Causes
of lung Cancer
Smoking
The incidence of lung cancer is strongly correlated with cigarette smoking, with
about 90% of lung cancers arising as a result of tobacco use. The risk of lung
cancer increases with the number of cigarettes smoked over time; doctors refer
to this risk in terms of pack-years of smoking history (the number of packs of
cigarettes smoked per day multiplied by the number of years smoked). For
example, a person who has smoked two packs of cigarettes per day for 10 years
has a 20 pack-year smoking history. While the risk of lung cancer is increased
with even a 10 pack-year smoking history, those with 30 pack-year histories or
more are considered to have the greatest risk for the development of lung
cancer. Among those who smoke two or more packs of cigarettes per day, one in
seven will die of lung cancer.
Pipe and cigar smoking can also cause lung cancer, although the risk is not as
high as with cigarette smoking. While someone who smokes one pack of cigarettes
per day has a risk for the development of lung cancer that is 25 times higher
than a nonsmoker, pipe and cigar smokers have a risk of lung cancer that is
about five times that of a nonsmoker.
Tobacco smoke contains over 4,000 chemical compounds, many of which have been
shown to be cancer-causing, or carcinogenic. The two primary carcinogens in
tobacco smoke are chemicals known as nitrosamines and polycyclic aromatic
hydrocarbons. The risk of developing lung cancer decreases each year following
smoking cessation as normal cells grow and replace damaged cells in the lung. In
former smokers, the risk of developing lung cancer begins to approach that of a
nonsmoker about 15 years after cessation of smoking. For more, please read the
Smoking and Quitting Smoking
article.
Passive smoking
Passive smoking, or the inhalation of tobacco smoke from other smokers sharing
living or working quarters, is also an established risk factor for the
development of lung cancer. Research has shown that non-smokers who reside with
a smoker have a 24% increase in risk for developing lung cancer when compared
with other non-smokers. An estimated 3,000 lung cancer deaths occur each year in
the U.S. that are attributable to passive smoking.
Asbestos fibers
Asbestos fibers are silicate fibers that can persist for a lifetime in lung
tissue following exposure to asbestos. The workplace is a common source of
exposure to asbestos fibers, as asbestos was widely used in the past for both
thermal and acoustic insulation materials. Today, asbestos use is limited or
banned in many countries including the Unites States. Both lung cancer and
mesothelioma (a type of cancer of the pleura or of the lining of the abdominal
cavity called the peritoneum) are associated with exposure to asbestos.
Cigarette smoking drastically increases the chance of developing an
asbestos-related lung cancer in exposed workers. Asbestos workers who do not
smoke have a fivefold greater risk of developing lung cancer than non-smokers,
and those asbestos workers who smoke have a risk that is 50 to 90 times greater
than non-smokers.
Radon gas
Radon gas is a natural, chemically inert gas that is a natural decay product of
uranium. It decays to form products that emit a type of ionizing radiation.
Radon gas is a known cause of lung cancer, with an estimated 12% of lung cancer
deaths attributable to radon gas, or 15,000 to 22,000 lung cancer-related deaths
annually in the U.S. As with asbestos exposure, concomitant smoking greatly
increases the risk of lung cancer with radon exposure. Radon gas can travel up
through soil and enter homes through gaps in the foundation, pipes, drains, or
other openings. The U.S. Environmental Protection Agency estimates that one out
of every 15 homes in the U.S. contains dangerous levels of radon gas. Radon gas
is invisible and odorless, but can be detected with simple test kits.
Familiar predisposition
While the majority of lung cancers are associated with tobacco smoking, the fact
that not all smokers eventually develop lung cancer suggests that other factors,
such as individual genetic susceptibility, may play a role in the causation of
lung cancer. Numerous studies have shown that lung cancer is more likely to
occur in both smoking and non-smoking relatives of those who have had lung
cancer than in the general population. Recent research has localized a region on
the long (q) arm of the human chromosome number 6 that is likely to contain a
gene that confers an increased susceptibility to the development of lung cancer
in smokers.
Lung diseases
The presence of certain diseases of the lung, notably chronic obstructive
pulmonary disease (COPD), is associated with a slightly increased risk (four to
six times the risk of a nonsmoker) for the development of lung cancer even after
the effects of concomitant cigarette smoking are excluded.
Prior history of lung cancer
Survivors of lung cancer have a greater risk than the general population of
developing a second lung cancer. Survivors of non-small cell lung cancers (NSCLCs,
see below) have an additive risk of 1-2% per year for developing a second lung
cancer. In survivors of small cell lung cancers (SCLCs) the risk for development
of second cancers approaches 6% per year.
Air pollution
Air pollution, from vehicles, industry, and power plants, can raise the
likelihood of developing lung cancer in exposed individuals. Up to 1% of lung
cancer deaths are attributable to breathing polluted air, and experts believe
that prolonged exposure to highly polluted air can carry a risk similar to that
of passive smoking for the development of lung cancer.
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