Diesel Exhaust Fluid (DEF) is a non-toxic solution made of 32.5% urea and 67.5% deionized water. It is used in vehicles equipped with Selective Catalytic Reduction (SCR) systems to reduce harmful nitrogen oxide (NOx) emissions. When injected into the exhaust stream, DEF reacts with NOx, converting it into harmless nitrogen and water vapor. DEF is stored in a separate tank in diesel vehicles and is consumed as the engine runs. Proper handling and storage are essential, as DEF can freeze at low temperatures and should be kept away from direct sunlight. Regular checks ensure that the fluid levels remain adequate for optimal emissions control.