Correct Answer: <10 microg/d

Many physicians believe that levels <10 microg/dl are safe. However, the scientific literature says othewise stating levels at or below <10 micropgs are responsible for compromising child intelligence.

References:

Canfield RL., Intellectual impairment in children with blood lead concentrations below 10 microg per deciliter., N Engl J Med. 2003 Apr 17;348(16):1517-26.

Jusko TA, Henderson CR, Lanphear BP, Cory-Slechta DA, Parsons PJ, Canfield RL., Blood lead concentrations < 10 microg/dL and child intelligence at 6 years of age, Environ Health Perspect. 2008 Feb;116(2):243-8.

Lanphear BP, Hornung R, Khoury J, Yolton K, Baghurst P, Bellinger DC, Canfield RL, Dietrich KN, Bornschein R, Greene T, Rothenberg SJ, Needleman HL, Schnaas L, Wasserman G, Graziano J, Roberts R., Low-level environmental lead exposure and children's intellectual function: an international pooled analysis, Environ Health Perspect. 2005 Jul;113(7):894-9.