Air quality: the standards and the index

Prelims and MainsCurrent affairs on this: Air pollution

Two instruments turn air into numbers. The National Ambient Air Quality Standards, notified by the Central Pollution Control Board (CPCB) in 2009 under the Air Act, set limits for twelve pollutants, with annual and 24 hour values: 40 and 60 micrograms per cubic metre for PM2.5, 60 and 100 for PM10, against WHO's 2021 guideline of 5 and 15 for PM2.5. The National Air Quality Index, launched in 2014, converts the day's readings into one number from 0 to 500 in six bands: Good (0 to 50), Satisfactory (51 to 100), Moderate (101 to 200), Poor (201 to 300), Very Poor (301 to 400) and Severe (401 to 500). It covers eight pollutants, PM10, PM2.5, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead; each gets a sub index and the worst one is the day's AQI, provided at least three pollutants are measured and one is particulate matter.

The National AQI bands

Band	AQI	PM2.5 (24 hour, µg/m³)	PM10 (24 hour, µg/m³)	Health effect (CPCB wording)
Good	0 to 50	0 to 30	0 to 50	minimal impact
Satisfactory	51 to 100	31 to 60	51 to 100	minor breathing discomfort to sensitive people
Moderate	101 to 200	61 to 90	101 to 250	breathing discomfort to people with lung or heart disease
Poor	201 to 300	91 to 120	251 to 350	breathing discomfort on prolonged exposure
Very Poor	301 to 400	121 to 250	351 to 430	respiratory illness on prolonged exposure
Severe	401 to 500	above 250	above 430	affects healthy people, serious for those with disease

The AQI is the worst sub index of the day; the 24 hour PM2.5 standard of 60 sits at the top of the Satisfactory band. The lit bands are the range GRAP works in.Source: CPCB National AQI, 2014; NAAQS 2009

The standards say what is safe; the index says how bad today is. The Graded Response Action Plan (GRAP) is keyed to the index bands; NCAP targets are set against the standards.
PM2.5, particles under 2.5 micrometres, is the pollutant that matters most for health because it reaches the alveoli and the blood; its main sources in North India are vehicles, biomass and stubble burning, construction dust and industry, with winter inversions trapping it.
Ozone at ground level is a secondary pollutant made from nitrogen oxides and volatile organic compounds in sunlight, which is why it peaks on hot afternoons and in summer, not in winter smog.
The index leaves out volatile organic compounds and carbon dioxide; a 2016 question turned on which pollutants are and are not in the AQI.
Monitoring: the CPCB's continuous ambient stations and the manual National Air Quality Monitoring Programme; the System of Air Quality and Weather Forecasting and Research (SAFAR) of IITM for four cities; the AQI band drives the GRAP stage three days ahead on forecast.

Mains: India's standards are three to eight times looser than WHO's and most cities still miss them, so the argument over air is not about the number but about enforcement at the source, which no index can do.

UPSC has asked

Prelims 2022: the WHO Air Quality Guidelines
Prelims 2016: the gases used to calculate the Air Quality Index

Further reading: WHO global air quality guidelines (WHO) · Ambient (outdoor) air pollution (WHO)