There is no safe level of lead. That's the problem. There is a threshold because they had to draw a line somewhere, but lead causes detectable impairment of b

There is no safe level of lead. That's the problem. There is a threshold because they had to draw a line somewhere, but lead causes detectable impairment of brain development in proportion to concentration, at any concentration. It's not like you can have 15ppb of lead in your blood and not suffer from it.