Ambient air quality criteria or standards are concentrations of pollutants in the air (usually outdoor air but sometimes indoor air) 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.
Contents
Various organisations have proposed criteria e.g. WHO, EU, US EPA and they 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
Important for any numerical standard is to ensure that averaging period, unit, and statistical measure are given (e.g. 98th percentile of hourlry means measured over a calendar year in micrograms per cubic metre (µg/m3)). Without all of these three aspects the criterion is confusing and 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 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)/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).
The Criteria
Below is a list of available criteria around the world. There is a lot of cross referencing (e.g. the International Finance Corp (IFC) has their own criteria but they are a copy of those specified by the WHO. It is important to check the reference as not all the related caveats/controlling parameters of the criterion can be put in the table. Also some criteria require certain specific ways of monitoring to demonstrate compliance.