An Overview of Lung Cancer Causes

Though most people associate lung cancer with smoking, there are many causes. There are people who have smoked their entire life and did not develop lung cancer. Likewise, there are many lifelong non-smokers who develop the disease. In fact, lung cancer in never-smokers is the sixth leading cause of cancer deaths in the United States.

We aren't certain about the exact causes of lung cancer, but several risk factors have been identified. Some of these you may be very familiar with, but others are less well known. Lung cancer is a multifactorial disease, meaning that there are often several factors that work together to either increase or decrease risk. We know that some exposures are more than additive in conferring risk. For example, exposure to asbestos plus smoking raises the risk for lung cancer much more than if the two risks were simply added together. At the same time, there are some practices—such as exercising and eating a healthy diet—that may lower your risk.

How Do Environmental Exposures Cause Lung Cancer?

Understanding how exposures in our environment or lifestyle may raise lung cancer risk can help in evaluating possible causes. Some exposures may increase risk by directly damaging DNA in cells. These cancer-causing substances are referred to as carcinogens. Other exposures may create chronic inflammation. Increased cell division to replace and repair tissue damage that happens as a result increases the risk that errors—DNA mutations—may occur. It is this accumulation of DNA mutations in lung cancer cells that results in a normal cell becoming a cancer cell.

Considering Cancer Causes

It can be frightening to talk about cancer causes, and the sometimes sarcastic remark,"Doesn't everything cause cancer?" has been well earned. In reality, however, it is not easy to cause a normal cell to become a cancer cell. Most of the time this change does not occur until a series of mutations (either inherited or from the environment) damages the DNA of a cell enough that it no longer responds to normal commands telling it to stop dividing.

We also have genes in our bodies that code proteins designed to repair damaged cells or remove them before they have the opportunity to become cancer cells. Mutations in these genes, known as tumor suppressor genes, lie behind some genetic predispositions to cancer.

Causes and Possible Causes of Lung Cancer

There are some causes of lung cancer we are fairly certain about, and others where the jury is still out. Let's take a look at some of the known causes, probable causes, and possible causes of lung cancer, as well as how substances are studied to assess their role in causing lung cancer.


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.

In addition to cigarette smoking, cigar smoking is a risk factor for 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.

Radon Exposure

Exposure to radon gas in the home is the second leading cause of lung cancer and the most common cause of lung cancer in non-smokers. Radon is an odorless, colorless gas that is released from the normal decay of uranium in the soil beneath our homes. This gas can enter homes through cracks in the foundation and walls, around sump pumps, pipes, and drains, and accumulates in the air that circulates in our homes.

The World Health Organization estimates that up to 15 percent of lung cancers worldwide are due to radon exposure. Radon-induced lung cancer is felt to be responsible for around 27,000 deaths yearly in the United States. To understand the significance of this number, compare these deaths to the roughly 40,000 breast cancer deaths each year.

The only way to know if your home has an elevated radon level is to do radon testing. Inexpensive radon kits are available at most hardware stores or online. If levels are elevated, radon mitigation performed by a certified professional can almost always solve the problem and remove this risk.

Though likely much less important than homes built on granite, radon exposure from granite countertops may also be a risk factor for lung cancer.

Secondhand Smoke

Secondhand smoke is a cause of lung cancer and is responsible for roughly 7,000 lung cancer deaths per year in the United States. Living with someone who smokes raises your risk of developing lung cancer by 20 percent to 30 percent. And just like with firsthand smoke, secondhand smoke increases the risk of heart disease and other cancers.

Occupational Exposures

On-the-job exposures to chemicals and substances is a significant cause of lung cancer. It's estimated that in the United States, between 13 percent and 29 percent of lung cancers in men have occupational exposures as a contributing factor (that number changes to roughly 5 percent for women.)

Some industrial chemicals associated with lung cancer include:

  • Asbestos
  • Arsenic
  • Chromium compounds
  • 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 Data Safety sheets that employers are required to provide on any chemicals you may be exposed to at work.

Air Pollution

Though air pollution has long been recognized as a health hazard, it's only recently that its role in causing lung cancer has come to light. This risk varies tremendously around the world but is thought to be a contributing cause of lung cancer in 5 percent of men and 3 percent of women in the United States.

Wood Smoke and Cooking Smoke

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.


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, 1.7 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.

Lung Diseases

People with some lung diseases appear to have an increased risk of developing lung cancer. People with chronic obstructive pulmonary diseases (COPD), such as emphysema, have an increased risk of lung cancer. COPD is an "independent risk factor" for lung cancer, meaning that having this disease raises lung cancer risk and that this risk is independent of smoking. Tuberculosis also appears to raise the risk of lung cancer and may also result in a delay in diagnosis. Having pulmonary fibrosis, such as idiopathic pulmonary fibrosis, significantly increases the risk of lung cancer. There is also some evidence that asthma may raise the risk of lung cancer, especially in non-smokers

Other Medical Conditions

Our immune system play an important role in fighting cancer. Medical conditions that damage the immune system, such as HIV/AIDS, appear to increase the risk of lung cancer, as do autoimmune diseases such as rheumatoid arthritis. In addition, immune suppression for organ transplantation raises risk.

People with several other types of cancer are more prone to develop lung cancer as well. Some of this may be related to the treatments for cancer (chemotherapy and radiation therapy can cause secondary cancers) or due to genetic changes that predispose people to cancer in general.


We don't often think of infections as a cause of cancer, but a tenth of cancers in the United States and 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.

Radiation Exposure

Both radiation in the environment and medical radiation have been linked to lung cancer. Medical radiation used as a cancer treatment—for example, to treat Hodgkin's disease or after a mastectomy for breast cancer—can raise risk, especially for those who receive these treatments at a young age.

Diet and Dietary Supplements

A diet high in processed (cured) and red meats may increase lung cancer risk, whereas a diet rich in fruits and vegetables is associated with a lower risk. Alcohol intake may also be associated with an increased risk. We hear a lot about using dietary supplements, but the use of these to lower cancer risk needs to come with a word of caution. Upon learning that a diet high in beta-carotene (a plant phytochemical) was associated with a lower lung cancer risk, researchers studied the effects of a beta-carotene supplement on a large number of people. In contrast to dietary beta-carotene, supplemental beta-carotene was instead linked to an elevated risk of the disease.

Gender Differences and Estrogen

We know that some lung cancer cells have estrogen receptors and that lung cancer risk can vary with reproductive history and hormone replacement therapy. We also know that lung cancer in women varies relative to lung cancer in men. That said, we are only beginning to learn what, if any, effect that estrogen has on lung cancer risk.

Studying Lung Cancer Causes

Much of the problem in recognizing the possible causes of lung cancer lies in the difficulty of evaluating environmental exposures.

One problem is the cancer latency period. Most exposures—such as cigarette moking—do not cause cancer right away. The cancer latency period is defined as the amount of time between exposure to a cancer-causing substance (carcinogen) and the time at which cancer is diagnosed. If cigarette smoking were just invented 10 years ago, we would probably not recognize that it can be a cause of lung cancer.

Another problem in evaluating causes lies in study design. To determine most accurately if a substance causes cancer, we need to design a study that observes a substance over time and controls for other factors which may be at play. These are called prospective studies. Most of what we know of cancer causes, however, comes from retrospective studies. These studies look at people who have cancer and reach back in time to attempt to understand the causes. Many prospective studies are in progress, but information from many of them may not be available for decades.

How widespread an exposure is may also play a role in whether or not we learn it is a lung cancer risk factor. An exposure such as smoking, which is common globally, is easier to study that an exposure that occurs in a much smaller percentage of the population.

A final concern worth mentioning is correlation versus 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.

Reducing Your Risk of Lung Cancer

What quickly becomes clear when learning about lung cancer is that we don't yet understand all of the risk factors. This is even more clear as we recognize that lung cancer is increasing in one group of people: young, never-smoking women.

Until we know more, a few basic practices may reduce your exposure to potential carcinogens:

  • Read labels on products you use, whether at home or at work.
  • Use gloves when you handle anything you wouldn't put in your mouth; the whole idea behind prescribing some medications in "patch form" is that our skin can readily absorb many chemicals.
  • Make sure to wear a mask when it is called for, and to use good ventilation when working with chemicals such as paints and cleaning products.

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