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Nitrogen Dioxide Designations

Learn About Nitrogen Dioxide Designations

On this page:

Nitrogen dioxide designations process

Nitrogen dioxide (NO2) is one of a group of highly reactive gasses known as "oxides of nitrogen," or "nitrogen oxides (NOx)." Current scientific evidence links short-term NO2 exposures, ranging from 30 minutes to 24 hours, with an array of adverse respiratory effects, including increased asthma symptoms, more difficulty controlling asthma, and an increase in respiratory illnesses and symptoms. Reducing levels of NO2 pollution is an important part of EPA’s commitment to a clean, healthy environment.

Title I of the Clean Air Act requires that within two years after EPA sets a new National Ambient Air Quality Standard (NAAQS), or revises an existing standard, we must designate areas in the United States as being in "attainment" (i.e., meeting) or "nonattainment" (i.e., not meeting) with the standard.

The Clean Air Act requires states to submit, and gives tribes the opportunity to submit, initial area designation recommendations within 12 months following promulgation of a new or revised NAAQS. For the 2010 NO2 standards, EPA designated as “nonattainment” any area that violated the 2010 NO2 standards based on air quality data from 2010-2012.  Although tribes are not required to provide recommendations, they are invited to do so. Some tribes participated in this process.

Basis for air quality designations

EPA’s final designations are based on air quality monitoring data, recommendations submitted by states and tribes and other technical information. EPA provided guidance for the NO2 designations process in the preamble to the NO2 NAAQS rule.