Uranium -238 as it exists in nature is unstable and gives off natural radioactivity of alpha and beta particles as it "decays" to the stable isotope of lead - 206. This natural radioactivity is present in a variety of rocks and minerals over wide spread areas of the world.
Four Interesting Isotopes:
Ra = Radium: This isotope is sometimes found in drinking water. A few years ago, before the western suburbs obtained the main drinking water supplies from Lake Michigan, radium was found in the well water, and in some cases was over the EPA limit.
Rn = Radon: This isotope is of concern because as a rare gas it can migrate upward through the rocks and enter houses and building through cracks in the floor or through the water in the sump pump drainage hole. It is estimated that about 25% of the houses in the Chicago area are over the EPA limit for radon gas.
Po = Polonium: This isotope should be of concern for the people that smoke. Radium further back in the series is found in small quantities mixed in with the phosphate fertilizer used to grow the tobacco plants. As a result some of the radium is found on the tobacco leaves. It gradually decays so the this particular isotope may get in the lungs attached to smoke particles. Once in the lungs it may become stuck there and emit alpha particles, which in turn is probably the cause of about half of the lung cancers due to smoking. The other half is caused by the carcinogenic chemicals produced by smoking the tobacco in the "tars".
Th = Thorium: In a plant in West Chicago, thorium was once extracted from from ores and used to make lantern mantles for campfire lanterns. The remains of the soil from this extraction are still very slightly radioactive. The plant has been closed for about 30 years. Some of this radioactive soil was spread around in parks, and home backyards in West Chicago. The rest in stored in large piles on the site of the old plant. There have been law suits going on for about 20 years, with the final result that this soil has been and still is being moved to a land fill in Utah.