The Workshop was the first step in the EPA review of NAAQS for the pollutants in question. Among regulations worldwide, the U.S. Clean Air Act is likely the single piece of legislation with the largest influence on atmospheric pollutants. The EPA sets NAAQS for six principal pollutants that are derived from numerous sources and are considered harmful to public health and the environment (the six pollutants are: ozone, particulate matter, carbon monoxide, sulfur oxides, nitrogen oxides, and lead). The Clean Air Act mandates that each of the six NAAQS be reviewed every five years, and each step of the process is conducted with close review by a committee of external experts, as well as input from all stakeholders.
The basis of a NAAQS revision is the Integrated Science Assessment, a comprehensive review, synthesis, and evaluation of the most policy-relevant science, where EPA identifies and assesses peer-reviewed information published up to the time of the standards review. All subsequent risk and policy assessment, followed by rulemaking, refers to this document. The Workshop provided a platform for subject matter experts in the general public to inform EPA of relevant data that has come to light since the last review of NAAQS for oxides of nitrogen and sulfur.
Dr. Dubois was one of three panelists tasked with discussing new developments in the influence of oxides of sulfur on mercury methylation. While the largest effect of oxides of sulfur on ecosystems is acidification, mercury methylation has attracted attention because of widespread concern about mercury found in fish caught in American lakes and rivers. As of 2010, there were 3,710 advisories for mercury, and 25 states had statewide advisories for mercury in freshwater lakes and/or rivers. 16.4 million lake acres and 1.14 million river miles were under advisory nationwide, which corresponds to 39% of the total lake acreage and 32% of total river miles. Mercury becomes a serious environmental neurotoxin when it is methylated by the action of sulfur-reducing aquatic microorganisms, and in the methylated form, starts accumulating in gradually higher concentrations at higher levels in the food chain. Several factors are known to influence the rate at which sulfur-reducing microorganisms produce methylmercury, including dissolved organic acids and pH, and it also has been established that when mercury is present, more atmospheric deposition of SO2 results in greater concentrations of methylmercury in sediment, in water, and in fish at the top of the aquatic food chain. However, an exact quantification of the influence of the various factors involved besides mercury and oxides of sulfur is still missing. The production of methylmercury in freshwater bodies increases when deposition of oxides of sulfur increases, and decreases when deposition decreases, but how much methylmercury is produced in various locations around the country cannot be reliably predicted. We do know, however, that where methylmercury is present in excessive amounts in fish, both mercury and oxides of sulfur have to excessive in the environment.
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