Diesel exhaust fluid stored in underground storage tanks is not covered by Underground Storage Tanks regulations
, according to an Environmental Protection Agency Office of UST memo to regional and state program managers.

According to a NATSO newsletter, DEF does not meet the definition of a hazardous substance, as it contains less than 2 percent ammonia and falls under the UST program's de minimus exemption.

DEF, the 32.5 percent solution of urea in water, is required in selective catalytic reduction technology, one of the processes for reducing nitrogen oxide emissions from heavy-duty diesel engines. SCR is one of the technologies that can be used to meet the EPA's emissions requirements in 2010.

According to NATSO, DEF may consist of a small amount of ammonia, which is the only component of DEF that could be considered hazardous. The international standard for DEF allows no more than 0.2 percent by weight of alkalinity, measured as ammonia, to be present in the solution.