Using air toxics data from 2003 through 2005, a research firm found that ambient concentrations of benzene, 1,3-butadiene, carbon tetrachloride, acetaldehyde, arsenic, and acrolein regularly exceeded EPA's health criteria [10 to the minus 6] at a majority of air quality monitoring stations.  Exceedances of acrolein were particularly severe; it was the only noncancer reference concentration at most monitoring sites.  The study notes the need for additional monitoring methods and locations to precisely gauge air toxic concentrations and their associated health risk.  The study also notes that some criteria for assessing the 10 to the minus 6 level were vague and inconsistent.