DEF is a nontoxic solution made up of purified water and pure automotive-grade urea. DEF is not a fuel, or fuel additive. In a Selective Catalytic Reduction system, DEF is injected into the tractor exhaust stream through a controlled ‘dosing’ module which varies the application rate. The DEF fluid vaporizes NOx emissions and converts them into harmless nitrogen gas and water vapor–natural components of the air we breathe.