A general indication of the Contemporary background levels of PCDDs, PCDFs, and coplanar PCBs in the ambient air over rural and remote areas of the United States.

Long-term measurements of the atmospheric concentrations of PCDDs, PCDFs, and coplanar PCBs were taken in rural and remote areas of the United States by the National Dioxin Air Monitoring Network (NDAMN). A total of 21 quarterly sampling moments occurred from June, 1998 to December, 2002 at 34 locations geographically distributed throughout the United States. Sampling sites were located in rural and remote areas to obtain background air concentrations of dioxin-like compounds. Results were reported as the toxic equivalent (TEQ) of the mix of PCDDs/ PCDFs (TEQDF) and the mix of coplanar PCBs (TEQPCB). At the studied rural sites the mean annual TEQDF for each of the NDAMN sampling years was 10.43, 11.39, 10.40, and 10.47 fg m(-3) for 1999, 2000, 2001, and 2002, respectively. There was no statistically significant difference in the rural mean TEQDF air concentrations across the sampling years (at 0.05 level of significance), although the mean concentration in sampling year 2000 increased 10% relative to the other sampling years. The 95th percent confidence interval of TEQDF air concentrations in rural areas of the United States is from 6.4 to 15.4 fg m(-3), indicating there is a 95% probability that the true mean falls within this range. Mean annual atmospheric concentrations (TEQDF) at the remote sites were 1.41, 0.99, 0.7, and 1.07 fg m(-3) in 1999, 2000, 2001, and 2002, respectively, The 95th percent confidence interval of TEQDF air concentrations suggest that the true mean annual atmospheric TEQDF concentration in remote areas of the United States is between 0.1 and 3 fg m(-3). The remote sites have average air TEQDF concentrations that are approximately 10 times lower than those of the rural sites. For the rural sites, there was close agreement in the mean annual air concentrations of coplanar PCBs (TEQPCB) among the years 1999, 2000, 2001, and 2002 (i.e., 0.62, 0.69, 0.59, and 0.7 fg m(-3), respectively). However, as is the case with PCDDs/PCDFs, there was a marked increase (i.e., approximately equal to 13%) in the annual rural mean air concentration in 2000 as compared to the other sampling years. The confidence intervals across the NDAMN sampling years suggests a 95% probability that mean TEQPcB atmospheric concentrations in rural and remote areas of the United States are within the range of 0.5-0.9 fg m(-3) and 0.1-0.5 fg m(-3), respectively. The congener distributional patterns of PCDDs/PCDFs in air were relatively constant at all locations, and match the profile of urban air. We propose the hypothesis that urban areas are regional sources of PCDDs/PCDFs and are affecting atmospheric levels in rural and remote areas of the United States.