Causes and Risk Factors of Lung Cancer

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

For some people at risk, lung cancer screening is now available. While knowing your risk factors is important, many people who develop the disease don't have any obvious risk factors, and lung cancer is actually increasing in young women who have never smoked. Anyone who has lungs can get lung cancer.

There are many common risk factors for lung cancer, though many people only think of smoking. This is unfortunate, as the focus on smoking sometimes overshadows other significant risk factors.

Smoking

Smoking is the number one cause of lung cancer, being responsible for around 80 percent of lung cancers in the United States. The risk of someone who smokes developing lung cancer is 13 to 23 times greater than 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 percent of women who develop lung cancer are lifelong non-smokers; worldwide, only 50 percent 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, but there is good evidence that the recently popularized 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 lung cancer 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 percent to 30 percent 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 published in the Journal of the National Cancer Institute 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.

Air Pollution

Both indoor and outdoor air pollution can raise the risk of lung cancer. 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. 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 at home and on the job, such as formaldehyde and asbestos, silica, chromium, is an important risk factor for lung cancer, especially when combined with smoking.

Occupational Exposure

On-the-job exposures to chemicals and substances is 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.

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 COPD (chronic obstructive pulmonary disease) 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 still be much more likely to develop lung cancer. Overall, the chance that someone who has COPD will develop lung cancer (aside from the smoking risk) 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 felt that pulmonary fibrosis increases the risk of lung cancer by 40 percent, and tuberculosis raises risk as well.

Medical Conditions 

People with certain cancers appear to have an increased risk of lung cancer (whether due to genetic causes, 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, and kidney cancer. In addition, people with HIV, autoimmune diseases such as rheumatoid arthritis, and organ transplant recipients also have an increased risk of developing lung cancer.

Infections

We don't often think of infections as a cause of cancer, but a tenth of cancers in the United States and about 25 percent worldwide are related to infectious diseases. Recent 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.

Uncommon Risk Factors

There are a number of less common, but still important risk factors to be aware of. Since lung cancer is considered a multifactorial disease, meaning that many factors may work together to either increase or decrease risk , an understanding of all factors should be considered when it comes to understanding your personal risk.

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 percent.

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. This gene was popularized as one of the "breast cancer genes" by Angelina Jolie , but what's heard less widely spoken of is that inherited BRCA2 mutations may increase lung cancer risk, especially in women who smoke.

Overall, approximately 8 percent 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.

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 lung, 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 percent 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 without the disease found that an elevated platelet count was associated with an increased risk. People with high counts were 62 percent more likely to develop non-small cell lung cancer and 200 percent 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.

Smoking and exposure to secondhand smoke, as previously mentioned, are two major risk factors that you can modify. 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. At the current time, roughly 40 percent of people are initially diagnosed when lung cancer has already progressed to stage 4—a stage at which curative surgery is not possible and the 5-year survival rate is 4 percent. In contrast, the survival rates for earlier stages of the disease which may be detected by screening are much higher.

Diet and Food Supplements

Cured meat (eg. 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 7 prospective and 3137 lung cancer cases, a slightly greater risk of lung cancer was indicated among people who consumed at least 30 g/day of alcohol.

A Word From Verywell

Knowing your risk factors is important for a few reasons. Understanding that a substance such as radon or an occupational chemical, or a practice such as smoking raises your risk, you have a better opportunity to avoid that risk. If you find you are at risk for lung cancer, you can be aware of the earliest symptoms of lung cancer, as well as asking your doctor if lung cancer screening is appropriate.