A recent scientific study reports that there has been substantial progress on emissions reduction, and supports the U.S. states’ right to set their own emissions standards. The study, sponsored by the U.S. EPA and conducted by the National Academies’ National Research Council, says in particular that California’s standards–which are generally stricter than the U.S. federal government’s–are still needed because of persistent pollution in parts of the state, said the committee that wrote the report. California’s standards also tend to spur the development of better emission-control technologies that benefit the rest of the nation, the committee noted. It did not comment on the state’s recent standards for greenhouse-gas emissions because they were adopted while the report was in progress, and because there are no federal standards to which they can be compared. The committee examined emissions standards governing so-called mobile sources, which include cars and light- and heavy-duty trucks; diesel-powered cranes, bulldozers, and tractors; and equipment such as lawnmowers that run on small gasoline engines. The committee concluded that despite the substantial progress in reducing emissions from mobile sources nationwide, more needs to be done to attain federal air-quality standards in many parts of the country. Like California, other states adopt tougher mobile-source emissions regulations to help meet EPA’s air-quality standards. However, because colder weather or variations in fuel composition can make cars or their emissions-control equipment operate differently, automakers often claim difficulty complying with California rules when they are adopted elsewhere, and legal disputes ensue. The committee said EPA could alleviate such disputes either by providing formal but non-binding guidance or by being given the power to grant or, in limited circumstances, deny a waiver allowing states to adopt California standards. The committee could not agree on which of these approaches would be more effective. It noted that defining EPA’s role in the state adoption process is a policy decision that goes beyond scientific concerns. Currently, California is required to obtain an EPA waiver for each new mobile-source emissions standard. To speed up the waiver process, which can take several years, the committee recommended that EPA expedite waiver requests it considers non-controversial and place a time limit on decisions for more controversial requests. Since new regulations cannot be implemented for two years after their adoption by the state, a time limit of that length would be appropriate for the waiver process, the committee said.