Exhaust from diesel engines is a major source of air pollution. Nitrogen oxides (NOx), particulate matter (PM) and other pollutants from diesel exhaust are known to impair human health, causing cancer, heart and lung damage, and impaired mental functioning.

As a result, Europe, the United States, Japan and China are all taking steps to mitigate the problem, enacting regulations aimed at making diesel engines more efficient and less polluting. These regulations have been enacted in a series of tiers or stages over the years, with each tier becoming more strict.