Ambient air quality criteria

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Ambient air quality criteria, or standards, are concentrations of pollutants in the air, and typically refer to outdoor air. The criteria are specified for a variety of reasons including for the protection of human health, buildings, crops, vegetation, ecosystems, as well as for planning and other purposes. There is no internationally accepted definition but usually "standards" have some legal or enforcement aspect, whereas "guidelines" may not be backed by laws. "Criteria/criterion" can be used as a generic term to cover standards and guidelines.

Various organisations have proposed criteria e.g. WHO, EU, US EPA. These criteria are often similar – but not always, even if they are proposed for the same purpose (e.g. the protection of human health).