Non-attainment area

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In the United States, a non-attainment area is a place where outdoor air does not meet the national pollution limits for one or more pollutants. The polluted pollutants tracked are ozone, fine and coarse particle pollution (PM2.5 and PM10), lead, carbon monoxide, sulfur oxides, and nitrogen oxides.

These limits come from the National Ambient Air Quality Standards (NAAQS) set under the Clean Air Act. For each pollutant, there are two standards: primary standards to protect human health, and secondary standards to protect public welfare, such as buildings, crops, and vehicles. The standards have been updated several times since 1970.

If an area exceeds the limit for a pollutant, it is in non-attainment for that pollutant. An area can be non-attainment for one pollutant and attainment for others, but many areas are non-attainment for multiple pollutants because these pollutants are often connected.

When the EPA updates or adds a standard, it checks whether the country meets the NAAQS across all areas. States and tribal governments must propose whether their areas are attainment, non-attainment, or unclassifiable within about a year. The EPA then designates each region within roughly two years.

If an area is designated non-attainment, the state or local government must develop a plan (State Implementation Plan, or SIP) to reduce pollution within about three years. Tribes are encouraged to create a plan, but not required.

An area can apply to be redesignated as attainment after three consecutive years of meeting the standard. If approved, the area becomes attainment/unclassifiable. If it later fails to meet the standard again, it can be redesignated as non-attainment.

SIPs come in different types. Infrastructure SIPs cover the entire state and show how the state will maintain and meet the NAAQS. Nonattainment area SIPs are tailored to the specific area. All SIPs must be approved by the EPA; if a state does not submit a SIP or the SIP is denied, the EPA can issue a Federal Implementation Plan (FIP).

The NAAQS are reviewed every five years to ensure they reflect current science and public health knowledge.

The EPA maintains a Green Book listing non-attainment areas by county, with data on when a county did not meet the standards. As of October 31, 2023, 258 counties were not attaining the standards for at least one pollutant, and since 1992 a total of 2,094 counties have been non-attainment for at least one pollutant for at least one year.