The units used in the reference differ because in each case the units relate to the different methods customarily used for measuring the "amount" of the toxin in question in public water supplies.

In many cases a difference can be equivocated by noting the enormous difference in the molecular weight of two toxins. For many substance it's the "number of molecules" that the body absorbs and retains that's most significant, so a much larger "amount" in grams of a high molecular weight substance like lead is required to get the same effect than is needed for substance with a very low molecular weight like fluorine.

The real difference in this case, however, is that lead is "toxic" in any amount, and since it is not significantly eliminated by normal body processes it accumulates over time with rather low but continuous exposure.

Some minimal presence of fluorine (fluoride) is necessary for good health. The benefits for prevention of tooth cavities are much debated, not with respect to the need for some but on the amounts needed; but benefits for maintaining of bone mass - prevention of osteoporosis and other age-related bone glacility - are well demonstrated.

Fluorine/fluoride begins to be toxic only after a fairly substantial basal-body-content is exceeded. Fluorine/fluoride in excess of body needs also is naturally removed at least partially by normal excretion so that it accumulates less rapidly, although the ability of the body to maintain appropriate levels in the presence of excessive input exposure is limited.

In both cases, the limits cited are levels that are acceptable for long term exposure to the two substances. Quite different numbers apply to the amounts retained in a person that will produce toxic effects.

The water standard for lead assumes that once a person ingests lead it remains in the body permanently, so the standard must be low enough to prevent significant adverse toxicity over a lifetime of exposure, from water and all other kinds of exposure.

The water standard for fluoride needs only to prevent ingestion at a rate that exceeds the rate of excretion (with some allowance for other sources of fluoride ingestion).

Lead is like your favorite rat poisen. Any amount is dangerous.

Fluoride is more like your favorite mood enhancer. Some helps, but an "overdose" will kill you. The standard must allow for mainenance of "good levels" without producing "toxic levels."

The numbers cited appear to be good enough, but the comparison, in the way it's expressed, is an apple-to-oranges comparison with the result expressed in units of plums.

John