There are ten dL in one litre, so an ideal amount of lead in a litre of blood would be no greater than 400 micrograms (ug)
There's 1000mL in a litre, so we're looking at 0.4ug of lead or less in a millilitre. 1mol of lead atoms weighs about 207.2g, therefore there's about 2.907 × 10^21 atoms per gram, or about 1.163×10^14 atoms in 0.04ug. Therefore there should be no more than 1.2×10^14 atoms of lead per millilitre.
