 "People walk on Kartavya Path on a smoggy morning after air quality dips, partly due to the use of firecrackers during the Diwali festival in New Delhi, India, October 22, 2025. File Image/Reuters")
The morning after Diwali, Delhi once again disappeared under a thick grey haze.
Firecracker residue, construction dust, and emissions mixed in the still air to create a toxic blanket that turned the national capital into the world’s most polluted major city.
Visibility fell sharply, the air smelled acrid, and residents complained of watery eyes and breathing discomfort.
By Tuesday, Delhi’s Air Quality Index (AQI) reached hazardous levels.
But confusion spread when different monitoring systems produced drastically different figures: while India’s Central Pollution Control Board (CPCB) placed the AQI at around 351, global tracker IQAir measured an astonishing 1,121, and in some parts of the city, even reported readings above 2,000.
The disparity raised a simple but urgent question: how can Delhi’s AQI be both 400 and 2000 at the same time?
The answer lies in the way each system defines, sources, and calculates air quality — and why both, in their own contexts, can be correct.
How Delhi’s air turned hazardous once again
Following the Diwali festivities, Delhi’s air quality took an expected but severe dip.
For nearly a week, the city’s AQI remained around 350, categorised as “very poor” by national standards.
The CPCB, India’s nodal agency for air quality monitoring, recorded AQI levels hovering below 400 across much of the Delhi-NCR region.
However, the global air quality platform IQAir, headquartered in Switzerland, displayed numbers that shocked many users — with some neighbourhoods such as Siri Fort showing AQI figures above 2,400 shortly after midnight on Diwali night.
At 12:30 am, CPCB data placed Siri Fort’s AQI at 272, while IQAir’s dashboard registered 2,449 for the same area.
At other locations like Mandir Marg, IQAir logged 489 at 11:30 am, driven largely by PM2.5 concentrations of 320 microgrammes per cubic metre, while CPCB’s monitors at that time reported 335 microgrammes per cubic metre — a negligible difference in concentration but a dramatic gap in the numerical AQI value.
The mismatch, experts explain, is rooted not in error but in the fundamental differences in methodology, scale, and equipment between India’s monitoring framework and international systems such as IQAir’s.
How AQI is measured
The Air Quality Index (AQI) is a globally used metric to communicate air pollution levels to the public.
It consolidates multiple pollutants into a single number to represent how clean or polluted the air is, and what that means for human health.
India’s CPCB calculates AQI based on eight pollutants — PM10, PM2.5, ozone (O₃), sulphur dioxide (SO₂), nitrogen dioxide (NO₂), carbon monoxide (CO), lead (Pb) and ammonia (NH₃).
Each of these has defined concentration ranges that correspond to health-based categories on a scale from 0 to 500, as outlined below:
0-50: Good
51-100: Satisfactory
101-200: Moderate
201-300: Poor
301-400: Very Poor
401-500: Severe
Any reading above 500 is still classified as “severe,” since the health impact is already critical at that threshold.
The index was developed in 2015 as part of India’s National Air Quality Index initiative to simplify pollution communication for citizens.
The IQAir platform, on the other hand, follows the US Environmental Protection Agency (EPA) model. While it also uses a 0-500 range, it allows readings to exceed 500 when pollution levels are exceptionally high.
Its grading system is more segmented at upper levels, designating values above 301 as “hazardous.”
Thus, when pollution crosses the upper limit of India’s index, CPCB’s readings stop rising numerically — even though pollution continues to worsen — while IQAir’s values keep scaling upward to reflect the additional intensity.
This alone explains why the same particulate concentration could yield vastly different AQI figures across platforms.
CPCB vs IQAir: How the data differs
Beyond scale, the most significant variation arises from data sources and technology.
The CPCB and its associated bodies — including the Delhi Pollution Control Committee (DPCC), the India Meteorological Department (IMD), and the Indian Institute of Tropical Meteorology (IITM) — use reference-grade analyser stations.
These are government-run facilities equipped with scientifically calibrated instruments that adhere to rigorous national standards.
IQAir, however, aggregates data not just from government networks but also from independent sensors, including those installed by private companies, community projects, and individuals.
These devices use sensor-based measurement rather than the analyser-based approach.
While sensors allow for wider geographic coverage and real-time updates, they are often low-cost, non-reference devices that rely on proprietary algorithms to estimate pollutant concentrations.
Because IQAir does not disclose the exact calibration and conversion methods it applies to raw data, experts caution against interpreting its high readings as directly comparable to government data.
The US-based system used by IQAir continues beyond 500 because it aims to display relative differences even at extremely polluted levels.
In contrast, India’s capped system considers anything above 500 to be a public health emergency, where additional precision offers little practical benefit.
To understand how these calculations diverge, consider a hypothetical example: if the concentration of PM2.5 — the fine particulate matter most harmful to human lungs — is measured at 1,100 microgrammes per cubic metre, the IQAir calculator could display an AQI of 2,043, while CPCB’s calculation for the same value would reach around 1,054 before being capped at 500.
IQAir’s system continues to quantify just how far beyond “severe” the pollution has gone, whereas CPCB’s scale is intentionally designed to stop once conditions are already dangerous to all.
Therefore, experts advise that instead of focusing on the number itself, citizens should pay attention to the category labels such as “very poor,” “severe,” or “hazardous.”
Both CPCB and IQAir currently reflect that Delhi’s air falls within the most dangerous bracket.
The bottom line: Numbers differ, danger doesn’t
The current episode is part of a long-standing seasonal pattern. Each year, as winter approaches, Delhi’s geography, weather, and human activity combine to trap pollutants close to the surface.
Low wind speeds, falling night temperatures, and the widespread burning of agricultural residue in Punjab and Haryana form a dense smog layer that the city’s atmosphere struggles to disperse.
CPCB’s AQI is standardised for national reporting, regulatory compliance, and policy response. It provides consistent, scientifically verified data that can guide government action.
IQAir, meanwhile, is useful for real-time updates and global comparisons, even if its data is not calibrated against local reference methods.
Ultimately, both platforms converge on the same warning — that Delhi’s air quality is deteriorating to levels that pose serious health risks for all, including healthy adults.
Delhi’s residents, therefore, should not be misled by the apparent contradiction between 400 and 2000.
Both figures reflect the same dire conclusion — that the capital’s air is unsafe to breathe.
Also Watch:
With inputs from agencies