Data (N = 9650) for US adults aged ≥20 years downloaded from National Health and Nutrition Examination Survey for 2003-2014 were analyzed to study trends in adjusted and unadjusted concentrations of selected perfluoroalkyl substances (PFAS), namely, perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), perfluorodecanoic acid (PFDA), perfluorohexane sulfonate (PFHxS), and perfluorononanoic acid (PFNA). Over 2003-2014, unadjusted concentrations of PFOA decreased by 50%, by 75% for PFOS, by 32% for PFDA, by 27% for PFHxS, and by 30% for PFNA. Females not only had substantially lower concentrations of every PFAS than males but rate of decrease was also higher for females than males, for example, 36.7% for females and 30.6% for males every two years for PFOS. For each survey period of two years, percent decrease in adjusted concentrations was 17% for PFOA, 33.5% for PFOS, 11.5% for PFDA, 6.3% for PFHxS, and 7.6% for PFNA. However, these trend data must be examined within the context of design changes in NHANES over 2003-2014 resulting in oversampling of Hispanics other than Mexican Americans starting 2007-2008 survey cycle and oversampling of non-Hispanic Asians starting 2011-2012 survey cycle. In order to examine how design changes may have affected computations of adjusted and unadjusted concentrations, the data were analyzed using the racial/ethnic categories prior to and after oversampling of Hispanics other than Mexican Americans and non-Hispanic Asians was put into place.
