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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 organizations 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).

Specifying the Criteria
It is important for any numerical standard that the averaging period, unit, and statistical measure are given (e.g. the 98th percentile of hourly means measured over a calendar year in micrograms per cubic metre (µg/m3)). Without these there is no common ground for a given criterion, making it confusing or even meaningless. Criteria can be set in different units (e.g. µg/m3, parts per billion by volume (ppbv), parts per billion by mass (ppb(mass)), parts per million (ppm)) and it is possible to convert between all of these units if you know the molecular mass of the pollutant and the temperature at which you want to convert. Different standard temperatures are used throughout the world and so it is important to state the temperature of conversion (if relevant). Most pollutants have ambient criteria in the parts per billion (ppb) or µg/m3 range. Some have smaller units (e.g. dioxins are often in pico-grams /m3); others have larger units (e.g. carbon monoxide (CO) in mg/m3). Particle pollution (e.g. PM10, PM1.0) is specified in units of mass (e.g. µg/m3) and not in units of volume (ppmv).

In the EU, the pollutants and their criteria are specified in Directive 2008/50/EC.

The Criteria
Below is a list of available air quality criteria from around the world. There is a lot of cross referencing between organizations (e.g. the International Finance Corp (IFC)) that have their own criteria. Ultimately, many criteria are based on those specified by the World Health Organization (WHO). It is important when complying with ambient air quality criteria to check the direct reference as well. Not all related caveats/controlling parameters of a criterion can be put in the table, hence, specified below are mostly the six criteria air pollutants most monitoring agencies specify. These agencies also list whether certain criteria must be monitored in a specific way to fit with compliance measures. * The level of the annual NO2 standard is 0.053 ppm. It is shown here in terms of ppb for the purposes of clearer comparison to the 1-hour standard level. * The previous SO2 standards (0.14 ppm 24-hour and 0.03 ppm annual) will additionally remain in effect in certain areas: (1) any area for which it is not yet 1 year since the effective date of designation under the current (2010) standards, and (2)any area for which an implementation plan providing for attainment of the current (2010) standard has not been submitted and approved and which is designated nonattainment under the previous SO2 standards or is not meeting the requirements of a SIP call under the previous SO2 standards (40 CFR 50.4(3)). A SIP call is an EPA action requiring a state to resubmit all or part of its State Implementation Plan to demonstrate attainment of the required NAAQS. * It is primary standards, providing public health protection, including protecting the health of "sensitive" populations such as asthmatics, children, and the elderly.

** It is secondary standards, providing public welfare protection, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings. * Final rule signed October 1, 2015, and effective December 28, 2015. The previous (2008) O3 standards additionally remain in effect in some areas. Revocation of the previous (2008) O3 standards and transitioning to the current (2015) standards will be addressed in the implementation rule for the current standards. * In areas designated nonattainment for the Pb standards prior to the promulgation of the current (2008) standards, and for which implementation plans to attain or maintain the current (2008) standards have not been submitted and approved, the previous standards (1.5 µg/m3 as a calendar quarter average) also remain in effect.