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The federal Clean Air Act (CAA) requires the State of California to obtain Environmental Protection Agency (EPA) permission in order to adopt “standards and other requirements relating to the control of emissions,” but expressly preempts all other states from adopting such regulations. Beginning January 1, 2007, California—which suffers the most severe air pollution in the country—will require all ships operating within twenty-four miles of the coast to limit emissions from auxiliary engines to levels that would be reached by using a certain low-sulfur fuel. If California’s new regulation falls within the CAA definition of “standards and other requirements,” it is invalid without EPA authorization. Courts have generally found that “in-use” regulations, which are applied to the operation of motor vehicles and ocean vessels, are not covered by the preemption provisions of the CAA. However, a closer examination of differences between the CAA’s treatment of motor vehicles and nonroad vehicles, along with a recent Supreme Court decision interpreting the definition of “standard,” indicates that California’s regulation is preempted unless EPA grants authorization.