11. Tritium activity in the environment over time

Figure 23 a 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....