Print PagePublically released in December 2008, the Integrated Science Assessment (ISA) for Oxides of Nitrogen and Sulfur (NOx and SOx)-Ecological Criteria is the scientific foundation for the review of the secondary National Ambient Air Quality Standards (NAAQS) for NOx and SOx. This review included the effects of nitrogen and sulfur deposition on U.S. ecosystems. Over 3000 scientific publications, newly available since the last assessments of NOx and SOx (1993 and 1982, respectively), were reviewed to characterize the ecological effects, which were organized into the broad categories of acidification, nitrogen enrichment and sulfur enrichment. How was ecological data used to determine causality and inform policy-makers?
Chemical and biological indicators of nitrogen and sulfur deposition that could be useful for developing a national air quality standard were identified from the peer-reviewed literature. Dose response relationships were characterized between deposition loads and indicators or between chemical and biological indicators. Threshold values of deposition (critical loads) and chemical indicators were evaluated. Hill’s causality criteria for relating pollutant exposure to human health effects were modified to apply to evaluating ecological effects associated with NOx and SOx exposure.
Results/Conclusions
The key causal relationships identified were that acidification (due to NOx, NHx and SOx) and nitrogen enrichment (due to NOx and NHx) occur in the U.S. The effects of acidifying deposition on ecosystems have been well studied over the past several decades, leading to the identification of vulnerable regions and the development of robust ecological models used for predicting soil and surface water acidification. Chemical indicators of acidification are well established, and their threshold values for the onset of biological effects are established in the literature. In contrast to acidification, a broadly applicable and well-tested predictive model of the ecological effects of N deposition is not available. However, there is substantial empirical information for specific ecosystems and endpoints. There are numerous chemical and biological indicators of N enrichment; however threshold values are not well established. Published studies typically did not subdivide N deposition exposure into reduced vs. oxidized N, thus causality determinations were made on the effects of total N deposition.
This abstract was reviewed by the National Center for Environmental Assessment, U.S. EPA and approved for publication. Approval does not signify that the contents necessarily reflect the view and policies of the Agency.
