We do take action on substances like mercury, which are present in the environment both as a result of human activity as well as naturally occurring. The wording in the act, focusing on those primarily from human activities, was to recognize that you wouldn't be able to virtually eliminate substances that are released as a result of naturally occurring events like fires, or naturally occurring phenomena in soils and things like that. It was more a translation of policy into legal terms.
On October 24th, 2006. See this statement in context.