What Air Quality Index tells you about air pollution?

What Air Quality Index tells you about air pollution?

AQI, which forms the basis of any debate over the increase or decrease in air pollution, allows measurement of multiple pollutants and is indexed on the worst-performing pollutant at a given time, which experts find confusing.

By Garima Prasher

Since 2015, India has been measuring air pollution using the National Air Quality Index (AQI), which averages pollution over the day or across a large area. It also takes into account several air pollutants like (Particle Pollution) PM 10 and PM 2.5. Experts now recommend that for the sake of simplicity and better clarity of data, we should take into account just one pollutant – PM 2.5.

“If you have to ask the question, “has air quality improved over time?”, I don’t know if I can use AQI to answer that. It is not a linear method. Measuring multiple pollutants might be helpful, but we should stick to dominant pollutant all the time, which in India is PM 2.5. For us to have a reasonable conversation with public and decision-makers, it is useful to focus on one pollutant and use concentration to measure air quality,” says Santosh Harish, fellow, Centre for Policy Research.

AQI, which forms the basis of any debate over the increase or decrease in air pollution, allows measurement of multiple pollutants and is indexed on the worst-performing pollutant at a given time, which experts find confusing.

In most of the northern cities, PM 2.5 level is high and experts are of the view that it should be used as an indicator of pollution level rather than measuring eight pollutants to deduce air quality. Moreover, if the idea is to raise awareness, talking about an expanded list of eight pollutants can get confusing.

Severe shortage of monitoring stations

According to the Central Pollution Control Board (CPCB) guidelines, India needs at least 4,098 Compact Air Quality Monitoring Stations (CAQMS) to measure AQI but we have only 200 such stations. Not just that, as many as 16 states and union territories are still awaiting their first CAQMS.

Moreover, most of the cities have just one station to represent the full array of pollutants. Statistically speaking, such samples might be insignificant and paint a wrong picture of air quality.


Two wheeler riders are most exposed to air pollution when stuck in long traffic jams

According to Brikesh Singh, coordinator, Clean Air Collective, spatial granularity is important as more number of monitors imply good data. “While data gathered by the low-cost sensors might not be as accurate as CAQMS, minor deviations do not matter when the pollution level is as high as ours. The countries that have been able to successfully tackle the issue have always encouraged and accepted the data generated by devices used by the general public. Such sensors offer enough data at low cost to citizens for running local campaigns, create awareness and spark conversation among people and citizen groups. If that happens across multiple pockets in a city, that can be a game-changer,” Singh explains.

With the average cost of a CAQMS being around Rs 1 crore, the case of low-cost, sensor-based monitors grows stronger. Santosh says it is important that the network of monitoring stations expands beyond a handful of cities and into the Indo-Gangetic belts and industrial areas.

Types of monitoring equipment

There are two types of monitoring equipment -- SEMs (Surface Emission Monitoring) and ambient monitoring equipment called CAQMS (Continuous Air Quality Monitoring Stations). SEMs is used for source pollution measurement. For example, pollution from industries and factories. Such data is directly uploaded to the CPCB site. It monitors emissions from the source and is different from AQI, although it measures the same parameters. AQI is calculated from data gathered at CAQMS which uses US EPA standardized equipment. As many as 200 such CAQMS are available across the country, with most of them being installed in and around Delhi.  

Data Table – source (CPCB, urbanemissions.info)





StatesNumber of sensors in 2017Number of sensors requiredNumber of sensors current
Jammu & Kashmir0910
Himachal Pradesh0430
Chandigarh071
Uttarakhand0640
Sikkim040
Arunachal Pradesh040
Nagaland0200
Manipur0250
Mizoram090
Tripura0210
Meghalaya0251
Assam01431
Chhattisgarh01030
Madhya Pradesh030313
Daman & Diu040
Dadra & Nagar Haveli040
Goa0110
Lakshadweep040
Puducherry0100
Andaman & Nicobar Island040
Jharkhand11401
Gujarat11975
Kerala11152
Odisha21692
Punjab31258
Rajasthan322610
Bihar32773
Tamil Nadu32555
Haryana412324
West Bengal519712
Andhra Pradesh51265
Karnataka520917
Telangana6976
Uttar Pradesh955824
Maharashtra930822
NCT of Delhi147738

(Author is Bengaluru - based freelance writer and a member of 101Reporters.com, a pan-India network of grassroots reporters.)