The new one-hour standard for NO₂ is set at a level of 100 parts per billion (ppb). This level defines the maximum allowable concentration anywhere in an area. EPA also is retaining the existing annual average standard of 53 ppb. NO₂ is formed from vehicle, power plant and other industrial emissions, and contributes to the formation of fine particle pollution and smog.

Short-term exposures to NO₂ have been linked to impaired lung function and increased respiratory infections, especially in people with asthma.

EPA is establishing new monitoring requirements in urban areas that will measure NO₂ levels around major roads and across the community. Monitors must be located near roadways in cities with at least 500,000 residents. Larger cities and areas with major roadways will have additional monitors. Community-wide monitoring will continue in cities with at least 1 million residents.

Working with the states, EPA will site at least 40 monitors in locations to help protect communities that are susceptible and vulnerable to elevated levels of NO₂.

EPA expects to identify or designate areas not meeting the new standard, based on the existing community-wide monitoring network, by January 2012. New monitors must begin operating no later than January 1, 2013. When three years of air quality data are available from the new monitoring network, EPA intends to redesignate areas as appropriate.