It would be more accurate to state EIS/CS requires a *minimum* of 99.9% purity in silver for the purpose of manufacturing EIS/CS for consumption.
This three nine and four nine silver is an age old argument that is pure myth. 99.9 is actually 99.98% on assay and people argue the toss over .2%?, ridiculous.
Any impurities contained within pure fine silver are impossible to eliminate due to the pure fine silver manufacturing process and are minuscule when one considers the amount of EIS/CS consumed by volume by the user, and for however long they wish to consume it. Nobody in their right mind would guarantee 100% on anything.
There is absolutely nothing wrong with 99.9% pure fine silver. Personal choice, and perhaps marketing schpeel, are the only factors which will influence an individual in purchasing 99.9% (actually 99.98%) or 99.99%
One will still be ingesting impurities from 99.99% pure fine silver. The only difference is the ppm or ppb of those impurities. When it's all broken down, how many and what percentage of impurities would one be consuming between the two in the grand scheme of things? Not enough to worry oneself about!!
If one wishes to pay that little extra for that 99.99% pure fine silver in the belief that 99.9% silver is either inferior or will somehow contaminate one more, then so be it, but it's pointless to do so in the scheme of things EIS/CS related.