Nitrogen oxide (NOx) emission standards for light duty vehicles in the U.S. 1975-2025
Emission standards for nitrogen oxides (NOx) from light duty vehicles in the United States have strengthened since emission standards took effect in 1975. That year, average emission standards were 3.1 grams of NOx per mile driven. Current emission standards under the Tier 3 program, which are phased in from model year (MY) 2017 to (MY) 2025, are set at 0.3 g/mi. This is an improvement of roughly 99 percent from 1975. Road vehicle NOx emissions in the U.S. have fallen significantly since emission standards were introduced.