Risk factors for lung cancer

A risk factor is something that increases the risk of developing cancer. It could be a behaviour, substance or condition. Most cancers are the result of many risk factors. Smoking tobacco is the most important risk factor for lung cancer.

The risk of developing lung cancer increases with age. More than half of all newly diagnosed lung cancer cases occur among people aged 60 years or older. Men develop lung cancer slightly more often than women.

Risk factors are generally listed in order from most to least important. But in most cases, it is impossible to rank them with absolute certainty.

There is convincing evidence that the following factors increase your risk for lung cancer.

Smoking tobacco, particularly cigarettes, is the main cause of lung cancer. There are many chemicals in tobacco smoke. Some of these chemicals are carcinogenic, which means that they cause changes to cells in the lung that can lead to the development of lung cancer.

About 72% of lung cancer cases in Canada are due to smoking tobacco. The risk of developing lung cancer increases with how long you have smoked, how old you were when you started smoking and the number of cigarettes you smoke each day. The risk is also higher if you smoke tobacco and have other risk factors.

Pipes, cigars, herbal cigarettes, hookahs, chewing tobacco, low-tar cigarettes and low-nicotine cigarettes also cause cancer and are not considered safe.

Second-hand smoke is what smokers exhale and what rises from a burning cigarette, pipe or cigar. It is also called environmental tobacco smoke (ETS). Breathing in second-hand smoke is called involuntary or passive smoking.

No amount of exposure to second-hand smoke is safe. It contains the same chemicals as smoke that is actively inhaled. People exposed to second-hand smoke have a higher risk for lung cancer. Second-hand smoke is a main risk factor for lung cancer among non-smokers.

Radon is a colourless, odourless, tasteless gas that comes from the natural breakdown of uranium in rocks and soil. In the outdoors, radon gas is diluted by fresh air, so it is not usually a concern. But radon can seep into buildings through dirt floors or cracks in the foundations. It may reach unsafe levels in enclosed, poorly ventilated spaces. Breathing in radon gas can damage cells that line the lungs.

Radon exposure increases the risk for lung cancer. Radon is the leading cause of lung cancer in non-smokers and the second leading cause of lung cancer in smokers.

The risk of developing lung cancer depends on how much radon you are exposed to and how long you are exposed to it. The risk for lung cancer from radon is much higher in people who smoke than in those who don’t.

Asbestos is the name of a group of minerals that occur naturally. It can be separated into long, thin fibres that are very fine. When someone breathes in these fibres, they can get trapped in the lungs.

Asbestos has been widely used in building materials and many industries. People who have the highest risk of asbestos exposure include:

• workers in asbestos mines
• automotive industry workers, including brake and clutch repair workers
• shipyard workers
• cement plant workers
• plumbing and heating tradespeople
• construction workers, painters, carpenters and electricians

Studies show that smokers who are exposed to asbestos have an even greater risk of developing lung cancer.

Certain chemicals are carcinogens, which means that they cause cancer. These chemicals can cause lung cancer in people who are exposed to them at work. In general the risk of developing lung cancer is even higher for smokers who are exposed to these chemicals.

Occupational exposure to the following chemicals increases the risk for lung cancer:

• arsenic and inorganic arsenic compounds
• beryllium and beryllium compounds
• cadmium and cadmium compounds
• chemicals used in rubber manufacturing, iron and steel founding and painting
• chloromethyl ethers and bis(chloromethyl) ether
• chromium (VI) compounds
• diesel engine exhaust
• mustard gas
• radioactive ores such as uranium and plutonium
• silica dust and crystalline silica
• some nickel compounds
• some types of polycyclic aromatic hydrocarbons (PAHs)
• bitumen used in roofing
• cobalt-tungsten carbide
• welding fumes

Some industries use many chemicals, so it is difficult for researchers to know which ones increase the risk for lung cancer. People who work in the following industries have the highest risk for lung cancer:

• rubber manufacturing
• iron and steel founding
• coal gasification
• coke production
• chimney sweeping
• commercial painting
• roofing and paving
• industries that use the Acheson process to create silicon carbide

Air pollution is chemicals, particles and other materials in the air in amounts that could damage the environment or harm the health or comfort of people, animals and plants. The types of pollutants in the air vary from place to place depending on sources of emissions in the area. Emissions can also move in from other regions.

There is strong evidence that exposure to outdoor air pollution causes lung cancer. The more air pollution you are exposed to, the greater your risk of developing lung cancer.

Research shows that the different components of outdoor air pollution cause cancer. These components include diesel engine exhaust, benzene, particulate matter and some PAHs.

People who had lung cancer have a higher risk of developing lung cancer again. You may also have a slightly higher risk for lung cancer if you have a first-degree relative (a brother, sister, child or parent) who had lung cancer. The increased risk could be due to a number of factors, such as shared behaviours (like smoking) or living in the same place where there are carcinogens (like radon).

Certain lung diseases or conditions can scar the lungs and increase the risk for lung cancer. Examples of these conditions include:

• chronic obstructive pulmonary disease (COPD), which is long-term damage to the lungs that is often caused by smoking
• emphysema and chronic bronchitis, which are types of COPD
• tuberculosis (TB), which is a lung infection caused by tuberculosis bacteria
• lung infection caused by Chlamydophila pneumoniae

People who were treated with radiation therapy to the chest for certain cancers, such as Hodgkin lymphoma or breast cancer, have a higher risk for lung cancer. These people have an even higher risk if they smoke.

People who were exposed to ionizing radiation during atomic bomb explosions or nuclear accidents have a greater risk of developing lung cancer.

Arsenic can get into drinking water from natural sources in the soil or from certain types of industries, such as mining. While experts don’t completely understand how arsenic causes changes to cells, studies from many different parts of the world show that high levels of arsenic in drinking water increase the risk for lung cancer. The risk is even greater in people who smoke.

HIV infection and AIDS can weaken the immune system. People with HIV/AIDS have a higher risk of developing many types of cancer, including lung cancer.

People who have an organ transplant take drugs to suppress their immune system so the body doesn’t reject the organ. Having a suppressed immune system increases their risk of developing lung cancer.

Some types of cooking and heating can release pollutants that increase the risk for lung cancer. The levels of these pollutants can be very high in spaces that have poor air flow.

Burning coal indoors for cooking and heating is most strongly linked to lung cancer. Burning wood and other fuels, such as dung or grass, and frying foods in oil at high temperatures can also increase the risk for lung cancer.

Systemic lupus erythematosus (SLE or lupus) is an autoimmune disease. Lupus can affect various parts of the body and cause inflammation of the skin, joints, blood vessels, nervous system and internal organs such as the heart, lungs and kidneys. People with lupus have a higher risk for lung cancer.

Beta carotene is a type of antioxidant. Some clinical trials found that people who smoked more than 1 pack per day and who were given high-dose beta carotene supplements had a greater risk of developing lung cancer.

The following factors have been linked with lung cancer, but there is not enough evidence to show they are known risk factors. More research is needed to clarify the role of these factors for lung cancer.

Researchers are trying to find out if the following chemicals increase the risk for lung cancer:

• bitumen used in paving
• dioxin used in pesticides
• strong chemical acid mists
• fibrous silicon carbide

Research shows that some families have a strong history of lung cancer, which may mean that they have a mutation in a certain gene that may cause lung cancer. Researchers are trying to find out if a specific gene or genes may increase the risk for lung cancer. Studies are also trying to find very small changes in genes (called genetic polymorphisms) that may increase the risk for lung cancer or make people, especially non-smokers, more sensitive to known risks for lung cancer.

The evidence suggesting a link between long-term smoking of cannabis (marijuana) and cancer is not as strong or as comprehensive as the evidence linking smoking tobacco and cancer. Some studies found that long-term recreational smoking of cannabis can increase the risk for lung cancer.

Research suggests that people who are not physically active may have a higher risk for lung cancer, whether they smoke or not.

Some studies report that people who eat a diet high in vegetables and fruit have a lower risk for lung cancer.

It isn’t known whether or not the following factors are linked with lung cancer. It may be that researchers can’t show a definite link or that studies have had different results. More research is needed to see if the following are risk factors for lung cancer:

• occupational exposure to synthetic fibres (such as glass wool)
• occupational exposure to vinyl chloride
• rheumatoid arthritis

In women, research is looking at the role of estrogen in the development of lung cancer. Studies show that reproductive factors, such as the number of children a woman has, age at menopause and removal of the ovaries, may increase a woman’s risk for lung cancer.

To make the decisions that are right for you, ask your healthcare team questions about risks.