I'm a word person and much as I would like it not to be so, when I read math equations my mind blurs. I thank you all for the impressive information but unless you were here diagramming it for me I'm just not going to learn much.

So my question remains: What is the conversion of parts per litre to parts per million? I just barely recognize that the equation has to be reduced to the same basis.

Further, is the writer correct that 4 ppm is "more than 250 times that of .015 per litre? Quote: "The toxicity rating for fluoride is 4, yet the MCL for fluoride is currently set at 4 parts per million, over 250 times the permissible level for lead."