Half of one tenth of sweet bugger all . . .

Fukushima-related radioactive materials measured across entire Northern Hemisphere [1]

Scarey headline, right?

Check the details:

    The CTBTO's radionuclide stations are designed to register minuscule amounts of radioactive particles and noble gases -- down to a number of a few atoms. The system's sensitivity is second-to-none -- it can detect a concentration of 0.1 g of radioactive Xenon evenly distributed within the entire atmosphere of Earth. A rooftop detector at the CTBTO's headquarters in Vienna still catches traces of emissions from the 1986 Chernobyl disaster.

They are literally counting atoms and the levels detected outside of the immediate area were way to low to pose any kind of serious health risk.

Precedure wise some of the US National laboratory tests amounted to putting fresh filters in truly massive high volume warehouse size HVAC systems, waiting for two weeks worth of fine particle dust to gather, shaking down the filters to concentrate particulate, and then letting that sit for 48 hours at the heart of an extremely sensitive gamma ray detector in order to get "target of interest" counts in the low hundreds swamped by a sea of always there regular background environmental radiation.

It's a triumph of modern instrumentation that signal was detected .. but not the smoking gun of increased cancer risk.

Although . . . if you do want to worry, there are still trace elements of those several hundred atmospheric atom bomb tests from the first part of the Cold War drifting about [2]

[1] (2011) https://www.sciencedaily.com/releases/2011/04/110407121343.h...

[2] https://www.epa.gov/radtown/radioactive-fallout-nuclear-weap...