It is often not possible to tell exactly what causes any cancer. The time between radiation exposure and the detection of cancer is known as the latent period. Cancers associated with high-dose exposure (greater than 50,000 mrem, or 500 mSv-500 times the NRC limit to the public) include leukemia, breast, bladder, colon, liver, lung, esophagus, ovarian, multiple myeloma and stomach cancers. Much of this information comes from survivors of the atomic bombs in Japan and people who have received radiation for medical tests and therapy. The data on links between radiation exposure and cancer are mostly based on populations receiving high level exposures. Living cells exposed to radiation could: (1) repair themselves, leaving no damage (2) die and be replaced, much like millions of body cells do every day or (3) incorrectly repair themselves, resulting in a biophysical change. The body is able to repair damage from radiation, chemicals and other hazards. For low levels of exposure, the biological effects are so small they may not be detected. We tend to think of the effects of radiation in terms of how it impacts living cells. NRC's radiation exposure limits are contained in Title 10 of the Code of Federal Regulations, Part 20. Exposure to adults working with radioactive materials must be below 5,000 mrem (50 mSv) per year. But the NRC does require its licensees to limit exposure to members of the public to 100 mrem (1 mSv) per year above background. The NRC does not regulate background radiation. Natural and man-made radiation may come from different sources, but both affect us in the same way. The pie chart below shows the sources that make up the average annual U.S. Some consumer products such as tobacco, fertilizer, welding rods, exit signs, luminous watch dials and smoke detectors contribute about 10 mrem per year. Other medical procedures make up another 150 mrem or so each year. Among the largest of these sources is a computed tomography scan, which accounts for about 150 mrem. Man-made sources of radiation from medical, commercial and industrial activities contribute roughly 310 mrem more to our annual exposure. Cosmic, terrestrial, and internal radiation account for the rest. Radon and thoron gases account for two-thirds of this exposure. resident receives an annual radiation exposure from natural sources of about 310 millirem (3.1 millisieverts or mSv). So people living in Colorado are exposed to more background radiation than residents of the coasts. It also has more terrestrial radiation from soils rich in naturally-occurring uranium. For example, Colorado, because of its altitude, has more cosmic radiation than the East or West Coast. Background levels can vary greatly from one location to the next. This natural radiation that is always present is known as "background" radiation. A lot of our exposure is due to radon, a gas from the Earth's crust that is present in the air we breathe. Capitol, which is largely built of granite, contains more radiation than most homes. Brick and stone homes have higher radiation levels than homes made of other materials such as wood. Some foods such as bananas and Brazil nuts naturally contain higher levels of radiation. It is in the air we breathe, the food we eat, the water we drink and the materials used to build our homes. It comes from outer space (cosmic), the ground (terrestrial) and even from within our own bodies. As a result, life has evolved in the presence of significant levels of ionizing radiation. It is in our environment and has been since the Earth was formed. Backgrounder on Biological Effects of Radiation
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