Peak Blue DEF or Diesel Exhaust Fluid is a nontoxic solution of 67.5% purified water and 32.5% automotive grade urea. DEF fluid is used in the Selective Catalytic Reduction System on diesel engines to reduce NOx. Peak Blue DEF meets the ISO 22241 quality standard and is an American Petroleum Institute certified diesel exhaust fluid. Peak Blue DEF is not flammable, toxic or harmful to the environment. DEF usage will be approximately 2% of the diesel fuel used.