Surprising is what it was to me, Stilly, but it can be made to make sense. There's a lot of air shot through pig-iron to burn off the carbon ("Bessemer convertor", nags my inner completist), and we're talking about very low but detectable levels of radiation.

I read somewhere else that residual radioactivity in the air gets picked up by vines, and measurable amounts end up in the resulting wine;* the levels have varied over the years, and have been declining since the Test Ban Treaties took effect. This, it was suggested, can be used to tell when a specific bottle of wine was made, whatever it says on the label.

* It was not mentioned whether drinking certain vintages causes one to glow in the dark.