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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, PM_1.0) 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.

Pollutant	Abbr.	Value	Units	Averaging Period	Statistical Measure	# exceedences	Organisation	Status	Applicability	Notes
Nitrogen dioxide	NO2	40	µg/m3	Annual1	Mean	None	WHO	Guideline	Global
Nitrogen dioxide	NO2	200	µg/m3	1 hour	Mean	None	WHO	Guideline	Global
Nitrogen dioxide	NO2	40	µg/m3	Annual	Mean	None	EU	Limit Value	EU Member States	Calendar Year
Nitrogen dioxide	NO2	53*	ppb	Annual	Mean	None	US EPA	Legal	United States
Nitrogen dioxide	NO2	100	µg/m3	Annual	Yrly mean of daily	None	SCENR	Legal	Qatar	Calendar Year
Nitrogen dioxide	NO2	40	µg/m3	Annual	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China
Nitrogen dioxide	NO2	100	µg/m3	Daily	Mean		New Zealand Ministry for the Environment	Guideline	New Zealand	Calendar Year
Nitrogen dioxide	NO2	113	µg/m3	Daily	Daily mean of hourly	None	Japanese Ministry of Environment	Legal	Japan	Quoted as 0.06 ppm
Nitrogen dioxide	NO2	150	µg/m3	Daily	99.7%ile	1 day/yr	SCENR	Legal	Qatar	Calendar Year
Nitrogen dioxide	NO2	80	µg/m3	Daily	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China
Nitrogen dioxide	NO2	100	ppb	1 hour	98% of 1-hour daily maximum concentrations, averaged over 3 years	None	US EPA	Legal	United States
Nitrogen dioxide	NO2	40	µg/m3	Annual	Mean	None	India Central Pollution Control Board (CPCB)	Legal	India	Calendar Year
Nitrogen dioxide	NO2	30	µg/m3	Annual	Mean	None	India Central Pollution Control Board(CPCB)	Legal	India - Ecologically sensitive areas	Calendar Year
Nitrogen dioxide	NO2	80	µg/m3	24 hour2	Mean	None	India Central Pollution Control Board (CPCB)	Legal	India	Calendar Year
Nitrogen dioxide	NO2	200	µg/m3	1 hour	99.79%ile	18 hrs/yr	EU	Limit value	EU Member States	Calendar Year
Nitrogen dioxide	NO2	200	µg/m3	1 hour	Mean	9/yr	New Zealand Ministry for the Environment	Legal	New Zealand	Calendar Year
Nitrogen dioxide	NO2	400	µg/m3	1 hour	99.7%ile	26 hrs/yr	SCENR	Legal	Qatar	Calendar Year
Nitrogen dioxide	NO2	200	µg/m3	1 hour	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China

* 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.

Pollutant	Abbr.	Value	Units	Averaging Period	Statistical Measure	# exceedences	Organisation	Status	Applicability	Notes
Sulphur dioxide	SO2	75	ppb	1 hour	99% of 1-hour daily maximum concentrations, averaged over 3 years	None	US EPA	Legal	United States*
Sulphur dioxide	SO2	0.5	ppm	3 hours	Mean	Once per year	US EPA	Legal	United States
Sulphur dioxide	SO2	20	µg/m3	Annual	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 1 Areas
Sulphur dioxide	SO2	60	µg/m3	Annual	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 2 Areas
Sulphur dioxide	SO2	50	µg/m3	Daily	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 1 Areas
Sulphur dioxide	SO2	20	µg/m3	Annual	Mean	None	India Cental Pollution Control Board (CPCB)	Legal	India - Ecologically sensitive areas
Sulphur dioxide	SO2	50	µg/m3	Annual	Mean	None	India Cental Pollution Control Board (CPCB)	Legal	India
Sulphur dioxide	SO2	80	µg/m3	24 hour	Mean	None	India Cental Pollution Control Board (CPCB)	Legal	India
Sulphur dioxide	SO2	150	µg/m3	Daily	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 2 Areas
Sulphur dioxide	SO2	150	µg/m3	1 hour	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 1 Areas
Sulphur dioxide	SO2	500	µg/m3	1 hour	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 2 Areas
Sulphur dioxide	SO2	125	µg/m3	24 hour	Any 24 hour period	None	WHO	Interim Target (IT) 1	Global	Was the guideline in 2000
Sulphur dioxide	SO2	50	µg/m3	24 hour	Any 24 hour period	None	WHO	Interim Target (IT) 2	Global	Calendar Year (1 Jan - 31 Dec)
Sulphur dioxide	SO2	125	µg/m3	24 hour	Any 24 hour mean	None	WHO	Interim Target (IT) 1	Global
Sulphur dioxide	SO2	50	µg/m3	24 hour	Any 24 hour mean	None	WHO	Interim Target (IT) 2	Global
Sulphur dioxide	SO2	20	µg/m3	24 hour	Any 24 hour mean	None	WHO	Guideline	Global
Sulphur dioxide	SO2	500	µg/m3	10 minute	Any 10 minute mean	None	WHO	Guideline	Global
Sulphur dioxide	SO2	1300	µg/m3	1 hour	99.99%ile	1 hr/year	Ras Laffan	Legal	Qatar	Calendar Year
Sulphur dioxide	SO2	350	µg/m3	1 hour	Mean	9/yr	New Zealand Min.for Environment	Legal	New Zealand
Sulphur dioxide	SO2	570	µg/m3	1 hour	Mean	None	New Zealand Ministry for the Environment	Legal	New Zealand
Sulphur dioxide	SO2	120	µg/m3	24 hour	Average		New Zealand Ministry for the Environment	Guideline	New Zealand

* 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.

Abbr.	Value	Units	Averaging Period	Statistical Measure	# exceedences	Organisation	Status	Applicability	Notes
PM10 "Coarse Particulate Matter"	150	µg/m3	24 hour	Average over 3 years	Once per year	US EPA	Legal	United States
PM10	125	µg/m3	24 hour	99th %ile	3 days/yr	WHO	Guideline	Global
PM10	50	µg/m3	24 hour	Mean	None	WHO	Guideline	Global
PM10	20	µg/m3	Annual	Mean	None	WHO	Guideline	Global
PM10	60	µg/m3	Annual	Mean	None	India - Central Pollution Control Board (CPCB)	Legal	India
PM10	10	µg/m3	24 hour	Mean	None	India - Central Pollution Control Board (CPCB)	Legal	India
PM10	50	µg/m3	24 hour	Mean	1/yr	New Zealand Ministry for the Environment	Legal	New Zealand
PM10	40	µg/m3	Annual	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 1 Areas
PM10	70	µg/m3	Annual	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 2 Areas
PM10	50	µg/m3	24 hour	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 1 Areas
PM10	150	µg/m3	24 hour	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 2 Areas
PM2.5 "Fine Particulate Matter"	35	µg/m3	24 hour	98% averaged over 3 years	None	US EPA	Legal	United States
PM2.5	25	µg/m3	24 hour	Any 24 hr mean	None	WHO	Guideline	Global
PM2.5	60	µg/m3	24 hour	mean	None	India Central Pollution Control Band (CPCB)	Legal	India
PM2.5	40	µg/m3	Annual	Mean	None	India - Central Pollution Control Board (CPCB)	Legal	Global
PM2.5	75	µg/m3	24 hour	Any 24 hr mean	None	WHO	Interim target (IT) 1	Global
PM2.5	50	µg/m3	24 hour	Any 24 hr mean	None	WHO	Interim target (IT) 2	Global
PM2.5	37.5	µg/m3	24 hour	Any 24 hr mean	None	WHO	Interim target (IT) 3	Global
PM2.5	12.0*	µg/m3	Annual	Annual mean over 3 years	None	US EPA	Legal	United States
PM2.5	15	µg/m3	Annual	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 1 Areas
PM2.5	35	µg/m3	Annual	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 2 Areas
PM2.5	15.0**	µg/m3	Annual	Annual mean over 3 years	None	US EPA	Legal	United States
PM2.5	10	µg/m3	Annual	Mean	None	WHO	Guideline	Global
PM2.5	25	µg/m3	24 hour	Mean	None	WHO	Guideline	Global
PM2.5	25	µg/m3	Annual	Mean	None	WHO	Interim target (IT) 2	Global
PM2.5	15	µg/m3	Annual	Mean	None	WHO	Interim target (IT) 3	Global
PM2.5	35	µg/m3	24 hour	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 1 Areas
PM2.5	75	µg/m3	24 hour	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 2 Areas

* 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.

Pollutant	Abbr.	Value	Units	Averaging Period	Statistical Measure	# exceedences	Organisation	Status	Applicability	Notes
Ozone	O3	0.07	ppm	8 hours	Annual 4th highest daily maximum 8-hour concentration, averaged over 3 years	3/yr	US EPA	Legal	United States*
Ozone	O3	100	µg/m3	8 hours	Any 8 hour mean	None	WHO	Guideline	Global
Ozone	O3	100	µg/m3	8 hours	Mean	None	India Central Pollution Control Board (CPCB)	Guideline	India
Ozone	O3	180	µg/m3	1 hour	Mean	None	India Central Pollution Control Band (CPCB)	Legal	India
Ozone	O3	160	µg/m3	8 hours	Any 8 hour mean	None	WHO	Interim target	Global
Ozone	O3	100	µg/m3	8 hours	Daily maximum 8-hour averaged concentration	None	Chinese Ministry of Ecology and Environment	Legal	China Category 1 Areas
Ozone	O3	160	µg/m3	8 hours	Daily maximum 8-hour averaged concentration	None	Chinese Ministry of Ecology and Environment	Legal	China Category 2 Areas
Ozone	O3	160	µg/m3	1 hour	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 1 Areas
Ozone	O3	200	µg/m3	1 hour	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China Category 2 Areas
Ozone	O3	235	µg/m3	1 hour	99.9%	8 hrs/yr	SCENR	Legal	Qatar	Calendar Year
Ozone	O3	120	µg/m3	8 hours	99.8%	2x8hrs/yr	SCENR	Legal	Qatar	Calendar Year
Ozone	O3	150	µg/m3	1 hour	Mean	None	New Zealand Ministry for the Environment	Legal	New Zealand

* 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.

Pollutant	Abbr.	Value	Units	Averaging Period	Statistical Measure	# exceedences	Organisation	Status	Applicability	Notes
Carbon monoxide	CO	35	ppm	1 hour	Mean	1/yr	US EPA	Legal	United States
Carbon monoxide	CO	40000	µg/m3	1 hour	Maximum	None	SCENR	Legal	Qatar	Calendar Year
Carbon monoxide	CO	04	mg/m3	1 hour	Maximum	None	India Central Pollution Control Band (CPCB)	Legal	India
Carbon monoxide	CO	02	mg/m3	8 hour	Maximum	None	India Central Pollution Control Band (CPCB)	Legal	India
Carbon monoxide	CO	10	mg/m3	1 hour	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China
Carbon monoxide	CO	9	ppm	8 hour	Mean	1/yr	US EPA	Legal	United States
Carbon monoxide	CO	10000	µg/m3	8 hour	Maximum	None	SCENR	Legal	Qatar	Calendar Year
Carbon monoxide	CO	10	µg/m3	8 hour	Running mean	1/yr	New Zealand Ministry for the Environment	Legal	New Zealand
Carbon monoxide	CO	4	mg/m3	24 hour	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China

Pollutant	Abbr.	Value	Units	Averaging Period	Statistical Measure	# exceedences	Organisation	Status	Applicability	Notes
Lead	Pb	0.15*	µg/m3	3 month	Rolling 3 month average	None	US EPA	Legal	United States
Lead	Pb	0.5	µg/m3	Annual	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China
Lead	Pb	0.5	µg/m3	Annual	Mean	None	India Central Pollution Control Board (CPCB)	Legal	India
Lead	Pb	1	µg/m3	24 hours	Mean	None	India Central Pollution Control Board (CPCB)	Legal	India
Lead	Pb	1	µg/m3	Season	Mean	None	Chinese Ministry of Ecology and Environment	Legal	China

* 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.

Footnotes

[1] Annual arithmetic mean of a minimum of 104 measurements in a year at a particular site, such as taken twice weekly as uniform 24-hour samples.

[2] 24/8/1 hourly monitored values, as applicable, shall be complied with 98% of the time, they may exceed the limits but not on two consecutive days of monitoring.

References

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