content: It has to do with when the steel was produced, not that it's under water per se.  We just have a rather plentiful source under water due to all the ships sunk in WWII.  You can use ANY steel, so long as it was produced prior to 1945.  It's all right in the article:  The level of background radiation in the air increased, and this had an effect on the production of steel, so that steel produced since 1945 has had elevated levels of radioactivity.  Here it is saying that the PRODUCTION of steel is what introduces background radiation, not simply "not being under water".  So where do you get steel, which is a man-made material, that was made before 1945? Primarily from the ocean, in sunken ships from WWII.  And here it explains that PRIMARILY we source old steel from the oceans, because we've sank a lot of boats over the years (especially during the last World War). So it really has nothing to do with being exposed to the air vs. the water.  They ONLY factor in play is when the steel was originally produced.