Central Idea

As Delhi-NCR and its environs grapple with worsening air pollution, the Air Quality Index (AQI) has gained prominence as a critical measure of air quality.

Understanding Air Quality Index (AQI)

AQI measures how safe the air around you is for breathing. Organizations that report AQI measure the density of various pollutants in the air (such as PM2.5, PM10, nitrogen dioxide, ozone, etc) at different monitoring stations.
The widely-used National Air Quality Index (NAQI) given by the Central Pollution Control Board is a 24-hour average.
Its unit is micrograms per cubic meter.
A particular amount of one pollutant may not be as harmful as the same amount of another pollutant.
So, each pollutant’s quantity in the air is adjusted to a common scale (say, 0 to 500) that works for all pollutants.
Finally, the pollutant with the worst sub-index determines the AQI for that time and location.

Air Pollutants covered:

(Pollutants that most of us NEVER heard of-)

Graded Response Action Plan (GRAP): AQI plays a pivotal role in shaping government policies to combat air pollution. When AQI levels in areas like Delhi-NCR deteriorate, emergency measures, such as Stage 3 of GRAP, are activated.
Immediate Action: For instance, the recent dip in AQI to the ‘severe’ category prompted immediate actions. Diesel four-wheelers not meeting BS-VI compliance were prohibited, and truck entry into Delhi was restricted. Petrol cars continued to operate under regular conditions.

The GRAP was conceived as a response to the alarming findings of a WHO study in 2014, which ranked Delhi as the most polluted city globally.
In 2016, the Supreme Court (M. C. Mehta vs. Union of India Case) approved GRAP after multiple expert consultations.
First GRAP was notified in January 2017 by the Ministry of Environment, Forest and Climate Change.

Starting in 2021, the Commission for Air Quality Management in NCR & Adjoining Areas (CAQM) has taken over the responsibility of implementing GRAP.
Prior to 2021, the Supreme Court-appointed EPCA would instruct states to enforce GRAP measures.
In 2020, the EPCA was disbanded and substituted with the Commission for Air Quality Management (CAQM).
The CAQM recommendations depend on the Air Quality Index (AQI) and meteorological predictions provided by the Indian Institute of Tropical Meteorology (IITM) and the India Meteorological Department (IMD).

Air Quality Stage	Range	Measures to be implemented
Stage I (Poor)	201-300	Enforce NGT/Supreme Court’s order on over-aged diesel/petrol vehicles.
Stage II (Very Poor)	301-400	Implement rigorous actions to combat air pollution at identified hotspots.
Stage III (Severe)	401-450	Impose strict restrictions on BS III petrol and BS IV diesel vehicles. Suspend physical classes in schools for primary grade children up to Class 5 in certain areas.
Stage IV (Severe Plus)	>450	Prohibit the entry of four-wheelers registered outside Delhi, except for electric vehicles, CNG vehicles, and BS-VI diesel vehicles.