Tritium activity in the environment over time
Artificial Radionuclide Background in French Metropolitan Territory

Figure 23a shows trends in tritium activity by volume in rainwater in the northern hemisphere since 1950. These trends are similar to those seen for carbon-14, with an increase in just a few years from the natural cosmogenic background, which was between 0.1 and 0.6 Bq.L^-1, to a maximum of 400 Bq.L^-1 in 1964 at the height of the fallout from atmospheric testing of American and Soviet nuclear weapons. The decrease observed since 1964 is mainly due to the radioactive decay of tritium (half-life 12.3...