A sample of water has no fluoride (F) ions in it. What is the maximum limit (in $mg~L^{-1}$) of soluble fluoride salt to be added to this water to convert it into safe drinking water?
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Too little fluoride causes dental problems, while excessive fluoride causes fluorosis. Safe drinking water generally contains about:
\[
1-2~mgL^{-1}
\]
fluoride ions.