Diesel exhaust has been found to contain a long list of toxic air contaminants. Among these pollutants, fine particle pollution is an important as a cause of diesel's harmful health effects. However, when diesel engines burn their fuel with high oxygen levels, this results in high combustion temperatures and higher efficiency, and these particles tend to burn off and reduce, but the amount of NOx pollution tends to increase.

NOx pollution can be reduced with diesel exhaust fluid, which is injected into the exhaust stream, and catalytically destroys the NOx chemical species. Exhaust gas recirculation which works by recirculating a portion of an engine's exhaust gas back to the engine cylinders also has very positive effects on NOx emissions.