Delhi AQI In 'Dangerous' Zone: How Is AQI Calculated? FAQs Answered

GRAP IV norms have been applied in Delhi with the air quality crossing 'danger' levels. The Indian Institute of Tropical Meteorology in its daily bulletin has forecast air quality to remain in the ‘very poor’ category in the next six days and cited that the Air Quality Index (AQI) is hovering near dangerous levels. But what does AQI mean? How is AQI calculated?

What is the Air Quality Index?

​The Air Quality Index​ (AQI) is a number that measures the quality of air. ​AQI is used to report daily air quality. According to the CPCB,​ Indian AQI ranges from 0-500, 0 being good and 500 being severe. There are eight major pollutants that are taken into account: particulate matter (PM 10 and PM 2.5), carbon monoxide (CO), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), ammonia (NH3), and lead (Pb).

How is AQI calculated?
The AQI is calculated using pollutant-specific sub-indices based on 24-hour average concentrations (8-hour average for CO and O₃), with the highest sub-index determining the overall AQI. A minimum of three pollutants must be monitored, including either PM2.5 or PM10, with at least 16 hours of data required. Even if full AQI data is insufficient, individual pollutant sub-indices are shared. Continuous monitoring stations use automated systems for real-time AQI updates based on a 24-hour rolling average, while manual stations use an AQI calculator for inputting data manually. This ensures consistent and accurate air quality assessments.

Indian AQI Range and Impacts:

- 0-50 (Good):​ Air quality poses minimal or no health risk as such.  
- 51-100 (Satisfactory): Air quality is acceptable but may cause minor breathing discomfort for sensitive individuals.  
- 101-200 (Moderate): May lead to breathing discomfort for children, the elderly, and individuals with lung or heart conditions.  
- 201-300 (Poor): Prolonged exposure can impact health, especially for those with heart issues, even with short exposure.  
- 301-400 (Very Poor): Long-term exposure can result in respiratory illnesses.  
- 401-500 (Severe): Affects both healthy and sensitive individuals, causing hazardous health impacts.

How ​is Air Quality ​monitored  

Air quality in India is monitored region-wise by three key bodies: the Central Pollution Control Board (CPCB), the Delhi Pollution Control Committee (DPCC), and researchers at the Indian Institute of Tropical Meteorology (IITM).  

These agencies collect pollutant concentration data from two types of monitoring stations: manual monitoring stations and Continuous Ambient Air Quality Monitoring Stations (CAAQMS). The main difference lies in their output timing. Manual stations, primarily used for chemical analysis, have an 8-hour turnaround time, while CAAQMS provides real-time AQI data, widely used for public updates across the country.  

AQI forecasts rely on weather conditions like temperature, wind, and precipitation, which influence pollution levels and transport. Calculations are based on pollutant-specific health benchmarks set by the EPA’s National Ambient Air Quality Standards. However, localized events like fires and non-monitored areas may affect accuracy.

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