Air1 Diesel Exhaust Fluid (DEF) is an ultra pure aqueous solution made of 32.5% high-purity urea and deionized water. Air1 DEF meets the thoughest industry standards and is API certified to guarantee high product quality. It is best used in new diesel powered engines with Selective Catalytic Reduction (SCR) technology. This emissions control liquid is contained in a separate DEF tank and is sprayed into the exhaust to chemically reduce the amount of diesel engine’s nitrogen oxide (NOx) emission levels during combustion, lowering the impact of air pollution caused by cars and trucks on the environment.