Arguments continue over perchlorate standards continue. The National Academy of Sciences has come out with a standard that splits the difference between the EPA proposal and industry proposals.
Perchlorate is not a carcinogen, nor something that accumulates in the body.
Perchlorate disrupts thyroid function by competitively inhibiting iodine uptake in a dose-dependent fashion. Because thyroid hormones play a major role in brain development, EPA and the NAS committee focused on making sure that fetuses and infants would be protected. The committee based its dose recommendation on the 2002 Greer study, in which healthy men and women were given perchlorate to determine at what dose iodine inhibition occurs (Environ. Health Perspect. 2002, 110, 927–937). That study found no significant inhibition at 7 µg/kg-day, a conclusion supported by four additional studies. The committee applied an uncertainty factor of 10 to protect the fetuses of pregnant women who might have hypothyroidism or iodine deficiency and hence came up with the 0.7µg/kg-day recommendation.
"Competitive inhibition" is what happens when a chemical looks enough like some other chemical that it gets in the way of reactions that use that chemical. In this case, perchlorate looks enough like iodine that the thyroid gland will grab hold of it and try to stick it into thyroxin and other molecules. It doesn't work, so it blocks the production of thyroid hormones until it falls out on its own and is replaced with real iodine.
As described, the NAS standard seems to be perfectly adequate.
As usual, these things involve trade-offs. You can lower perchlorate levels below the standard, but it costs more to do so. The resources you spend lowering perchlorate levels are resources that won't be available for other things that might bring much greater benefit.
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