Published: 27 May 2026 | Author: Team JanVayu | Reading time: 5 min
Every morning, millions of Indians check one number to decide whether to open a window or put on a mask. AQI 150, AQI 300, AQI 500. It drives headlines, group chats and school closures. Few people, including many journalists and policymakers, know exactly what it measures, what it leaves out, or why the same AQI in Delhi and Denver can describe very different air.
AQI is the maximum, not the average
Most people get this wrong. The Air Quality Index is not an average of all pollutants. It is the worst of eight separate sub-indices, one each for PM2.5, PM10, NO2, SO2, CO, O3, NH3 (ammonia) and Pb (lead) (CPCB National AQI methodology, 2014). A monitoring station calculates a sub-index for each pollutant and reports the highest as "the AQI." The pollutant behind that highest value is called the dominant pollutant.
It matters which one it is. An AQI of 200 driven by PM2.5, the fine particles that lodge deep in the lungs, calls for a different response from an AQI of 200 driven by ground-level ozone, which irritates the airways and worsens asthma. The number is the same and the danger is different.
The dominant pollutant problem
PM2.5 gets nearly all the public attention, with good reason. It is the deadliest air pollutant globally, responsible for an estimated 1.72 million deaths in India in 2022 (Lancet Countdown 2025). All air pollution (ambient, household and ozone together) caused more than 2 million deaths in India in 2023 (State of Global Air 2025). Looking only at PM2.5, though, hides other pollutants that are growing:
- PM10, or coarse dust, can be the dominant pollutant where road and construction dust is heavy, even when PM2.5 looks manageable.
- NO2 (nitrogen dioxide) comes largely from vehicle and combustion emissions and is a precursor to ground-level ozone. PM2.5 usually sets the headline AQI, so NO2 rarely makes the news.
- O3 (ozone) is India's growing summer problem. PM2.5 peaks in winter, but ozone rises in hot months, when sunlight turns NOx and VOCs into a harmful secondary pollutant. A CSE analysis of 25 cities (1 March to 10 May 2026) found 15 of them recorded summertime ozone averages well above the NAAQS of 100 µg/m³.
When you see an AQI number, ask which pollutant is driving it.
Two scales, two answers
India uses the CPCB National AQI scale, which divides air quality into six categories: Good (0-50), Satisfactory (51-100), Moderate (101-200), Poor (201-300), Very Poor (301-400), and Severe (401-500). The US EPA scale, used by platforms like WAQI and IQAir, has different breakpoints and different category names.
The gap matters. A PM2.5 concentration of 90 µg/m³ is classified as "Moderate" on the Indian CPCB scale and "Unhealthy" on the US EPA scale. It is the same air and the same particles, but one scale calls it tolerable and the other tells you to limit outdoor exertion. The CPCB scale is more lenient across the range. When a politician says air quality is "Moderate", check which scale they are using.
India's own standards are loose
India's National Ambient Air Quality Standards (NAAQS) set the annual average PM2.5 limit at 40 µg/m³. The WHO guideline, updated in 2021 and based on the latest epidemiological evidence, is 5 µg/m³. That is an 8-fold gap. Air that India counts as acceptable is hazardous by global public health standards.
In practice, a city can meet NAAQS while its residents breathe PM2.5 at levels that the WHO says measurably raise the risk of lung cancer, heart disease, stroke and childhood respiratory illness.
What the number hides
AQI is calculated from 24-hour averages (or shorter rolling windows for some pollutants), which smooths out spikes. A neighbourhood downwind of a brick kiln might see PM2.5 reach 400 µg/m³ for three hours during firing, then fall to 80 for the rest of the day. The 24-hour average looks "Poor", while at 6 AM the air was "Severe".
Averaging also hides differences between places. A city's official AQI often comes from one or two monitoring stations, which may not represent every neighbourhood. In CPCB's 2024 daily bulletins, 221 of the 264 cities with a usable year, meaning at least 180 reported days (84%), had a median of fewer than three stations (JanVayu analysis of the CPCB bulletin data). Siting is a problem as well: Newslaundry's 2025 field check of 25 Delhi monitoring stations found that 88% flouted CPCB's siting criteria, and a CAG audit tabled in the Delhi Assembly in April 2025 found 13 of 24 DPCC stations it verified in 2020 were sited too close to trees (Newslaundry, 1 April 2025, a press report on the CAG audit).
What you should actually do
- Look at the raw µg/m³ values as well as the AQI. PM2.5 of 60 µg/m³ is 12 times the WHO guideline, whatever colour band a scale puts it in.
- Check the dominant pollutant. If it is ozone, staying indoors with the windows closed helps more than a mask. If it is PM2.5, a well-fitted N95 is your best protection.
- Compare sources. CPCB, WAQI, IQAir and community sensors (Sensor.Community) can show different numbers for the same city. JanVayu's Data Source Selector shows all four side by side.
- Do not rely on a single station. Check several, or use a platform such as JanVayu that combines the city network.
The AQI was built to turn a complicated picture into one number that anyone can act on. That is a sound aim, and a single number still leaves things out.
Explore the full breakdown in JanVayu's Understanding AQI panel.
Sources: CPCB National AQI methodology (2014); US EPA AQI Technical Assistance Document (2018); WHO Global Air Quality Guidelines (2021); Lancet Countdown 2025; CAG performance audit of air pollution control in Delhi, tabled in the Delhi Assembly, 1 April 2025, as reported by Newslaundry; IQAir World Air Quality Report 2025.