As mentioned in Chapter 3, there is a much higher number of radioactive isotopes than stable isotopes. Radioactive isotopes, including natural and artificial ones, are present in the environment, causing part of the background radiation. Radiation of the natural radioactive isotopes has affected living organisms during their evolu­tion. Obviously, living organisms could adapt to natural background radiation. The increase in radiation that results from the anthropogenic-generated radioactive iso­tope effects must always be compared to natural radioactivity when its effects on living organisms are discussed.