![]() 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. Radon comes from the decay of uranium, and uranium is an ion in solid minerals but radon is an inert gas so is free to move once it comes into existence (replaces the uranium parent).Backgrounder on Biological Effects of Radiation It leaves the mineral crystal and migrates upward like all low-density gases. Uranium is a fairly uncommon element and usually occurs as a replacement element for the primary cation (positively charged element) in the mineral. Typically, the potassium spots in a structure are where the uranium ends up as a minor constituent (potassium and uranium are similar atom sizes). Both are very large ions so both require a big space in the structure or they simply do not get included. (we are not talking about the fairly rare places where uranium concentrations are so high that actual uranium minerals are formed). There is a strong correlation between potassium content and uranium content in natural rocks. The uranium content is still way lower than potassium content, though. The rocks won't be highly radioactive (unlike actual uranium ore would be). Radon can become a problem when it is either generated in larger amounts in a source rock, or because the radon gets blocked after it migrates for a while. Rocks like granite which are relatively rich in potassium right at formation, and rocks that come from weathering of such potassium rich rocks, tend to have elevated uranium contents, and thus tend to be associated with elevated radon in an area. Many shales come from erosion of uranium-enriched parent rock so the shale itself will have higher than typical uranium contents. This is far from a strict rule though, lots of shales have basically no uranium at all.
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