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 ionization.
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 miners.
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 safe.
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 exposure.
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 sources:
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 radiation exposure.
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.
Natural background radiation
We are all exposed to some amount of radiation just from being on this planet. This is known as background radiation. For most people, background radiation accounts for most of their exposure to ionizing radiation during the year. It comes from several different sources.
Cosmic rays: Cosmic rays are radioactive particles that hit the earth from outer space. They come from the sun and from other stars. The earth's atmosphere blocks a portion of these rays, but some of them reach the ground. Because the atmosphere blocks some cosmic rays, exposure is greater at higher altitudes. For example, people who live in Denver, Colorado, which is at a high elevation, are exposed to slightly more cosmic rays than people living at sea level. People are also exposed to higher levels of cosmic rays during airplane flights. Airline pilots and flight attendants, who spend many hours at high elevations, are exposed to more of these rays, but it is not clear if they have an increased risk of cancer because of it.
Radiation in the earth: People are also exposed to small amounts of radiation from radioactive elements that occur naturally in rocks and soil. Some of these may end up in building materials used in houses and other structures. Tiny amounts of radiation may even be found in drinking water and in some plant-based foods as a result of being in contact with the soil. For people who smoke, tobacco can account for a significant portion of the yearly radiation they receive.
Radon: The largest source of natural background radiation for most people is radon. This is an odorless, colorless gas that is formed from the breakdown of radioactive elements in the ground. Radon levels are usually higher inside buildings and homes, especially in levels closer to the ground such as basements. Radon levels can vary a great deal, depending on where you live.
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