Background Radiation

Radiation is simply energy released in the form of electromagnetic waves or particles. With reference to health, radiation generally refers to high-energy forms of radiation that have the potential to cause death of the cell or mutations in the DNA. Radiation has always been present everywhere on earth; indeed, radiation accounts for part of the baseline mutation rate in nature and so contributed to the evolution of life as we know it. The natural radiation that everyone is exposed to is referred to as background radiation and is the baseline rate against which other radiation exposures are compared.

There are three general outcomes when radiation energy is transferred to cells and disrupts biological molecules: (1) the cell dies, (2) the cell repairs itself properly and there is no residual damage, or (3) the cell repairs itself improperly resulting in a change. While cell death can be exploited by medicine to kill tumor cells with high-dose radiation therapy, permanent changes in DNA from low-dose radiation over years can sometimes result in cancer.

Although radiation exposure in certain professions is higher due to X-rays from medical devices or radiation from nuclear reactors, the low natural baseline level of radiation all around us—background ra-diation—constitutes the vast majority of the average person's radiation exposure. Background radiation comes from (1) extraterrestrial sources such as cosmic rays from outer space, (2) terrestrial sources such as radon gas, and (3) within one's body itself because of the decay of mildly unstable isotopes of potassium40 and carbon-14.

Many things can contribute to the amount of background radiation an individual is exposed to at a given time: flying in an airplane where there is less atmosphere to shield against cosmic rays, being inside buildings, certain foods such as bananas that contain high levels of phosphate, or living in a particular geographic area that has more radioactive isotopes. However, people normally spend small amounts of time in airplanes or doing jobs involving minimally radioactive substances such as phosphate fertilizer, so these enhanced exposures are usually insignificant compared to the normal background radiation. To put background radiation into perspective, a commercial round-trip flight across the United States is equivalent to adding about a month of average background radiation, a total body computerized tomography (CT) scan is equivalent to four years of average background radiation exposure, mammography is equivalent to about four months of background radiation, and a simple chest X-ray [Page 200]is comparable to background radiation exposure for just 10 days.

Radon and its decay products are the most important sources of background radiation. Although it had been known that those working in mines with high levels of radon were at risk for lung cancer, studies in the 1970s first demonstrated that indoor radon exposures in basements of houses could be as high as those in the mines; subsequent studies have shown that outdoor levels, although less than indoor, also represent a significant portion of the average person's radon exposure. The link between radon exposure and lung cancer is now well established, and in the United States, radon is the second leading cause of lung cancer after cigarettes.

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