The Air Quality Index (AQI) is a standardized tool used by government agencies to communicate the status of air quality to the public in an easily understandable format. It transforms complex, technical data regarding various pollutant concentrations into a single number, a nomenclature, and a color code.
Evolution and Governance in India
- Launch: The National Air Quality Index was launched in October 2014 as part of the “Swachh Bharat Abhiyan” with the tagline “One Number-One Color-One Description.”
- Development: Developed by the Central Pollution Control Board (CPCB) in consultation with IIT Kanpur and environmental medical professionals.
- Objective: To provide a real-time assessment of air quality and to facilitate quick protective measures during episodes of high pollution.
The Eight Pollutants of AQI
While the National Ambient Air Quality Standards (NAAQS) monitor 12 pollutants, the AQI is calculated based on the short-term (up to 24-hourly averaging period) concentrations of eight specific pollutants:
- Particulate Matter 10 (PM10)
- Particulate Matter 2.5 (PM2.5)
- Nitrogen Dioxide (NO2)
- Sulphur Dioxide (SO2)
- Carbon Monoxide (CO)
- Ground-level Ozone (O3)
- Ammonia (NH3)
- Lead (Pb)
Note: To calculate the AQI for a specific location, data for at least three pollutants must be available, one of which must be either PM10 or PM2.5.
AQI Categories and Health Impact
The AQI scale runs from 0 to 500. As the AQI value increases, the level of air pollution and the health concern also increase.
| AQI Category | Range | Color Code | Associated Health Impacts |
| Good | 0–50 | Dark Green | Minimal impact. |
| Satisfactory | 51–100 | Light Green | May cause minor breathing discomfort to sensitive people. |
| Moderate | 101–200 | Yellow | May cause breathing discomfort to people with lung/heart disease. |
| Poor | 201–300 | Orange | May cause breathing discomfort to most people on prolonged exposure. |
| Very Poor | 301–400 | Red | May cause respiratory illness to the people on prolonged exposure. |
| Severe | 401–500 | Dark Red | Affects healthy people and seriously impacts those with existing diseases. |
Calculation Methodology
The AQI is determined based on the “Sub-index” system.
- A sub-index is calculated for each of the eight pollutants based on their concentration and the linear relationship with a predefined scale.
- The overall AQI for a given time and location is the highest (worst) sub-index among the pollutants measured.
- For example, if the sub-index for PM2.5 is 250 (Poor) and all others are below 100, the final AQI will be 250.
AQI vs. NAAQS: Key Differences for Prelims
It is crucial to distinguish between these two monitoring tools as they serve different administrative purposes.
| Feature | Air Quality Index (AQI) | National Ambient Air Quality Standards (NAAQS) |
| Purpose | Real-time public health communication. | Long-term policy and regulatory compliance. |
| Pollutants | 8 pollutants. | 12 pollutants (Includes Benzene, Arsenic, etc.). |
| Reporting | Numerical index (0–500). | Concentration values (μ g/m3). |
| Averaging | 24-hour (or 1-hour for O3 and CO). | Annual and 24-hour averages. |
SAFAR (System of Air Quality and Weather Forecasting and Research)
SAFAR is a separate but complementary system developed by the Ministry of Earth Sciences (MoES).
- Objective: It provides location-specific information on air quality and weather in near real-time and a 1–3 day advance forecast.
- Implementation: Currently operational in major metropolitan cities like Delhi, Pune, Mumbai, and Ahmedabad.
- Scope: It monitors additional parameters like UV Radiation, Mercury, and Skin-like Temperature (Weather).
Critical Facts and Trivia
- The “Cleanest” Cities: Under the National Clean Air Programme (NCAP) and Swachh Vayu Sarvekshan, cities are ranked based on their AQI improvement.
- Winter Spikes: During winter in North India, the AQI often hits the “Severe+” category (above 500) due to a combination of stubble burning, low wind speeds, and temperature inversion.
- Global Variation: Different countries use different AQI scales. For instance, the US EPA AQI uses a different set of concentration breakpoints compared to the Indian CPCB AQI.