Why was the radioactivity concentration in Japan high in the past?
The radioactivity concentration was high in the past due to atmospheric nuclear tests conducted from 1945 to the 1980s or the Chernobyl Nuclear Power Plant accident that occurred in 1986.
Why is the average value shown at a very low position in the chart?
The number of samples that exhibited a high radioactivity concentration value is much smaller than the number of samples that exhibited a low radioactivity concentration value. As a result, the average value became smaller.