**2. Radioactive contamination**

Radioactive contamination started on the planet in 1945, when the first nuclear test was performed in Alamo Gordo, New Mexico, followed by the war actions in Hiroshima and Nagasaki. Since then, radioactive contamination at global level has been variable, depending on repeated nuclear tests, few accidents such as Three Mile Island and Chernobyl, and minor failures in nuclear power plants. These contaminants are produced mainly by fission products from 235U, which according their fission yielding and half lives, they remain radioactive during a time span from seconds to a great number of eons (1 eon = 1 x 109 years). But certainly, burned nuclear fuels which are under control and stored accordingly the safest techniques to guarantee they will always be confined and never disseminated in the environment, same case that residues of artificially produced radioisotopes used in medicine, industry or any other purpose, they should not be considered as radioactive contaminants, as much as they are under safe enough surveillance. So, approximately 30-40% all of known radioisotopes are fission products, which when they come into environment by deliberate nuclear explosion, severe accident or failure in nuclear plant, they represent the so called radioactive contamination. From this perspective, it seems that radioactive contamination has been growing up from its beginning, with rather short equilibrium periods. Also, if it is considered that sea water represents approximately 80% of planet surface, plus the action of wind, rain and rivers current, the main repository of radioactive contamination should be the sea. However, radioactive contamination is only added to natural radioactivity. From the first elements in the Periodic Table: 3H, 10Be and 14C, natural radioisotopes are either continuously produced by nuclear reactions in the earthly atmosphere, or they were created at same time that non radioactive ones, in the mixture of isotopes forming elements such as 40K, 50V and 87Rb. And then from Bi to beyond uranium elements, every isotope is radioactive with no exception. Therefore, it seems that to properly quantify the importance at planet level of any radioactive contamination, it should be done on the basis of radioactivity already present since the planet birth, whose decaying becomes the most evident sign of earth evolution and it is still taking place. In this way, 0.0118% isotopic abundance, 1.28 x 109 years half life, 40K is the natural radioisotope most abundant in the earth crust and also in the numerous salts dissolved in sea water. So, the radioactivity due to 40K might be the most suitable measurement, in order to have one basis of natural radioactivity to be compared with that of any artificial radioisotope. Among these, the fission product 137Cs presents the highest yielding in the fission of 235U , and it is the most common radioactive pollutant found in nuclear accidents due to its half life equal to 30.07 years, and γ rays easy to detect with higher efficiency due to a low energy equal to 662 Kev. Figure 1 represents the fission products yielding from 235U vs. mass number (A) and Fig. 2 represents percentage of elements on earth vs atomic number (Z).
