Urea is also commonly used in Diesel Exhaust Fluid (DEF), which is a solution of 67.5% deionized water and 32.5% high-purity urea. DEF is injected into the exhaust stream of diesel engines to reduce Nitrogen Oxide (NOx) emissions, which are harmful pollutants that contribute to air pollution and can cause respiratory problems.

When DEF is injected into the hot exhaust stream, the urea in the solution breaks down into ammonia and carbon dioxide, which react with the NOx in the exhaust to form harmless nitrogen gas and water vapor. This process is known as Selective Catalytic Reduction (SCR) and is a highly effective way to reduce NOx emissions from diesel engines.

Urea is DEF is carefully controlled to ensure the proper chemical reaction takes place, and the solution must meet strick quality standards to ensure optimal performance and avoid damage to the engine.