# Ambient air quality criteria

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).

## Specifying the criteria

It is important for any numerical standard that 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/m$^3$)). 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/m$^3$, 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 the molecular mass of the pollutant and the temperature are known. 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/m$^3$ range. Some have smaller units (e.g. dioxins are often in picograms/m$^3$); others have larger units (e.g. carbon monoxide (CO) in mg/m$^3$). Particle pollution (e.g. PM$_{10}$, PM$_{1.0}$) is specified in units of mass (e.g. μg/m$^3$) 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 around the world. There is a lot of cross referencing between organizations (e.g. the International Finance Corp (IFC)) that have their own criteria but 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, and therefore those specified below are mostly those that most monitoring agencies specify. These agencies also list whether 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 | NO$_2$ | 40 | μg/m$^3$ | Annual$^1$ | Mean | None | WHO | Guideline | Global | |
| Nitrogen dioxide | NO$_2$ | 200 | μg/m$^3$ | 1 hour | Mean | None | WHO | Guideline | Global | |
| Nitrogen dioxide | NO$_2$ | 40 | μg/m$^3$ | Annual | Mean | None | EU | Limit Value | EU Member States | Calendar Year |
| Nitrogen dioxide | NO$_2$ | 53 | ppb | Annual | Mean | None | US EPA | Legal | United States | Primary and Secondary |
| Nitrogen dioxide | NO$_2$ | 100 | μg/m$^3$ | Annual | Yrly mean of daily | None | SCENR | Legal | Qatar | Calendar Year |
| Nitrogen dioxide | NO$_2$ | 40 | μg/m$^3$ | Annual | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China | |
| Nitrogen dioxide | NO$_2$ | 100 | μg/m$^3$ | Daily | Mean | | New Zealand Ministry for the Environment | Guideline | New Zealand | Calendar Year |
| Nitrogen dioxide | NO$_2$ | 113 | μg/m$^3$ | Daily | Daily mean of hourly | None | Japanese Ministry of Environment | Legal | Japan | Quoted as 0.06 ppm |
| Nitrogen dioxide | NO$_2$ | 150 | μg/m$^3$ | Daily | 99.7%ile | 1 day/yr | SCENR | Legal | Qatar | Calendar Year |
| Nitrogen dioxide | NO$_2$ | 80 | μg/m$^3$ | Daily | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China | |
| Nitrogen dioxide | NO$_2$ | 100 | ppb | 1 hour | 98% of 1-hour daily maximum concentrations, averaged over 3 years | None | US EPA | Legal | United States | Primary |
| Nitrogen dioxide | NO$_2$ | 40 | μg/m$^3$ | Annual | Mean | None | India Central Pollution Control Board (CPCB) | Legal | India | Calendar Year |
| Nitrogen dioxide | NO$_2$ | 30 | μg/m$^3$ | Annual | Mean | None | India Central Pollution Control Board(CPCB) | Legal | India - Ecologically sensitive areas | Calendar Year |
| Nitrogen dioxide | NO$_2$ | 80 | μg/m$^3$ | 24 hour$^2$ | Mean | None | India Central Pollution Control Board (CPCB) | Legal | India | Calendar Year |
| Nitrogen dioxide | NO$_2$ | 200 | μg/m$^3$ | 1 hour | 99.79%ile | 18 hrs/yr | EU | Limit value | EU Member States | Calendar Year |
| Nitrogen dioxide | NO$_2$ | 200 | μg/m$^3$ | 1 hour | Mean | 9/yr | New Zealand Ministry for the Environment | Legal | New Zealand | Calendar Year |
| Nitrogen dioxide | NO$_2$ | 400 | μg/m$^3$ | 1 hour | 99.7%ile | 26 hrs/yr | SCENR | Legal | Qatar | Calendar Year |
| Nitrogen dioxide | NO$_2$ | 200 | μg/m$^3$ | 1 hour | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China | |
| Pollutant | Abbr. | Value | Units | Averaging Period | Statistical Measure | # exceedences | Organisation | Status | Applicability | Notes |
| Sulphur dioxide | SO$_2$ | 75 | ppb | 1 hour | 99% of 1-hour daily maximum concentrations, averaged over 3 years | None | US EPA | Legal | United States* | Primary |
| Sulphur dioxide | SO$_2$ | 500 | ppb | 3 hours | Mean | Once per year | US EPA | Legal | United States | Secondary quoted as 0.5 ppm |
| Sulphur dioxide | SO$_2$ | 20 | μg/m$^3$ | Annual | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 1 Areas | |
| Sulphur dioxide | SO$_2$ | 60 | μg/m$^3$ | Annual | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 2 Areas | |
| Sulphur dioxide | SO$_2$ | 50 | μg/m$^3$ | Daily | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 1 Areas | |
| Sulphur dioxide | SO$_2$ | 20 | μg/m$^3$ | Annual | Mean | None | India Central Pollution Control Board (CPCB) | Legal | India - Ecologically sensitive areas | |
| Sulphur dioxide | SO$_2$ | 50 | μg/m$^3$ | Annual | Mean | None | India Central Pollution Control Board (CPCB) | Legal | India | |
| Sulphur dioxide | SO$_2$ | 80 | μg/m$^3$ | 24 hour | Mean | None | India Central Pollution Control Board (CPCB) | Legal | India | |
| Sulphur dioxide | SO$_2$ | 150 | μg/m$^3$ | Daily | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 2 Areas | |
| Sulphur dioxide | SO$_2$ | 150 | μg/m$^3$ | 1 hour | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 1 Areas | |
| Sulphur dioxide | SO$_2$ | 500 | μg/m$^3$ | 1 hour | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 2 Areas | |
| Sulphur dioxide | SO$_2$ | 125 | μg/m$^3$ | 24 hour | Any 24 hour period | None | WHO | Interim Target (IT) 1 | Global | Was the guideline in 2000 |
| Sulphur dioxide | SO$_2$ | 50 | μg/m$^3$ | 24 hour | Any 24 hour period | None | WHO | Interim Target (IT) 2 | Global | Calendar Year (1 Jan – 31 Dec) |
| Sulphur dioxide | SO$_2$ | 125 | μg/m$^3$ | 24 hour | Any 24 hour mean | None | WHO | Interim Target (IT) 1 | Global | |
| Sulphur dioxide | SO$_2$ | 50 | μg/m$^3$ | 24 hour | Any 24 hour mean | None | WHO | Interim Target (IT) 2 | Global | |
| Sulphur dioxide | SO$_2$ | 20 | μg/m$^3$ | 24 hour | Any 24 hour mean | None | WHO | Guideline | Global | |
| Sulphur dioxide | SO$_2$ | 500 | μg/m$^3$ | 10 minute | Any 10 minute mean | None | WHO | Guideline | Global | |
| Sulphur dioxide | SO$_2$ | 1300 | μg/m$^3$ | 1 hour | 99.99%ile | 1 hr/year | Ras Laffan | Legal | Qatar | Calendar Year |
| Sulphur dioxide | SO$_2$ | 350 | μg/m$^3$ | 1 hour | Mean | 9/yr | New Zealand Ministry for the Environment | Legal | New Zealand | |
| Sulphur dioxide | SO$_2$ | 570 | μg/m$^3$ | 1 hour | Mean | None | New Zealand Ministry for the Environment | Legal | New Zealand | |
| Sulphur dioxide | SO$_2$ | 120 | μg/m$^3$ | 24 hour | Average | | New Zealand Ministry for the Environment | Guideline | New Zealand | |
| Abbr. | Value | Units | Averaging Period | Statistical Measure | # exceedences | Organisation | Status | Applicability | Notes |
| PM$_{10}$ "Coarse Particulate Matter" | 150 | μg/m$^3$ | 24 hour | Average over 3 years | Once per year | US EPA | Legal | United States | Primary and Secondary |
| PM$_{10}$ | 125 | μg/m$^3$ | 24 hour | 99th %ile | 3 days/yr | WHO | Guideline | Global | |
| PM$_{10}$ | 50 | μg/m$^3$ | 24 hour | Mean | None | WHO | Guideline | Global | |
| PM$_{10}$ | 20 | μg/m$^3$ | Annual | Mean | None | WHO | Guideline | Global | |
| PM$_{10}$ | 60 | μg/m$^3$ | Annual | Mean | None | India Central Pollution Control Board (CPCB) | Legal | India | |
| PM$_{10}$ | 10 | μg/m$^3$ | 24 hour | Mean | None | India Central Pollution Control Board (CPCB) | Legal | India | |
| PM$_{10}$ | 50 | μg/m$^3$ | 24 hour | Mean | 1/yr | New Zealand Ministry for the Environment | Legal | New Zealand | |
| PM$_{10}$ | 40 | μg/m$^3$ | Annual | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 1 Areas | |
| PM$_{10}$ | 70 | μg/m$^3$ | Annual | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 2 Areas | |
| PM$_{10}$ | 50 | μg/m$^3$ | 24 hour | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 1 Areas | |
| PM$_{10}$ | 150 | μg/m$^3$ | 24 hour | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 2 Areas | |
| PM$_{2.5}$ "Fine Particulate Matter" | 35 | μg/m$^3$ | 24 hour | 98% averaged over 3 years | None | US EPA | Legal | United States | Primary and Secondary |
| PM$_{2.5}$ | 25 | μg/m$^3$ | 24 hour | Any 24 hr mean | None | WHO | Guideline | Global | |
| PM$_{2.5}$ | 60 | μg/m$^3$ | 24 hour | mean | None | India Central Pollution Control Board (CPCB) | Legal | India | |
| PM$_{2.5}$ | 40 | μg/m$^3$ | Annual | Mean | None | India Central Pollution Control Board (CPCB) | Legal | Global | |
| PM$_{2.5}$ | 75 | μg/m$^3$ | 24 hour | Any 24 hr mean | None | WHO | Interim target (IT) 1 | Global | |
| PM$_{2.5}$ | 50 | μg/m$^3$ | 24 hour | Any 24 hr mean | None | WHO | Interim target (IT) 2 | Global | |
| PM$_{2.5}$ | 37.5 | μg/m$^3$ | 24 hour | Any 24 hr mean | None | WHO | Interim target (IT) 3 | Global | |
| PM$_{2.5}$ | 12.0 | μg/m$^3$ | Annual | Annual mean over 3 years | None | US EPA | Legal | United States | Primary |
| PM$_{2.5}$ | 15 | μg/m$^3$ | Annual | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 1 Areas | |
| PM$_{2.5}$ | 35 | μg/m$^3$ | Annual | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 2 Areas | |
| PM$_{2.5}$ | 15.0 | μg/m$^3$ | Annual | Annual mean over 3 years | None | US EPA | Legal | United States | Secondary |
| PM$_{2.5}$ | 10 | μg/m$^3$ | Annual | Mean | None | WHO | Guideline | Global | |
| PM$_{2.5}$ | 25 | μg/m$^3$ | 24 hour | Mean | None | WHO | Guideline | Global | |
| PM$_{2.5}$ | 25 | μg/m$^3$ | Annual | Mean | None | WHO | Interim target (IT) 2 | Global | |
| PM$_{2.5}$ | 15 | μg/m$^3$ | Annual | Mean | None | WHO | Interim target (IT) 3 | Global | |
| PM$_{2.5}$ | 35 | μg/m$^3$ | 24 hour | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 1 Areas | |
| PM$_{2.5}$ | 75 | μg/m$^3$ | 24 hour | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 2 Areas | |
| Pollutant | Abbr. | Value | Units | Averaging Period | Statistical Measure | # exceedences | Organisation | Status | Applicability | Notes |
| Ozone | O$_3$ | 0.07 | ppm | 8 hours | Annual 4th highest daily maximum 8-hour concentration, averaged over 3 years | 3/yr | US EPA | Legal | United States* | Primary and Secondary |
| Ozone | O$_3$ | 100 | μg/m$^3$ | 8 hours | Any 8 hour mean | None | WHO | Guideline | Global | |
| Ozone | O$_3$ | 100 | μg/m$^3$ | 8 hours | Mean | None | India Central Pollution Control Board (CPCB) | Guideline | India | |
| Ozone | O$_3$ | 180 | μg/m$^3$ | 1 hour | Mean | None | India Central Pollution Control Board (CPCB) | Legal | India | |
| Ozone | O$_3$ | 160 | μg/m$^3$ | 8 hours | Any 8 hour mean | None | WHO | Interim target | Global | |
| Ozone | O$_3$ | 100 | μg/m$^3$ | 8 hours | Daily maximum 8-hour averaged concentration | None | Chinese Ministry of Ecology and Environment | Legal | China Category 1 Areas | |
| Ozone | O$_3$ | 160 | μg/m$^3$ | 8 hours | Daily maximum 8-hour averaged concentration | None | Chinese Ministry of Ecology and Environment | Legal | China Category 2 Areas | |
| Ozone | O$_3$ | 160 | μg/m$^3$ | 1 hour | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 1 Areas | |
| Ozone | O$_3$ | 200 | μg/m$^3$ | 1 hour | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China Category 2 Areas | |
| Ozone | O$_3$ | 235 | μg/m$^3$ | 1 hour | 99.9% | 8 hrs/yr | SCENR | Legal | Qatar | Calendar Year |
| Ozone | O$_3$ | 120 | μg/m$^3$ | 8 hours | 99.8% | 2x8hrs/yr | SCENR | Legal | Qatar | Calendar Year |
| Ozone | O$_3$ | 150 | μg/m$^3$ | 1 hour | Mean | None | New Zealand Ministry for the Environment | Legal | New Zealand | |
| 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 | Primary |
| Carbon monoxide | CO | 40 | mg/m$^3$ | 1 hour | Maximum | None | SCENR | Legal | Qatar | Calendar Year |
| Carbon monoxide | CO | 4 | mg/m$^3$ | 1 hour | Maximum | None | India Central Pollution Control Board (CPCB) | Legal | India | |
| Carbon monoxide | CO | 2 | mg/m$^3$ | 8 hour | Maximum | None | India Central Pollution Control Board (CPCB) | Legal | India | |
| Carbon monoxide | CO | 10 | mg/m$^3$ | 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 | Primary |
| Carbon monoxide | CO | 10 | mg/m$^3$ | 8 hour | Maximum | None | SCENR | Legal | Qatar | Calendar Year |
| Carbon monoxide | CO | 10 | mg/m$^3$ | 8 hour | Mean | 1/yr | New Zealand Ministry for the Environment | Legal | New Zealand | |
| Carbon monoxide | CO | 30 | mg/m$^3$ | 1 hour | Mean | 1/yr | New Zealand Ministry for the Environment | Legal | New Zealand | |
| Carbon monoxide | CO | 4 | mg/m$^3$ | 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/m$^3$ | 3 month | Rolling 3 month average | None | US EPA | Legal | United States | Primary and Secondary |
| Lead | Pb | 0.5 | μg/m$^3$ | Annual | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China | |
| Lead | Pb | 0.5 | μg/m$^3$ | Annual | Mean | None | India Central Pollution Control Board (CPCB) | Legal | India | |
| Lead | Pb | 1 | μg/m$^3$ | 24 hours | Mean | None | India Central Pollution Control Board (CPCB) | Legal | India | |
| Lead | Pb | 1 | μg/m$^3$ | Season | Mean | None | Chinese Ministry of Ecology and Environment | Legal | China | |

## 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.