Emission standards are specific limits to the amount of pollutants that can be released into the environment by a particular machine. The most widely known standards regulate automobiles emissions, but there are a wide variety of regulations for industry as well. The emissions most commonly monitored are nitrogen (NOx) and sulfur oxides, particulates, carbon monoxide (CO), and volatile hydrocarbons.
In the US, there are two main standards, the Federal Standard set by the Environmental Protection Agency (EPA) and the more strict standards of the California Air Resource Board (CARB). States that follow the CA standard are often referred to as CARB states.
The standards have been implemented in three phases with increasingly strict requirments.
Phase 1 (1994 to 1997) - Vehicles over 8500 lbs had to meet set emission standards.
Phase 2 (1999-2003) - Vehicles now were sorted into "bins" and manufactureres were required to meet overall fleet emission standards
Phase 3A (2010 to 2016) - The current phase requires an increasingly high standard. Fleet mileage for cars will have to average 42 mpg, and trucks will have to average 26 mpg by the end of the phase in 2016.