients are also at greater risk.

Infections

Most 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

Just because two things are correlated does not mean that one causes the other. An example used often is that 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. The link between HPV and lung cancer mentioned above is one in which we don't yet know if there is causation, even though there is sometimes correlation.

Genetics

Overall, approximately 8% of lung cancer cases are considered hereditary. Genetic factors are more likely to be at play when lung cancer develops in non-smokers, women, and people under the age of 60.

Having a first-degree relative (a mother, father, sibling, or child) with lung cancer doubles the risk of developing lung cancer, whereas having a second-degree relative with lung cancer (aunt, uncle, nephew, or niece) raises your risk by around 30%.

With an increased understanding of genetics, some of the factors responsible for this risk are being identified. An example of this is the tumor suppressor gene known as BRCA2. Well-known for its association with breast cancer, 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 medications increase bradykinin in the lungs, which has been known to stimulate the growth of lung cancer, and also result in the accumulation of substance P, which has been associated with the growth of cancer.

A large (more than 300,000 people) 2018 study found that people who used ACE inhibitors were 14% more likely to develop lung cancer. The risk was associated with longer-term use and did not become apparent until at least five years of use, with the greatest risk associated with more than 10 years of use. Drugs in this category that were studied 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 supports, such as cessation aids.

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

Diet and Food Supplements

Cured meats (e.g., sausage, pressed duck, cured pork, etc.), deep-fried cooking, and chili have been associated with an increased lung cancer risk. Although some studies indicate carotenoids decrease lung cancer risk, results have been ambiguous, and some have even indicated that high-dose supplements of vitamin A can be harmful.

Alcohol

From a pooled analysis of seven prospective trials and 3,137 lung cancer cases, a slightly greater risk of lung cancer was indicated among people who consumed at least 30 grams (0.6 ounces) per day of pure alcohol.

Lung Cancer Screening

Currently, lung cancer screening is recommended for people who are between the ages of 55 and 74 who have at least a 30 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.

A Word From Verywell

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 for earlier stages of the disease, which may be detected by screening, are much higher.

Knowing your risk factors is essential to your prevention efforts. This awareness should also encourage you to take note of early symptoms of lung cancer and get them checked out as soon as possible, as well as ask your doctor if lung cancer screening is appropriate for you.

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