The Journal of the American Medical Association published an article showing a 78% drop in blood levels among Americans from 1976 to 1991.  Here is the [abstract][1], access to the full paper requires payment.  The authors attribute the drop primarily to the removal of lead from gasoline.

[Here's][2] an article that's not on a pay site which cites the same information, and describes a very strong correlation between the dropping blood lead levels and the reduction in use of leaded gasoline.

A more recent study found a significant drop in infant blood lead levels in Santiago, Chile, which began phasing out leaded gasoline in 1993.  [Here is the abstract.][3]

Another more recent [study][4] was done in Korea, and again showed very significant drops in blood levels correlated with the phaseout of leaded gasoline.


  [1]: http://jama.jamanetwork.com/article.aspx?articleid=376894
  [2]: http://chemcases.com/converter/converter-20.htm
  [3]: http://www.ncbi.nlm.nih.gov/pubmed/16189990
  [4]: http://journals.lww.com/epidem/Abstract/2003/09001/Unleaded_Gasoline_and_Lead_Level_in_Human_Blood_.171.aspx