referred to as NOx United States Environmental Protection Agency information page on the NOx SIP Call NOx poisonous, highly reactive smokestack emissions about 50% smokestack emissions smokestack emissions information page on the NOx SIP Call nitrogen oxide nitric oxide (NO) nitrogen-oxide (NOx) epa.gov http://www3.epa.gov/region1/airquality/nox.html http://www3.epa.gov/region1/airquality/nox.html