The US conducted 331 atmospheric tests according to the film.
Referring to my old PoliSci research notes (during the Regan - Chernenko/Andropov era) suggested about 172 Soviet atmospheric tests and China about 18
So around 500 all tolled that can be verified or the research material at the time indicated.
Deduce about 5-8% that cannot, you're left with a significant number. Thats a lot of Cesium & Stontium floating about. Fortunately the earth's a big place so there's about one 1 milli-billi-microgram per 10,000 sq. klicks or something.
This, apparently, is the surest way to tell if an old master is forged. ALL paintings done since 1946 will have small small trace of radioactive material in the paint (very, very,
very, small - so don't throw out your kids finger paintings).
I got the last bit out of a big book so you know its true.
PJH