Exposure and Cancer Part 2
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Ionizing radiation has enough energy to knock electrons off
of atoms or molecules. This is called ionization. Ionized
molecules are unstable and quickly undergo chemical changes.
If ionizing radiation passes through a cell in the body, it
can lead to mutations (changes) in the cell's DNA, the part
of the cell that contains its genes (blueprints). This could
contribute to cancer, or to the death of the cell. The
amount of damage in the cell is related to the dose of
radiation it receives. The damage takes place in only a
fraction of a second, but other changes such as the
beginning of cancer may take years to develop.
Types of ionizing radiation include x-rays, gamma rays, some
high-energy UV rays, and particles given off by radioactive
materials such as alpha particles and protons. These forms
of radiation have different energy levels and can penetrate
cells to different extents, but all are capable of causing
Does ionizing radiation cause cancer?
Ionizing radiation is a proven human carcinogen (cancer
causing agent). The evidence for this comes from many
different sources, including studies of atomic bomb
survivors in Japan, people exposed during the Chernobyl
nuclear accident, people treated with high doses of
radiation for cancer and other conditions, and people
exposed to high levels of radiation at work, such as uranium
Most studies on radiation and cancer risk have looked at
people exposed to very high doses of radiation in the
settings above. It is harder to measure the much smaller
increase in cancer risk that might come from much lower
levels of radiation exposure. Most studies have not been
able to detect an increased risk of cancer among people
exposed to low levels of radiation. For example, people
living at high altitudes, who are exposed to more natural
background radiation from cosmic rays than people living at
sea level, do not have noticeably higher cancer rates.
Still, most scientists and regulatory agencies agree that
even small doses of ionizing radiation increase cancer risk,
although by a very small amount. In general, the risk of
cancer from radiation exposure increases as the dose of
radiation increases. Likewise, the lower the exposure is,
the smaller the increase in risk. But there is no threshold
below which ionizing radiation is thought to be totally
Although radiation exposure affects the occurrence of
various types of cancer, it does not affect their
aggressiveness (tendency to grow and spread).
Types of cancer linked to ionizing radiation
Ionizing radiation increases the risk of certain types of
cancer more than others.
The thyroid gland and bone marrow are particularly sensitive
to radiation. Leukemia, a type of cancer that arises in the
bone marrow, is the most common radiation-induced cancer.
Leukemias may appear as early as a few years after radiation
Other types of cancer can also result from radiation
exposure, although they may take longer to develop (usually
at least 10 to 15 years).
Some of the other cancers most strongly linked to radiation
exposure in studies include:
• Lung cancer
• Skin cancer
• Thyroid cancer
• Multiple myeloma
• Breast cancer
• Stomach cancer
These are not necessarily the only cancer types that may be
linked to radiation, however.
The types of cancer linked to radiation are also affected by
the part of the body that is exposed. For example, people
who get pelvic radiation therapy would not be expected to
have higher rates of cancers in the head and neck because
these areas weren't exposed to radiation.
Other factors may also play a role in how likely a person
exposed to radiation is to develop cancer. Age is one such
factor - children's growing bodies are more sensitive to
radiation than adults. A person may also have gene changes
that make their cells more vulnerable to radiation damage,
which might in turn raise their risk more than in someone
without these gene changes.
Sources of ionizing radiation
People may be exposed to ionizing radiation from 3 main
• Natural background radiation comes from cosmic rays from
our solar system and radioactive elements normally present
in the soil. This is the major contributor to worldwide
• Medical radiation comes in the form of diagnostic x-rays
and other tests, as well as from radiation therapy.
Radiation therapy is currently used to treat some types of
cancer and involves dosages many thousand times higher than
those used in diagnostic x-rays.
• Non-medical, man-made radiation can come from workplace
and other sources, and is also a result of above ground
nuclear weapons testing that took place before 1962.
Radiation Exposure and Cancer Part 1
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