In summary, the presence of radiation in our everyday lives comes from natural sources, such as cosmic rays and the decay of uranium in the earth, and from man-made sources such as medical x-rays, industrial gamma rays, and air travel.

Both natural and man-made sources each contribute approximately 50 percent to the total average effective dose to individuals in the United States. Of the 50 percent manmade sources, 2 percent is from consumer sources and 48 percent is from medical sources.

This lesson discussed radiation sources. In the next lesson, you'll learn more about the biological effects of radiation.