Nitrogen Dioxide

Nitrogen dioxide (NO₂) is a  yellowish brown, highly reactive gas that is the primary ingredient in formation of ground-level ozone.  It is formed from high temperature combustion such as in power plants and automobile engines. In addition to its role in ozone formation, recent scientific evidence links short-term NO₂ exposures, ranging from 30 minutes to 24 hours, with adverse respiratory effects including airway inflammation in healthy people and increased respiratory symptoms in people with asthma.  Studies also show a connection between breathing elevated short-term NO₂ concentrations, and increased visits to emergency departments and hospital admissions for respiratory issues, especially asthma.

All of Connecticut has been in attainment of the annual National Ambient Air Quality Standard (NAAQS) for nitrogen dioxide since well before the Clean Air Act was amended in 1990.  On January 22, 2010, the U.S. Environmental Protection Agency (EPA) finalized revisions  to the NAAQS for NO₂, adopting a 1-hour standard of 100 ppb and retaining the existing annual standard of 53 ppb. In 2018, EPA concluded this standard was still adequate to protect public health and retained this standard. 

EPA has designated all of Connecticut as “unclassifiable/ attainment” for the 1-hour NO₂ NAAQS. See below for the documentation regarding designations.

EPA Retains the Existing NO₂ Standard Without Revision. April 18, 2016 (PDF)

Infrastructure SIP revision for the 2010 NO2 NAAQS, January 2, 2013

EPA's Rulemaking Finalizing NO2 Designations (PDF)

EPA's NO2 Designation Letter, January 20, 2012 (PDF)

Governor Rell's NO2 Designation Request, December 17, 2010 (PDF)

Air Quality Trends - Nitrogen Dioxide

Additional Information

EPA Nitrogen Oxides (NOx) Resources

National Ambient Air Quality Standards (NAAQS)

Content last updated June 12, 2018