The CDC recommends that levels of lead in adults be no greater than 40 ug per dL of blood. At that level a) how many grams of lead are in 1.0 liter of blood, and b) how many atoms of lead would be in a milliliter of blood?

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.