Another example: EPA allows up to 15 µg/L of lead in drinking water. But lower levels are known to be dangerous:
“Researchers now know, blood lead levels in children as low as five micrograms per deciliter — the Centers for Disease Control's "level of concern" — can lead to IQ deficits and increases in behavior problems like ADHD and conduct disorder.
In adults, low-level exposure of 10 micrograms per deciliter can cause high blood pressure and kidney problems.”
And the CDC has said that no level of lead in blood is safe.