That daily three-digit number, the Air Quality Index or AQI, has become a crucial part of life in Delhi. But what does a score of 350 truly mean? Here’s a simple guide to how authorities turn complex air data into a single, vital number.
One Number, Eight Pollutants
The National Air
Quality Index, launched in 2014, simplifies air quality into one easy-to-understand figure. To do this, monitoring stations track eight key pollutants: Particulate Matter (PM10 and PM2.5), Nitrogen Dioxide (NO2), Sulphur Dioxide (SO2), Carbon Monoxide (CO), Ozone (O3), Ammonia (NH3), and Lead (Pb). These pollutants are chosen because of their significant impact on public health. Continuous monitoring stations across the city automatically capture this data, forming the backbone of the entire system.
Calculating a Score for Each Pollutant
The system doesn't just average out the pollutants. Instead, each of the eight pollutants gets its own individual score, called a sub-index. A specific formula converts the measured concentration of a pollutant—say, the micrograms of PM2.5 per cubic meter of air—into a value on a scale from 0 to 500. This is done using predefined 'breakpoints' set by the Central Pollution Control Board (CPCB). For a final AQI to be calculated for any monitoring station, data must be available for at least three pollutants, one of which must be either PM2.5 or PM10.
The 'Worst Offender' Rule
This is the most crucial part of understanding the final AQI number. The overall AQI is not an average of all the pollutant sub-indices. Instead, the final AQI reported for a station is simply the highest of all the individual sub-indices calculated at that moment. For example, if the sub-index for PM2.5 is 350, Ozone is 110, and CO is 80, the reported AQI will be 350. The pollutant with the highest score is declared the 'dominant' or 'primary' pollutant. This 'worst-first' approach ensures that the index always reflects the biggest immediate threat to air quality, preventing a dangerously high level of one pollutant from being hidden by lower levels of others.
From 'Good' to 'Severe': Making Sense of the Colours
The AQI scale is divided into six colour_coded categories to provide clear health guidance. These are: Good (0–50), Satisfactory (51–100), Moderate (101–200), Poor (201–300), Very Poor (301–400), and Severe (401–500). Each category comes with a health advisory. 'Good' air has minimal impact, while 'Satisfactory' may cause minor breathing discomfort to sensitive people. As the numbers climb, the risks increase. 'Very Poor' air can lead to respiratory illness on prolonged exposure, while 'Severe' air can affect even healthy people and seriously impact those with existing diseases. On Monday, November 2nd, 2026, Delhi's AQI entered the 'very poor' category for the first time this season, with a reading of 312.
What the AQI Tells You (And What It Doesn't)
The AQI is a powerful tool for public health communication, providing a snapshot based on recent data—typically a rolling average over the past 24 hours for most pollutants (and 8 hours for CO and Ozone). It's what guides policy actions like the Graded Response Action Plan (GRAP) in Delhi. However, it has limitations. The city–wide AQI you see in news headlines is often an average of many different station readings, so the air in your specific neighbourhood could be better or worse. The index is a simplified number designed for quick communication; it's not a complete diagnostic of the complex mixture of pollutants you might be breathing.
