As on-road air pollutant emissions decline over time, air quality improves. Reductions in air pollutant concentrations are impressive, with concentrations of CO decreasing by 60 percent since 2000. All criteria pollutants have shown significant decreases over the last decade, showing a steady trend of improvement.
On-Road Air Pollution Concentration Reductions (2000–2013)
Sources: U.S. Environmental Protection Agency, Clearinghouse for Inventories and Emissions Factors (CHIEF), Current Emission Trends Summaries, https://www.epa.gov/air-emissions-inventories/air-pollutant-emissions-trends-data