[jabl]

When you turn iron ore into steel with blast furnaces and steel making (e.g. basic oxygen process) you blast it with atmospheric air (or oxygen made from atmospheric air), and tiny amounts of impurities in the air (such as fallout from atmospheric nuclear testing) get embedded in the steel.

Yes, this is the same air you're breathing. No, the levels are not high enough to be a health issue. But it's an issue for very sensitive scientific instruments and such.

[pimlottc]

Also to note that, in theory, you could use purified oxygen for the forging and create new steel similar to low-background radiation steel, but it would be ridiculously expensive given how much oxygen is used.