Ask three websites how bad Delhi's air is on Friday afternoon, and you get three different answers: 424, 352 and 185.
It is the same city, the same haze and roughly the same hour. None of them is necessarily wrong. Each is using a different ruler, a different stretch of time and a different corner of the city.
-
Indian Markets Turning Corner After Two Tough Years, Says Sandeep Tandon -
NDTV Profit Ignite 2026: Experts See Opportunity In Equities, Stay Bullish For Long-Term -
Buy Right, Sit Tight Does Not Work for Individual Stocks, Says Dinshaw Irani -
'Worst Is Behind': Quant MF Sees Market Bottom, Flags 'Extreme Euphoria' In Midcaps -
India's Mutual Fund Industry Could Become A $1 Trillion Opportunity, Say Fund Managers -
'Foreign Investors Are Baraatis': Raamdeo Agrawal's Take On LTCG Tax -
Stay Invested Through Market Cycles: Raamdeo Agrawal at NDTV Profit Ignite -
Ramesh Damani Remains Bullish On India For Next 20-30 Years, Sees Bargains Amid Market Weakness -
CAS Came After A Long Consultation Process: SEBI Chairman Tuhin Kanta Pandey AT NDTV PROFIT IGNITE -
Aim To Cut FPI Onboarding Time From Mth To Five Days: SEBI Chairman Tuhin Kanta Pandey AT NDTV PROFI
The Three Readings
At 12:05 p.m., the Central Pollution Control Board (CPCB) dashboard showed 424 ("Severe") for its Dwarka Sector 8 station, with fine dust (PM2.5) as the prominent pollutant.
At 12:30 p.m., IQAir put Delhi at 185 on the US scale ("Unhealthy"), with PM2.5 at 104.1 µg/m³.
At 1:51 p.m., AQI.in showed 352 ("Severe") for Dwarka Sector 10, with PM2.5 at 187 µg/m³ and PM10 at 359 µg/m³.
Different Rulers
An AQI converts a dust reading into a score from 0 to 500. Each country builds its scale around its own safe limits, so the same air gets a different score.
India's index, launched in 2014, has six bands from "Good" to "Severe". The US has its own bands and cut-offs, which it tightened in May 2024. IQAir adopted the change the same day.
An AQI of 100 is meant to match a country's own 24-hour safe limit. India's limit for PM2.5 is 60 µg/m³, while the World Health Organization's updated guideline is 15 µg/m³. Two scales pegged to different limits are like two teachers marking the same paper against different pass marks.
ALSO READ | Delhi Air Pollution: Capital Inches Towards 'Severe' AQI Ahead Of Diwali
A rough calculation using published breakpoints shows the effect. The 104.1 µg/m³ that IQAir scored 185 would land in the mid-200s ("Poor") on India's scale. The words differ too, with India's top band called "Severe" and the US one "Hazardous".
Different Clocks
The CPCB figure reflects the past 24 hours. That is like a team's run rate over the whole innings. IQAir uses hourly averages, which it labels "US AQI+", closer to the current over.
The US government's AirNow site weights up to 12 hours. When air worsens or clears fast, the numbers drift apart. Fine dust also tends to build up at night and ease when winds pick up, so readings swing through the day.
Different Places
The CPCB number is for one station. AQI.in showed a neighbouring one. IQAir's Delhi figure blends 30 stations from five contributors. A hotspot reads far worse than a citywide average, much as one lane's traffic says little about the whole city.
ALSO READ | Delhi-NCR Air Pollution Like Smoking 30 Cigarettes a Day', Supreme Court Told
Different Instruments
Government monitors are costly, carefully calibrated machines. IQAir adds cheaper sensors, which US EPA scientists have found can overestimate fine dust, especially in humid air.
Platforms apply correction formulas, and the choice can shift the final number. Stations also go offline or freeze, so a site may show an hours-old reading, so always check the "last updated" time.
One Villain Sets The Score
India tracks eight pollutants and shows only the worst one. In Delhi that is usually PM2.5, fine enough to enter the lungs and bloodstream. Analysts have also noted that capping hourly values at 500 can pull the 24-hour average below what the air actually was on extreme days.
Small differences in maths add to the gap. Each reading is slotted between fixed breakpoints and run through a formula, and sites may round differently. The US method trims PM2.5 to one decimal place instead of rounding it.
Why It Matters
An AQI covers outdoor air only. It says nothing about indoor air, which depends on windows, cooking and any purifier.
It can also drive policy. Delhi's Graded Response Action Plan (GRAP), a set of emergency curbs, is linked to the city's index, with Stage III starting at "Severe" (401-450) and Stage IV above 450. A different index could mean a different decision on curbs.
So Which Number Is Right?
Each is right for the question it answers. Compare like with like: the same site, station and time. Many scientists prefer the raw PM2.5 reading in µg/m³, which does not depend on any scale.
Essential Business Intelligence, Sharp Market Insights, Practical Personal Finance Advice, Daily Fuel, Gold and Silver Prices and Latest Stories — On NDTV Profit.