Noida: The air quality index (AQI) in Delhi reached a high of 450 on November 3, and has been topping the 300 mark almost consistently between October 27 and November 8 this year. Yesterday, for the first time in 15 days, the AQI was below 300, at 260.


While Delhi's air quality, on which mainstream media focuses, improved slightly, smaller towns--Ambala, Khanna, Kurukshetra and Ludhiana--had severely polluted air (AQI over 400) on November 9, according to the air quality index bulletin published by the Central Pollution Control Board (CPCB).

But what does having an AQI over 400 really mean?

For one, it means that people are breathing air that has an average concentration of 250 micrograms of PM 2.5 per cubic metre (µg/m3) in a day, according to the AQI-to-concentration converter. PM 2.5 refers to inhalable particulate matter that can enter the bloodstream and has several health impacts. A level of 250 µg/m3 is four times the permissible amount--60 µg/m3 over a 24-hour period--according to the standards defined by the CPCB.

It also means that the concentration of pollutants in the air "affects healthy people and seriously impacts those with existing diseases".

In our #TIL explainer, we look at the methodology behind the AQI and its role in monitoring air pollution.


What number does the AQI report?

The Air (Prevention and Control of Pollution) Act of 1981 (Air Act) empowers the CPCB to lay down the standards for air pollutants. It is the mandate of the CPCB to collect data on these pollutants, while the State Pollution Control Boards (SPCBs) enforce these standards and impose penalties on polluters as per the Air Act.

To detect pollutants, the CPCB has 883 manual monitors as on September 15, 2022, from which it records observations once a day, according to the CPCB website. With its size, population and aggravating air pollution, India needs 1,600 to 4,000 air quality monitors, we reported in December 2021.

The AQI is a metric that converts the concentration of a pollutant in the atmosphere into a number that conveys the potential hazard it poses for the residents, according to Sunil Dahiya, an analyst at the Helsinki-based think-tank, Centre for Research on Energy and Clean Air, working on effects of and solutions for air pollution.

"To calculate the AQI, we need the concentration of any three pollutants, one of which has to be the particulate matter (PM 2.5 and PM 10). The other two can be SOx (oxides of sulphur), NOx (oxides of nitrogen), CO (carbon monoxide) or Ozone," Dahiya explained.

A formula, defined under the National Air Quality Index guidelines, is used to convert the concentrations of these pollutants into a "sub-index", and the AQI uses the highest value of these sub indices, explained Tanushree Ganguly of the New Delhi-based policy research organisation, the Council on Energy, Environment and Water (CEEW).


In the case of the CPCB, the AQI is based on a 24-hour average concentration of six of the 12 pollutants that the CPCB monitors. Other pollutants that the CPCB monitors are lead and ammonia on a daily basis, but these are not reported in the AQI. BenzoPyrene, Benzene, Arsenic and Nickel have annual concentration standards.


Open fires in paddy fields or factories: Who is the bigger polluter?

The Punjab Preservation of Subsoil Act of 2009 asked that farmers sow paddy later, when the winds are calmer, to conserve groundwater. This in turn delays the harvest, after which farmers burn paddy stubble. Now this period coincides with the onset of winter, causing smog during this time, explained Ganguly. However, she said, stubble burning is not the only reason for the elevated AQI.

It is power plants, industry and vehicles that are the biggest emitters of PM 2.5 , SOx and NOx, found a 2021 review of emission data by CEEW.

Of the 172 cities in India for which AQI is reported daily, PM 2.5 was the chief pollutant in 103 of the cities on November 10. Fossil fuel exhaust from vehicles is the chief source of PM 2.5, as per this 2016 paper. Power plants emit SOx and NOx, which turn into PM 2.5 on reacting with water. Stubble burning too creates PM 2.5, and its contribution to PM 2.5 emissions increases around this time of the year, but transport and industry are bigger polluters, according to Ganguly.

Because Delhi's air gets the most attention, and especially around stubble burning, other sources of pollution and the fact that large swathes of the country are affected goes largely unnoticed and unreported, says Ganguly.

In December 2015, IndiaSpend had set up a network of low-cost real-time air quality monitors to democratise critical air quality data, and our analyses informed the discourse around air pollution and mitigation efforts (see here and here, for instance). These findings, disputed by the Delhi government at the time, were later validated by academic studies (see here).


Air pollution kills slowly and has long-term impacts

Air pollution kills over 6.5 million people globally every year, according to a May 2022 Lancet study. For perspective, Covid-19 killed 6.5 million people worldwide between January 2020 and November 2022, according to the World Health Organization.

"Air pollution is not like Covid-19, in the sense that you don't die immediately on exposure to a pollutant, unlike what we saw during the Covid-19 pandemic. It [pollution] kills you slowly," said Dahiya.

"PM 2.5 is small enough to enter the bloodstream and is carried to all organs from the deepest part of our lungs. They can cause respiratory illnesses, heart disease, increased cerebrovascular risk (which affects blood flow to the brain)," said Dr Poornima Prabhakaran, Director of the Centre for Environment and Health at the Public Health Foundation of India. She added that pregnant women and children are especially vulnerable as these pollutants can cause adverse pregnancy outcomes, impact foetal growth and development and affect child development.

Ambient and household air pollution was found to cause diseases of the central nervous system, aggravate heart disease, cause inflammation and rupturing of the appendicitis.

"To reduce the detrimental effects of air pollution, people, especially those with COPD [Chronic Obstructive Pulmonary Disease, a disease of the lungs in which the air flow to the lungs is reduced] or asthma, should be aware of the air quality and take extra measures such as reducing the time outdoor and wearing masks when necessary," said researchers from China in a 2018 study.


From data to policy: How AQI sets off decisions to control air pollution

On November 3, the AQI in Delhi was 450, which is classified as "severe" on the National AQI scale. Noida, Greater Noida, Ghaziabad, Gurugram and other cities close to the National Capital Region too reported AQIs greater than 400, according to the CPCB bulletin.

The Commission for Air Quality Management (CAQM), which was constituted in 2020 by the Ministry of Environment, Forest and Climate Change, is the body responsible for air quality management in Delhi and surrounding areas of Rajasthan, Haryana, Rajasthan, Punjab and Uttar Pradesh.

On November 2, CAQM ordered the implementation of Stage IV of the Graded Response Action Plan (GRAP). The GRAP is a list of steps to be taken by the state governments around New Delhi when air quality starts to deteriorate. When the AQI crosses 400, state governments can call for a ban on construction and mining activities, restrict movement of vehicles that cause pollution and shut down factories.

This implementation of Stage IV of GRAP was revoked on November 6, when the AQI reached 339, as per the CPCB bulletin.

However, the benefits for any of the steps taken under GRAP are unclear: Steps like permitting cars with either odd or even number plates to ply on a day, such as that implemented by the Delhi government, will not reduce pollution, Sagnik Dey, coordinator at the Centre of Excellence for Research on Clean Air at the Indian Institute of Technology, Delhi, told IndiaSpend in an interview in 2019.

"The truth is that most Indian cities have bad air quality throughout the year, except during the monsoon months, and the concentrations of pollutants are several times higher than the Indian standards themselves," Dahiya told IndiaSpend.

Of the 172 cities for which the CPCB reports AQI, 21 cities' air was rated "very poor" on November 10, and that in 39 cities was rated "poor". Only 13 Indian cities reported "good" quality air on November 10, according to the CPCB bulletin.

Other cities are covered by the National Clean Air Programme, which aims to reduce emissions in 122 cities by 20-30% compared to 2017 levels by 2024. The NCAP in its current form is a means to collect data on air quality with no means to implement these standards, we reported in 2020.

Dahiya says that in addition to better data gathering, we have to do more than just restrict vehicles and regulate farm stubble burning for brief periods of time. "It is the big institutional polluters like coal-based power plants and industries who need to be brought to book, but instead we keep giving the power plants extensions to reduce emissions."

If we rely on waiting for millions of individual small polluters to change their dependence on diesel engines, or stubble burning, and ignore big polluters, we cannot expect to do away with this problem soon, said Dahiya. "It needs action across all sectors."

We welcome feedback. Please write to respond@indiaspend.org. We reserve the right to edit responses for language and grammar.

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