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. For more detailed information on radon and its possible health effects, see our document, Radon.
Ionizing radiation is used in the diagnosis and treatment of some medical conditions. This can be in the form of radiation that penetrates from outside the body, or radioactive particles that are swallowed or inserted into the body.
Imaging tests: Certain types of imaging tests, such as x-rays, CT scans, and nuclear medicine tests (such as PET scans and bone scans) expose people to low levels of radiation in order to create internal pictures of the body. (MRI and ultrasound exams do not use ionizing radiation.)
The increased risk of cancer from exposure to any single test is likely to be very small. Still, concerns have been raised in recent years as the average amount of radiation a person is exposed to from medical tests has risen. Children’s growing bodies are especially sensitive to radiation.
Because of the very small but real risk, and the fact that radiation exposure from all sources can add up over one’s lifetime, imaging tests that use radiation should only be done if there is a good medical reason to do so. The usefulness of the test must always be balanced against the possible risks from exposure to the radiation. In some cases, other imaging tests such as ultrasound or MRI may be an option. But if there is a reason to believe that an x-ray or CT scan is the best way to look for cancer or other diseases, the patient will most likely be helped more than the small dose of radiation can hurt.
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