National Air Quality Index
National Air Quality Index (AQI) transforms complex air quality data of eight pollutants into a single number (index value), nomenclature and colour.
National Air Quality Index (AQI) was launched on 17 October 2014 to disseminate information on air quality in an easily understandable form for the general public. The measurement of air quality is based on eight pollutants, namely,
- Particulate Matter (size less than 10 µm) or (PM10),
- Particulate Matter (size less than 2.5 µm) or (PM2.5),
- Nitrogen Dioxide (NO2),
- Sulphur Dioxide (SO2),
- Carbon Monoxide (CO),
- Ozone (O3),
- Ammonia (NH3), and
- Lead (Pb)
for which short-term (up to 24-hourly averaging period) National Ambient Air Quality Standards are prescribed. It may be noted that ambient air quality standards are specified separately in India for around 12 pollutants including the 8 that constitute the Air Quality Index.
AQI has six categories of air quality. These are: Good, Satisfactory, Moderately Polluted, Poor, Very Poor and Severe. The AQI values and corresponding ambient concentrations (health breakpoints) for the identified eight pollutants are as follows:
|AQI Category, Pollutants and Health Breakpoints|
|AQI Category (Range)↓||→ Categories for the various readings of the pollutant based on the health breakpoints or health impacts|
|Satisfactory (51-100)||51-100||31-60||41-80||51-100||1.1-2.0||41-80||201-400||0.5 –1.0|
|Moderately polluted (101-200)||101-250||61-90||81-180||101-168||2.1- 10||81-380||401-800||1.1-2.0|
|Very poor (301-400)||351-430||121-250||281-400||209-748*||17-34||801-1600||1200-1800||3.1-3.5|
|Severe (401-500)||430 +||250+||400+||748+*||34+||1600+||1800+||3.5+|
*One hourly monitoring (for mathematical calculations only)
The AQI Index values and their associated health impacts are as follows:
|AQI||Associated Health Impacts|
|Good (0–50)||Minimal Impact|
|Satisfactory (51–100)||May cause minor breathing discomfort to sensitive people.|
|Moderately polluted (101–200)||May cause breathing discomfort to people with lung disease such as asthma, and discomfort to people with heart disease, children and older adults.|
|Poor (201–300)||May cause breathing discomfort to people on prolonged exposure, and discomfort to people with heart disease|
|Very Poor (301–400)||May cause respiratory illness to the people on prolonged exposure. Effect may be more pronounced in people with lung and heart diseases.|
|Severe (401-500)||May cause respiratory impact even on healthy people, and serious health impacts on people with lung/heart disease. The health impacts may be experienced even during light physical activity.|
AQI is considered as ‘One Number- One Colour-One Description’ for the common man to judge the air quality within his vicinity. The formulation of the index was an initiative under Swachh Bharat Mission (Cleanliness Mission), based on the recommendations of IIT Kanpur and the Expert Group formed in this regard. The earlier measuring index in this regard was limited to three indicators, while the current measurement index had been expanded with five additional parameters.
Construction of the AQI
Based on the measured ambient air concentrations, corresponding standards and likely health impact (known as health breakpoints), a sub-index is calculated for each of the pollutants. A sub-index is a linear function of concentration (e.g. the sub-index for PM2.5 will be 51 at concentration 31 µg/m3, 100 at concentration 60 µg/m3, and 75 at concentration of 45 µg/m3)
The formula for calculating a sub index is as follows:
Sub Index for a pollutant = Upper limit of the previous AQI category to which the pollutant’s current reading would have fallen +[(current reading – upper limit of the previous reading category of the pollutant)* (width or interval of the AQI category for the current level of reading / width or interval of the current reading category of the pollutant)]
For PM2.5 reading coming at a concentration of 150 µg/m3 , the sub index for PM2.5 would be = 300+[(150-120)*100/130] = 323
For PM2.5 reading coming at a concentration of 45 µg/m3 , the sub index for PM2.5 would be = 30+[(45-30)*50/30] = 75
The worst sub-index reflects overall AQI. For instance, if the sub index of PM2.5 =75, SO2 = 63, NO2 =38 then the AQI will be 75 which is the same as the value of the sub index of PM2.5. .
The excel sheet for calculating AQI, as uploaded by CPCB, may be seen here.
The live city wise readings on AQI can be seen here.
The Sub-indices for individual pollutants at a monitoring location are calculated generally using its 24-hourly average concentration value (8-hourly in case of CO and O3) and health breakpoint concentration range (e.g. AQI at 6 am on a day will incorporate data from 6am on previous day to the current day). However, all the eight pollutants may not be monitored at all the locations. Hence, overall AQI is calculated only if data are available for minimum three pollutants out of which one should necessarily be either PM2.5 or PM10. Else, data are considered insufficient for calculating AQI. But the sub-indices for monitored pollutants are calculated and disseminated, even if data are inadequate for determining AQI. Further, a minimum of 16 hours’ data is considered necessary for calculating sub index.
AQI index values can vary depending on the time of the day. AQI reflects the status of the worst pollutant in that city. i.e. higher reading in one city can be due to high concentration of PM whereas in some other city it may be due to SO2. Even if the other 7 pollutants have a benign reading, if one pollutant is in the “poor” category then AQI will be in the “poor” category.
The web-based system designed to provide AQI on real time basis is an automated system that captures data from monitoring stations (at present available only for 10 cities but it is planned for 46 cities which has over 1 million population and to 20 state capitals) on a continuous basis without human intervention, and displays AQI based on running average values. For manual monitoring stations, an AQI calculator is developed wherein data can be fed manually to get AQI value.
International comparison of air quality
WHO also measures air quality across major cities of the globe which may be seen here. The WHO database contains results of ambient (outdoor) air pollution monitoring from almost 1600 cities in 91 countries. Air quality is represented by annual mean concentration of particulate matter (PM10 and PM2.5, i.e. particles smaller than 10 or 2.5 microns). The world's average PM10 levels by region range from 26 to 208 μg/m3, with a world's average of 71 μg/m3.
- Press Release of Ministry of Environment and Forest dated 17 October 2014
- Air Quality Index – Central Pollution Control Board