99.9 or three nine is so close to 99.99 or four nine silver that the difference isn't worth worrying about. You'll probly find three nine is somewhere around 99.98% in purity which is only a gnats whisker off 99.99. They can't list it as 99.99 because it falls just below that assay. Any impurities are so minimal in amount in the scheme of things that they ain't gunna hurt anything anyway, and that's right, the water used and any other possible introduced contaminants should be of more concern than the silver. One will find that the 99.9 is not the *exact* %, it's usually a bees whisker off 99.99 in percentage of purity, usually assayed and listed as 99.9+ or 'plus' % purity.
People needn't get caught up in the marketing or media hype, three nine silver is fine if four nine is a tad too expensive or unprocurable for anyone. Just make sure it's *pure fine silver* and not jewellery or sterling silver that's all.