Urea is used in Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR) reactions to reduce the nitrogen oxide| pollutants in exhaust gases from combustion from diesel, dual fuel, and lean-burn natural gas engines. The BlueTec system, for example, injects a water-based urea solution into the exhaust system. Ammonia () produced by the hydrolysis of urea reacts with nitrogen oxides () and is converted into nitrogen gas () and water within the catalytic converter. The conversion of noxious  to innocuous  is described by the following simplified global equation:
When urea is used, a pre-reaction (hydrolysis) occurs to first convert it to ammonia:
Being a solid highly soluble in water (545 g/L at 25 °C), urea is much easier and safer to handle and store than the more irritant, caustic and hazardous ammonia (), so it is the reactant of choice. Trucks and cars using these catalytic converters need to carry a supply of diesel exhaust fluid, also sold as AdBlue, a solution of urea in water.