Any substance or condition that increases cancer risk is referred to as a risk factor. The most important risk factor for developing lung cancer is smoking. However, most cancers are the result of many risk factors.
*Risk factors are generally listed in order from most significant to least significant. In most cases, it is impossible to rank the relative significance of individual risk factors with absolute certainty.
The risk of developing lung cancer increases with age. More than half of all newly diagnosed lung cancer cases occur among people aged 70 years or older. Men develop lung cancer slightly more often than women.
The following factors are known to increase the risk of developing lung cancer.
Smoking tobacco
Smoking tobacco, particularly cigarettes, is the main cause of lung cancer.
- Tobacco smoke contains many harmful chemicals that can cause cancer (are carcinogenic). Smoking affects a person’s health and causes genetic changes in the cells of the lung that lead to the development of lung cancer.
- Smoking is related to more than 85% of lung cancer cases in Canada.
- The risk of developing lung cancer is influenced by how long a person smoked, their age when they started smoking and the number of cigarettes smoked each day.
- When smoking is combined with other risk factors, the risk of lung cancer is increased.
Other types of tobacco products such as low-tar and low-nicotine cigarettes, pipes, cigars, herbal cigarettes, hookahs and chewing tobacco also cause cancer and are not considered safe.
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Second-hand smoke
Second-hand smoke is what smokers exhale and what rises from a burning cigarette, pipe or cigar. People exposed to second-hand smoke have an increased risk of lung cancer. Second-hand smoke is a main risk factor for lung cancer among non-smokers.
- Second-hand smoke is also called environmental tobacco smoke (ETS), or involuntary or passive smoking.
- Second-hand smoke contains the same chemicals as smoke that is actively inhaled.
- No amount of exposure to second-hand smoke is safe.
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Radon
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 a home or building through dirt floors or cracks in basement foundations. It may reach unsafe levels in enclosed, poorly ventilated homes or buildings because of seepage into the basement. Breathing in radon gas can damage cells that line the lungs.
- Radon exposure increases the risk of 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 a person is exposed to, how long they are exposed as well as whether or not they smoke. The risk from radon is much higher in people who smoke than in those who don't.
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Asbestos
Asbestos is group of minerals that occur naturally. Asbestos has been widely used in building materials and many industries. Exposure to asbestos fibres in the air that people breathe increases the risk of lung cancer.
- The risk of asbestos exposure is highest for people who work with asbestos, such as miners or those who work with it in manufacturing.
- Studies have shown that the combination of smoking and asbestos exposure is especially hazardous. People who are exposed to asbestos and also smoke are at even greater risk of developing lung cancer.
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Occupational exposure to chemical carcinogens
The risk of lung cancer is increased among people who work in certain industries and occupations. Occupational exposure to the following cancer-causing chemicals increases the risk of lung cancer:
- arsenic and inorganic arsenic compounds
- beryllium and beryllium compounds
- cadmium and cadmium compounds
- chemicals in rubber manufacturing, iron and steel founding, and painting – includes numerous chemicals and the exact ones that increase the risk of lung cancer are not known
- chloromethyl ethers and bischloromethylether
- chromium (VI) compounds
- cobalt-tungsten carbide
- diesel engine exhaust
- mustard gas
- polycyclic aromatic hydrocarbons (PAHs) –through occupational exposure to soot (chimney sweeping), coal gasification, coke production, coal tar pitches (roofing, paving) and aluminum production
- radioactive ores such as uranium and plutonium
- silica dust and crystalline silica
- some nickel compounds
These are rare causes of lung cancer because normally a person would only come across these chemicals in large amounts if they worked with them.
In general, for many of these substances, the risk of developing lung cancer is even higher for people who smoke.
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Personal or family history of lung cancer
People who have had lung cancer have an increased risk of developing a second lung cancer. First-degree relatives (siblings, children or parents) of people who have had lung cancer may have a slightly higher risk of developing lung cancer themselves.
- The increased risk among first-degree relatives 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).
- Studies of families with a strong history of lung cancer have found that the increased risk might be due to a mutation in a lung cancer gene.
- Other studies have shown that the risk of lung cancer in a family increases if a family member developed the disease at an early age.
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Arsenic
Drinking water with high levels of arsenic increases the risk of developing lung cancer. The risk is further increased in people who smoke.
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Previous lung disease
People who have had lung diseases or conditions that scar the lungs have an increased risk of lung cancer. Examples of these conditions include:
- tuberculosis (TB) – a lung infection spread by inhaling tuberculosis bacteria
- chronic obstructive pulmonary disease (COPD) – a long-term disease (including chronic bronchitis and emphysema) that damages the lungs and is usually caused by smoking
- lung infection caused by Chlamydophila pneumoniae
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Exposure to radiation
The risk of lung cancer increases for people who have had previous exposure to ionizing radiation.
- People who have been treated with radiation therapy to the chest for cancers such as Hodgkin lymphoma or breast cancer are at increased risk for lung cancer. The risk is further increased in people who smoke.
- People in Japan who were exposed to ionizing radiation during atomic bomb explosions are at greater risk of developing lung cancer.
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Indoor burning of coal
Indoor burning (combustion) of solid fuels such as coal has been linked to lung cancer. Cooking and heating with coal on open fires or traditional stoves results in high levels of indoor air pollution, especially in poorly ventilated spaces. Indoor smoke contains a range of health-damaging pollutants. There is sufficient evidence that indoor combustion of coal increases the risk of lung cancer.
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Outdoor air pollution
Outdoor air pollution appears to increase the risk of lung cancer. Pollutants in the air vary among locations because of local emissions and emissions that move in from other regions. Some research suggests that air pollution in cities has stronger cancer-causing effects on the lungs of smokers.
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Weakened immune system
People with a weakened immune system have an increased risk of developing lung cancer. The immune system can become weakened for a number of reasons, such as:
- drugs that suppress the immune system (called immunosuppressants) – These drugs are used to prevent rejection of an organ after a transplant.
- HIV infection and AIDS – People with HIV/AIDS have a higher risk of developing many types of cancer.
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Lupus
Systemic lupus erythematosus (SLE or lupus) is an autoimmune disorder. 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.
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Possible risk factors
The following factors have some association with lung cancer, but there is not enough evidence to say they are known risk factors. Further study is needed to clarify the role of these factors for lung cancer.
- smoking marijuana – The evidence suggesting a link between smoking marijuana and cancer is not as strong or as comprehensive as the evidence linking tobacco and cancer. Not all studies have found a consistent relationship between long-term recreational smoking of marijuana and an increased risk of cancer.
- indoor burning of wood – The evidence shows that there is a potential risk of lung cancer from burning wood indoors. Indoor burning (combustion) of solid fuels such as wood results in high levels of indoor air pollution, especially in poorly ventilated spaces.
- high-temperature frying – The evidence shows that cooking oil emissions from high-temperature frying (such as deep-frying, pan-frying and stir-frying) are probably carcinogenic to humans.Some studies have shown an increased risk for lung cancer in people who fry foods in oil at high temperatures. The risk is influenced by the frequency and duration of high-temperature frying.
- diet – More research is needed to clarify the role that diet plays in lung cancer risk. Some studies have reported a decreased risk of developing lung cancer in people who eat a diet high in vegetables and fruit. Some studies have shown a reduced risk of lung cancer with a high intake of selenium. Other studies have shown that smokers who take beta carotene supplements may have an increased risk of lung cancer.
- physical inactivity – People who are not physically active may be at a higher risk for lung cancer, but the research is not conclusive.
- occupational exposure to certain chemicals – There may be some association between lung cancer and occupational exposure to vinyl chloride, dioxin, cobalt-tungsten carbide, or strong inorganic acid mists, but more research is needed to be sure. Research to date does not provide a conclusive link between pesticides and cancer, but the evidence does suggest a possible association with lung cancer.
- removal of both ovaries – Some studies have suggested that removal of both ovaries (bilateral oophorectomy) may increase a woman's risk of developing lung cancer. More research is needed to understand this risk.
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Unknown risk factors
The following are factors for which there is not enough evidence or the evidence is inconclusive. In other words, it can't be determined for sure whether these risk factors are or are not associated with lung cancer.
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See a list of questions to ask your doctor about risks. Find out how to reduce the risk of lung cancer.