EPA regulates the emissions from mobile sources by setting standards for the specific pollutants being emitted. EPA established progressively more stringent emission standards for carbon monoxide, hydrocarbons, nitrogen oxides, and particulate matter, starting in the mid-1970s for on-road vehicles and in the early 1990s for nonroad engines and equipment. Emissions standards set limits on the amount of pollution a vehicle or engine can emit. Once EPA sets emission standards for a particular engine and/or vehicle category, manufacturers must produce engines that meet those standards within the timeframe of the corresponding implementation schedule.