In addition to greenhouse gases (GHGs), fuel-burning vehicles emit other pollutants that are harmful to human health or the environment. Examples include nitrogen oxides (NOX), sulfur oxides (SOX), particulate matter (PM), and volatile organic compounds (VOCs). These are termed "conventional" pollutants. Though some conventional pollutants have climate impacts (for instance, black carbon, a type of PM, contributes to global warming), people are primarily concerned about conventional pollutants for their impacts on human health and the local or regional environment near where the emissions occur.
Conventional pollutants may be harmful in and of themselves - for example, human exposure to PM increases the number of premature deaths due to respiratory and cardiac diseases. They also may form secondary pollutants in the atmosphere. For example, NOX and VOCs, in combination with sunlight, produce ozone (O3), a pollutant that can damage lung function and increase the risk of respiratory infections.
Even before mitigating climate change became a concern, many countries implemented standards limiting conventional pollutant emissions from vehicles, to improve urban air quality and limit public health harms. Standards typically vary by vehicle type (car, light truck, heavy truck, bus, etc.) and are defined in mass of pollutant per unit distance traveled by the vehicle (such as grams per mile). This metric allows a variety of compliance options (such as exhaust after-treatment technologies, improved fuel efficiency, lightweighting of the vehicle, etc.), allowing flexibility to vehicle manufacturers.
Since the year 2000, the United States' conventional pollutant standards for vehicles have treated vehicles and fuels as a system, rather than looking narrowly at vehicles. This engages fuel manufacturers in the process of complying with the standard and can lead to synergies. For example, removal of sulfur from gasoline reduces SOX emissions and also allows vehicles' catalytic converters to work more efficiently to remove other conventional pollutants.