Good point ! I hadn't thought of it that way but in my mind if one is going to put a concentrated solution on the skin then it must be as pure as possible because the object is to saturate our deficient body with Magnesium & not other impurities. On the other hand I am thinking if it ( magnesium flakes for instance) is used in a large body of water, i.e. bathtub , then the concentration levels would be a heck of a lot less. Mmmmm...but it is definitely food for thought.
I can only comment on what I find & because I use the purest foodgrade/pharmaceutical grade magnesium chloride powder there is no way I could put it into a solution using ordinary tap water & not feel I had "spoilt" the brew .The flakes, well that is entirely different to me as the purity is not food or pharmaceutical grade anyway & I don't feel guilty about putting that into the bath tub.
In a perfect world it would be better if all the water was as clean as possible & some people do have purifers/filters connected to their bathroom supplies. In reality we should be aware that our skin is the largest organ of our body, so putting tainted water onto ourselves is very questionable.
What sort & how many toxins are we absorbing just by showering everyday ???
Why not just let the flakes dissolve as I suggested then you would not be adding any extra contaminants.You would find it would also be more concentrated too, so less would be used anyway.