India AQI vs US AQI: Why the Same Air Gets Two Different Numbers

Your purifier says 90. Your weather app says 150. Neither is wrong. They're using different math.

If you've checked two air quality apps at the same time, in the same room, and gotten two different verdicts, you're not imagining it. India and the United States each publish an Air Quality Index on a 0–500 scale, both built from the same pollutant concentrations, and they routinely produce different numbers. The gap is real and documented. It has nothing to do with sensor accuracy.

This post covers how the two scales are built, where they diverge most, and what to do about it when deciding on ventilation, purifier settings, or outdoor time for kids. To check a real reading for your own city under the Indian scale, use the live AQI checker, which pulls directly from CPCB stations.

Same formula, different thresholds

The Central Pollution Control Board (CPCB) and the US Environmental Protection Agency (EPA) calculate AQI the same way: a piecewise linear function that converts a pollutant's concentration into a 0–500 index, then reports whichever pollutant scores highest. For most Indian cities most of the year, that's PM2.5, fine particulate matter small enough to reach deep into the lungs.

The formula is identical. The breakpoints, where each country draws the line between categories, are not.

The actual breakpoint tables

Here is India's CPCB scale for PM2.5, in micrograms per cubic metre (24-hour average):

Source: CPCB National Air Quality Index breakpoint tables.

And here is the US EPA scale, current as of the rule that took effect on 6 May 2024:

Source: US EPA, Reconsideration of the NAAQS for Particulate Matter, effective 6 May 2024.

Which scale is your app actually using

Most of the real-world confusion traces back to one of these:

• IQAir AirVisual defaults to the US EPA scale everywhere, including in India. Settings lets you switch between US and Chinese scales, but CPCB isn't an option. This is why AirVisual's number often looks rougher than what's quoted on the news.
• CPCB's AQI portal (airquality.cpcb.gov.in) and most weather apps quoting "India AQI" use the Indian breakpoint table above, reported as a 24-hour rolling average.
• SAFAR, run by the Ministry of Earth Sciences for Delhi, Pune, Mumbai, and Ahmedabad, uses the same Indian breakpoints as CPCB but reports a real-time figure instead of a 24-hour average. That timing difference is a documented reason SAFAR and CPCB can show very different numbers for the same city on the same morning, especially right after a spike like Diwali, though differences in monitoring stations and update frequency can also play a part.
• Smart air purifiers sold in India typically show a raw PM2.5/PM10 number or a manufacturer-specific label rather than a labelled CPCB or EPA index. We can't verify which standard any given purifier's display follows, so check the manual before comparing it to a phone app.

When two sources disagree, it's almost always a different breakpoint table or a different averaging window, not a broken sensor.

See the gap for yourself

Enter a raw PM2.5 number, the kind your monitor or purifier shows alongside its index score, and see how each scale labels it.

Where the gap is widest

The two scales diverge almost everywhere, not just in the middle. At very low concentrations the US scale already reads far higher than India's, since its "Good" ceiling sits at 9 µg/m³ against India's 30. The gap then widens further through the middle of the range, the everyday range most Indian cities actually live in, before narrowing again only as both scales approach their shared ceiling of 500 near the highest, most hazardous concentrations.

At 55 µg/m³, comfortably inside Mumbai's and Chennai's typical winter range, CPCB calls it "Satisfactory" while the US scale already calls it "Unhealthy for Sensitive Groups." Delhi runs far higher: CREA's analysis of CPCB data put Delhi's winter 2025-26 average PM2.5 at 163 µg/m³, well past "Unhealthy" on either scale. For a city-by-city look at how this played out across a single month, see our April 2025 air quality analysis. Same monitoring data, two government agencies, and depending on which city and which scale, very different messages about whether vulnerable people should take care.

Around 80 µg/m³ the relationship flips. India's higher categories (Poor, Very Poor, Severe) are compressed into a narrower concentration range than the equivalent US bands, so the Indian number climbs faster from there. By 110 µg/m³, not unusual in Delhi on a bad winter day, the Indian index has clearly overtaken the US one, even though both agree the air is unhealthy.

Why India's scale is built this way

This isn't an oversight. When CPCB first designed the PM2.5 sub-index, it noted there were no India-specific health studies available, so the lower breakpoints were adapted from those already proposed by the US EPA and China for the higher categories. The scale communicates risk against India's own ambient air quality standards, which are themselves less strict than WHO guidelines. The US scale was revised in 2024 specifically because newer evidence shows health effects at lower PM2.5 levels than previously assumed.

Neither agency is hiding anything. India's AQI tells you where you stand against India's regulatory thresholds; the US AQI tells you where you stand against the US's, which are currently tighter. Comparing the two numbers without knowing this is like comparing a score out of 10 to one out of 20.

What to actually do with this

For real decisions, ventilation, purifier settings, outdoor time for kids, the index number is a translation layer, and translation loses information. The more useful number is the one underneath both scales: raw PM2.5 in µg/m³.

• Most monitors and purifiers that show an AQI number also show the raw µg/m³ reading somewhere in the app, often under details or history. Use that for comparisons across devices and over time.
• If you only have an index number, check which scale it's using before reacting, especially with imported monitors, which often default to the US scale.
• Treat both scales as legitimate but differently calibrated. Indian "Satisfactory" at 55–60 µg/m³ isn't the same claim as US "Good." It's a statement about where that concentration sits in India's own regulatory bands, not a universal health verdict.
• For ventilation calls specifically, indoor PM2.5 matters more than the outdoor index. Our window-opening guide and indoor vs outdoor air pollution cover when outdoor readings actually translate into indoor exposure.

The takeaway isn't that one country's index is right and the other wrong. An index number, by itself, is the least informative thing your monitor shows you. The raw concentration is what your lungs actually respond to.

Related reading

• Live AQI checker — current CPCB readings for any Indian city, with raw pollutant values alongside the index.
• April 2025 air quality analysis (India) — a city-by-city look at PM2.5 levels and what the same numbers mean under each scale.
• Indoor vs outdoor air pollution — why the outdoor AQI is only half the story for what your lungs actually breathe.
• Should I open my windows? — how to use an AQI reading to decide whether to ventilate.
• Air purifier buying checklist (India) — the eight-step process for matching a purifier to your room and your PM2.5 baseline.