Same Air, Three Scores: Why Delhi’s Pollution Looks Different On Each AQI Scale

On Diwali night, Delhi's Nehru Nagar recorded a PM2.5 concentration of 812 micrograms per cubic metre (µg/m³) between 9 pm and 10 pm. At 10 pm, the Central Pollution Control Board (CPCB) reported an Air Quality Index (AQI) of 387 for the station. The same hourly PM2.5 concentration, run through the US formula, translates to 1,469, while the UK's index gives it 10, the highest possible score.

The discrepancy is not a difference in how much pollution is in the air, but in how countries calculate and communicate it. India, the US and the UK use different AQI scales, pollutant thresholds and calculation methods. Even when they draw from the same pollution concentration, the resulting scores can diverge sharply.

The averaging period is another important distinction. CPCB's official AQI for particulate matter is based on a rolling 24-hour average, which can smooth out sharp pollution spikes. Many private applications, meanwhile, convert hourly or near-real-time concentrations using the US AQI formula. This is why Delhi's official readings remained in the 400s as screenshots showing AQI figures in the thousands circulated online.

What Is An Air Quality Index?

An AQI is a numerical tool used to communicate the health risks associated with air pollution. Monitoring stations measure the concentration of pollutants such as PM2.5, PM10, ozone, nitrogen dioxide, sulphur dioxide and carbon monoxide. These measurements are then converted into a score using a country's prescribed formula.

PM2.5 refers to fine particulate matter measuring 2.5 micrometres or less in diameter. These particles can penetrate deep into the lungs and are associated with serious health risks.

The index is not a universal scale. Governments determine the concentration thresholds corresponding to different AQI categories, the duration over which readings are averaged and the point at which the index stops increasing. Consequently, the same PM2.5 concentration can produce vastly different AQI figures across countries.

It is important to distinguish between the three measurements: an hourly PM2.5 concentration tells us how much particulate matter was present during a particular hour; a 24-hour average reflects the concentration over an entire day; and an AQI converts a pollutant concentration into a score. The three are related, but they are not interchangeable.

Why Does India's AQI Stop At 500?

India's National Air Quality Index was introduced in April 2015. It was developed by an expert group comprising doctors, air quality specialists, academics and representatives of state pollution control boards, with technical assistance from IIT Kanpur.

The index has six categories: Good (0-50), Satisfactory (51-100), Moderate (101-200), Poor (201-300), Very Poor (301-400) and Severe (401-500).

For PM2.5, the 24-hour concentration corresponding to the Good category extends up to 30 µg/m³. The Satisfactory category goes up to 60 µg/m³, India's national ambient air quality standard for PM2.5.

The Severe category begins at a PM2.5 concentration of 250 µg/m³. CPCB's calculation reaches an AQI of 500 at 380 µg/m³, after which the sub-index does not increase further.

The cap is intended to communicate health risks and trigger pollution control measures rather than provide an unlimited numerical representation of pollution. CPCB has described the index as a tool for effectively communicating air quality to the public.

However, this means an official AQI of 500 does not reveal whether the PM2.5 concentration is 380 µg/m³ or significantly higher. The index effectively places all such concentrations within the same upper limit.

How Do The US And UK Scales Differ?

The US Environmental Protection Agency (EPA) uses a six-category AQI system, ranging from Good to Hazardous. Unlike India's index, the American scale does not stop at 500.

The EPA revised its PM2.5 thresholds in May 2024. Under the updated system, the Good category ends at 9 µg/m³, while the Hazardous category begins at 225.5 µg/m³. An AQI of 500 corresponds to a PM2.5 concentration of 325.4 µg/m³.

For concentrations beyond this threshold, the EPA extends the calculation along the line between AQI values 301 and 500. This allows the index to continue rising into four-digit figures. Under the current formula, Nehru Nagar's hourly PM2.5 concentration of 812 µg/m³ translates to an AQI of 1,469. Under the older US formula, the same concentration would have produced a score of 706.

The UK's Daily Air Quality Index operates on a much smaller scale, ranging from 1 to 10. Its categories are Low, Moderate, High and Very High. A 24-hour PM2.5 average of 71 µg/m³ or more receives a score of 10.

The UK's index is therefore designed to communicate the severity of pollution through a capped category, rather than distinguish between increasingly extreme concentrations numerically.

The US and UK figures calculated from Nehru Nagar's hourly concentration are illustrative comparisons, not official daily AQI readings. Their prescribed AQI methods use longer averaging periods for particulate matter.

Why Do Apps Show AQI In The Thousands?

Many private air quality applications use the US AQI formula, even when displaying Indian locations. Platforms such as IQAir, AQI.in and the World Air Quality Index Project use US-based calculations, though their data sources and averaging periods differ.

IQAir labels its readings "US AQI+" and uses hourly averages. AQI.in labels its readings "AQI (US)" and draws from community monitors. The World Air Quality Index Project applies the US formula to the latest station readings, which it says may be unvalidated at the time of publication.

This is why government readings can remain in the 400s while private applications display four-digit numbers. The platforms may be using the same underlying pollution concentration but converting it through a different formula and averaging period.

The EPA, however, advises against applying its AQI formula to short-term readings. It states that a full 24 hours is required to obtain a valid AQI value and cautions against using hourly or real-time sensor data for this purpose.

The difference is especially pronounced during pollution spikes. An hourly concentration captures a brief period of intense pollution, while a 24-hour average also includes the comparatively cleaner hours before or after it. The resulting AQI values therefore describe different time windows.

Different Averaging Periods, Stations And Sensors

The duration over which pollution is measured is a major contributor to the discrepancy.

CPCB calculates its AQI using a 24-hour average for particulate matter. A sharp increase in pollution at night can therefore be diluted by relatively cleaner hours earlier in the day. On Diwali night, Nehru Nagar's 10 pm index still included afternoon readings.

Private applications may use hourly averages or even shorter intervals, capturing sudden spikes more directly. The US EPA's AirNow system, meanwhile, uses a weighted average of up to 12 hours for particle pollution.

The location of monitoring stations matters, too. CPCB's reading may represent a single station, while an application could combine readings from multiple locations across Delhi. A monitor near firecracker activity or a traffic-heavy road may record substantially higher pollution than one elsewhere.

The instruments themselves can produce differences. Government monitoring stations generally use reference-grade equipment, while low-cost sensors such as those used by PurpleAir measure particles through laser light scattering.

Humidity can cause these sensors to overestimate PM2.5 because particles absorb moisture and scatter more light. Researchers studying PurpleAir sensors alongside a reference monitor at the US embassy in New Delhi between 2018 and 2020 found that uncorrected readings overestimated pollution during humid post-monsoon and winter periods. Seasonally adjusted readings came within approximately 25% of the reference instrument, according to a 2023 study published in Atmospheric Measurement Techniques.

Which AQI Should Delhi Residents Rely On?

For assessing the severity of Delhi's pollution and understanding government-issued health advisories, CPCB's official AQI remains the primary reference. However, its 500-point ceiling limits its ability to communicate the extent of extreme pollution.

For comparisons across countries or applications, the raw PM2.5 concentration in µg/m³ can be more useful, provided the readings come from comparable monitoring instruments and averaging periods.

The World Health Organisation recommends that 24-hour PM2.5 exposure should not exceed 15 µg/m³. Nehru Nagar's hourly concentration of 812 µg/m³ was more than 54 times that guideline. However, this is a comparison between an hourly concentration and a 24-hour exposure guideline, not a direct measure of daily exposure.

Ultimately, an AQI is a communication tool, not a direct measurement of pollution. When comparing readings, residents should check the scale being used, the pollutant measured, the monitoring station, the averaging period and the time of the last update.

A reading of 387 in India, 1,469 in the US and 10 in the UK can describe the same highly polluted air when calculated from the same concentration. The numbers differ, but the underlying health risk does not disappear because one country's index has reached its ceiling.