Well, the natural radiation is mainly cosmic rays interacting with the earth's atmosphere, and we get a good bit of radiation on the earth's surface. The closer you are to the cosmos, the more radiation you get. So that if you're up in an airplane, you'll get considerably more than on the Earth's surface. Or people living at 5,000 feet, as I do, will get more than people living at sea level. But it's a part of the human environment just as air is, or anything else. It's most unremarkable.