I've known about this for a long time, but only in the past couple years started to appreciate how much of the background radiation has gone away.<p>> <i>World anthropogenic background radiation levels peaked at 0.11 mSv/yr above natural levels in 1963, the year that the Partial Nuclear Test Ban Treaty was enacted. Since then, anthropogenic background radiation has decreased to 0.005 mSv/yr above natural levels.</i><p>Im curious how much of the decay is half life related, and how much is that the particles have deposited themselves onto the planet, gotten out of the air.