New Delhi: Delhi’s air quality plunged sharply a day after Diwali, leaving the city shrouded in haze and making it one of the world’s most polluted major cities. Residents reported difficulty in breathing, itchy eyes, and general discomfort. While Swiss-based air-quality platform IQAir recorded the city’s AQI at a staggering 1,121, India’s Central Pollution Control Board (CPCB) reported a much lower figure of 351. Both readings, however, are correct within their respective frameworks.
Understanding AQI
The Air Quality Index (AQI) is a tool designed to communicate air pollution levels in simple terms, helping the public understand associated health risks. CPCB calculates AQI based on eight pollutants: PM10, PM2.5, ozone (O3), sulfur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), lead (Pb), and ammonia (NH3). Mohan George, consultant at the Centre for Science and Environment, explains, “AQI uses color-coded ranges to indicate whether the air is good, satisfactory, or very poor, guiding citizens on necessary precautions.” For instance, an AQI above 400 falls in the “severe” category, hazardous even for healthy individuals.
CPCB vs IQAir
The key difference lies in methodology and scale. CPCB’s AQI scale is capped at 500, with six categories ranging from ‘good’ (0-50) to ‘severe’ (401-500). IQAir, however, has a tighter scale, labeling anything above 301 as “hazardous.” Both platforms identify the dominant pollutant usually PM2.5 in Northern India during winter but their calculation methods differ, leading to varying AQI values.
Data Sources and Calculation
CPCB relies on government-operated, reference-grade monitoring stations, including DPCC, IMD, and IIT Madras. IQAir incorporates data from these stations as well as additional sensor-based monitors from embassies, companies, and individuals. For example, if PM2.5 concentration reaches 1,100 micrograms per cubic meter, IQAir could report an AQI of 2,043, while CPCB’s calculation would show 1,054.
Which Reading to Trust?
Experts recommend focusing on the AQI category—“very poor,” “severe,” or “hazardous”—and pollutant concentrations rather than the numeric value alone. For instance, on Tuesday morning, IQAir recorded PM2.5 at 320 µg/m³ near Mandir Marg (AQI 489), while CPCB measured 335 µg/m³. Both indicate dangerously polluted air, underlining the need for precautionary measures such as limiting outdoor exposure and using masks or air purifiers.
In essence, whether 350 or 1,100, the takeaway remains the same: Delhi’s air quality is hazardous, and public health measures are essential.
