Air quality standards for transportation aim to limit the emission of harmful pollutants from vehicles, such as nitrogen oxides, particulate matter, and volatile organic compounds, to protect public health and the environment.

The Environmental Protection Agency (EPA) is the primary federal agency responsible for setting and enforcing air quality standards, including those for transportation.

Air quality standards for transportation are typically divided into two categories: emission standards, which regulate the amount of pollutants emitted from vehicles, and fuel standards, which specify the allowable levels of pollutants in transportation fuels.

Emission standards for transportation primarily focus on reducing nitrogen oxides (NOx), which are a major contributor to smog and acid rain.

Fuel standards aim to reduce air pollution by limiting the levels of harmful pollutants, such as sulfur and aromatic compounds, in transportation fuels.

Tier 2 fuel standards, introduced in the United States, were designed to significantly reduce the sulfur content in gasoline, leading to cleaner emissions from vehicles.

Tier 3 fuel standards for gasoline aim to reduce the aromatic content, which contributes to the formation of smog and other air pollutants.

Tier 4 fuel standards for diesel fuel are designed to significantly reduce particulate matter emissions from diesel engines, improving air quality.

The California Air Resources Board's Low-Carbon Fuel Standard (LCFS) aims to reduce greenhouse gas emissions from the transportation sector by promoting the use of low-carbon alternative fuels.

The World Health Organization (WHO) is the primary international organization that develops global air quality standards, including those for transportation.

The WHO's air quality guidelines for transportation focus on limiting air pollution from vehicles by setting standards for harmful pollutants such as particulate matter, nitrogen oxides, and ozone.

The WHO's air quality guidelines for transportation primarily focus on reducing particulate matter (PM) emissions from vehicles, as PM is a major contributor to air pollution-related health risks.

The Euro emission standards are significant because they apply to all new vehicles sold in the European Union, ensuring a high level of air quality across the region.

The Euro 5 emission standard, introduced in 2009, required diesel vehicles to be equipped with particulate filters to reduce emissions of harmful PM.

The Euro 7 emission standard, which is expected to be implemented in 2025, focuses on reducing nitrogen oxide (NOx) emissions from vehicles, particularly in real-world driving conditions.