The Fourth National Report on Human Exposure to Environmental Chemicals, Updated Tables, September 2013 provides nationally representative biomonitoring data that has become available since the publication of the Fourth National Report on Human Exposure to Environmental Chemicals, 2009. The Updated Tables, September 2013 includes all the updates previously provided in Updated Tables, July 2010 through Updated Tables, March 2013. The Updated Tables, September 2013 present data for 91 chemicals measured in serum pooled samples that have not been reported in any previous Updated Tables. Since publication of the Fourth Report, 2009, 201 chemicals have updated tables and 49 chemicals have been added, for a total of 250 chemicals presented in these Updated Tables.