That's why I did not say "natural background levels of Cs 137". There are some Cs-137 all over the planet now. You are right, it is not natural, it is all man-made. But it is there now.Cs 137 is a *fission* product, ie almost entirely man made from either atmospheric weapons testing or in power reactors. Comparing Cs 137 to some natural background levels of Cs 137 is nonsensical.