Well, at least for Germany it was the actual nuclear fallout over large areas of the country after Chernobyl. Which is btw still measurable today. [1] That's a pretty scary thing to happen to you and one just has to accept that these are the actual lived experiences of people that form their opinions.

[1] https://www.bfs.de/EN/topics/ion/environment/foodstuffs/mush...

Radiation detectors can detect very low levels of radiation (far below any measurable health effects, for instance), so claiming we can still detect fallout from Chernobyl doesn't really say anything.