Definition - What does Emission Standards mean?
Emission standards dictate the maximum amount of a certain pollutant allowed to be discharged into the atmosphere from a single source, either mobile or stationary. These standards are provided by the United States Environmental Protection Agency (EPA) and apply to any on road and off road engines. EPA has authority to regulate engine emissions and air quality in general in the U.S. based on the clean air act (CAA) recently amended in 1990.
Safeopedia explains Emission Standards
Different vehicle and off-road engines are separated into different categories based on the CAA and EPA's standards to manage different sized engines. For example, there are tier 1, 2 and 3 for cars and light trucks. California has the right to separately adopt it's own more stringent regulations for vehicle emissions. There are also different standards for larger engines, such as with railways and buses, as well as diesel engines. Once EPA sets standards for a specific kind of engine category, manufacturers must produce engines that comply with these standards, and within the specified time-frames of an implementation program that corresponds with manufacture.