There seems to be an inconsistency between what US EPA is doing and what its Children’s Health Protection Advisory Committee is recommending.
EPA adopted this reference dose from a report published by the National Academy of Sciences. This value is 0.007 mg/kg-day. For a person weighing 70 kg (154 lbs) and consuming 2 liters (67.6 oz) of tap water per day, this dose is equivalent to 24.5 ppb in drinking water.
About the reference dose, Phibbs writes:
A reference dose is the level of daily exposure at which people would not be harmed.
If exposures at or below the reference dose do not harm people, then it’s safe. But if it’s safe, then why is CHPAC asking the Agency to set the maximum allowable level of exposure below the reference dose?