Why India's AQI of 387 Could Reach 1,500 in the US and Just 10 in the UK: Explained

The air quality government monitor at Nehru Nagar in Delhi during Diwali night clocked 812 micrograms of fine particles PM2.5 in 1m3 of air, which is the fine dust that penetrates deep into the lungs. Central Pollution Control Board (CPCB) issued an air quality index (AQI) at 10pm for the station, which is 387. The formula used by the US government converts 812 to 1,469 while the UK's index puts it at 10, the highest point on its scale.

As the night progressed, images of the apps and home monitors showing Delhi air quality readings at 1,000+ and 2,000+ started to appear on social media, while the official data remained in the 400s. Many of these were from apps with different scales, and averaging over shorter time periods.

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What is an AQI?

A monitor is used to measure the amount of pollutant in the air, measured in micrograms per cubic metre (µg/m³). An AQI is a conversion of that measurement that is given by bands set by a government. The starting point and averaging period for the readings, and the ending point of the scale are at the discretion of the governments. Therefore, the air at one location at one time can yield very different scores for two different indices.

Why is there a limit of 500 in India's index?

The National Air Quality Index (NAQI) was announced by the Government of India in April 2015. It was prepared by a team of experts comprising doctors, air quality experts, academics and state pollution boards and technical assistance from IIT Kanpur for the CPCB. It has six bands: good (0 to 50), satisfactory (51 to 100), moderate (101 to 200), poor (201 to 300), very poor (301 to 400) and severe (401 to 500).

The PM2.5 concentration in terms of PM2.5 averaged over 24 hours falls under “good” up to 30 micrograms per cubic meter (µg/m³), which is half of India's national standard of 60 micrograms per cubic meter (µg/m³), and falls under “satisfactory” up to 60 micrograms per cubic meter (µg/m³).

The next threshold was set at 90 µg/m³, where the increased risk of heart disease evened out in a global study of disease, and for levels above this the committee used the thresholds recommended in the US and China, the report states.

The severe band recommended the air may “have respiratory effects on healthy people”.

In the CPCB's report the severe band for PM2.5 is represented as “250+”. The report does not provide a concentration with an index of 500 and has nothing to say about values above 500. The data provided by CPCB only goes up to 500. The PM2.5 sub-index peaks at 380 µg/m³, according to CPCB's calculation, and does not exceed 500.According to the CPCB's calculation, the sub-index of PM2.5 peaks at 380 µg/m³ and will not go beyond 500.

A senior official of the CPCB had told HT when at least four of the city's 38 stations reported an AQI of 500, in November 2019, that the values could be anything above 500, it could even be 900 µg/m³, because the values were not recorded beyond 500.

Part of the reasoning for the cap, according to officials, is that the index is meant to warn people about the health risk and set off pollution curbs. The index, CPCB said, is meant to be a “tool for effective communication of air quality status to people”. What counts is the PM (particulate matter) concentration, the official who spoke to HT in 2019 added, “Based on this, measures are being taken to reduce pollution.”

HT's analysis of data showed that on Diwali night last year, around 93 per cent of the hourly readings of PM2.5 at Delhi's stations had crossed CPCB's severe band of 250µg/m³, which is above 600 per cent of the standard.About 93 per cent of hourly readings of PM2.5 at stations across Delhi have crossed the CPCB's severe band of 250µg/m³, which is 600 per cent above the standard, on Diwali night last year, its analysis of data showed.

Why were the figures of Nehru Nagar so low at 387?

CPCB also computes an index for each station based on their average values over the last 24 hours, meaning that if the station has a large increase during nighttime hours, it may be compensated by the lower values during the day. After an evening of festivities, on Diwali evening at 10pm, average Nehru Nagar included the afternoon hours. After that hour, the station continued to be non-functional and CPCB was probably continuing to calculate its index from the readings it had.

For comparison of the worst hours from year to year, HT analysed the Diwali night beyond the last band of CPCB's sub-index calculation for PM2.5. That year the average of six of the longer-running stations for the worst hour was 655, compared to 812 in 2021 and 752 in 2025.

What are the scales in the UK and US?

The Daily Air Quality Index (AQI) for the United Kingdom is from 1 to 10 with categories of low, moderate, high and very high. It gives 10 to any 24-hour average of PM2.5 at 71 µg/m³ or more. The Diwali reading of Nehru Nagar was over 11 times higher.

The U.S. Environmental Protection Agency's (EPA) index also has six classifications: good, fair, moderate, poor, very poor and hazardous. The agency tightened it from May 6, 2024: “good” now ends at 9 µg/m³ of PM2.5, down from 12, and “hazardous” begins at 225.5 µg/m³, down from 250.5. An index of 500 now corresponds to 325.4 µg/m³.

The US index doesn't end at 500. “All AQI values greater than 300 fall into the Hazardous category (including values greater than 500),” EPA notes in its technical guidance to the agencies that report the index, and advises the agencies to compute values on the AQI range of 301–500. On that line, adding one extra µg/m³ will increase the index by approximately 2 points. In the previous scale, they each contributed approximately 2/3 of a point.

Therefore the score of Nehru Nagar 812 on the existing scale is 1469 and on the old scale is 706.

The US index is designed to be read over a day, as is India's. EPA states that it takes 24 hours to get an AQI value.

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Where does the four-digit numbers come from?

Over 95% of the apps that reported thousands for Delhi over Diwali night work on the US scale, and many do so with readings of less than an hour.

IQAir designates its number as U.S. AQI⁺ and states that it's based on hourly averages. Among its 19 contributors of data, listed in its Delhi page, are CPCB and the Delhi Pollution Control Committee, as well as embassies, schools and individuals—meaning that some of the Delhi figures begin as government readings.

The latest station readings are used in the World Air Quality Index Project's formula, which operates the website aqicn.org, and it claims that “data are not validated at the time of publication”.

The US index is derived from the raw data from PurpleAir's laser particle counters and is displayed on the map. AQI.in, operated by a Delhi based firm Purelogic Labs, reports its number as AQI (US), relies on “community monitors” and even has its own band names, severe being the band number 201 to 300. India's air quality service, offered by Google, provides an index of air quality for Indian cities as its default index.

The government's hourly readings—operating its own equipment—yield four-digit numbers when run through the US formula. On Diwali night, Pusa recorded 881 µg/m³ while the worst ever on Diwali last year (when stations averaged 1,137 µg/m³) gets 2,117.

EPA does not recommend that an index be developed from such short readings. Its guidance is that it is not valid to calculate an AQI value using shorter-term (eg hourly) data, and it does not recommend using realtime data, particularly 1-minute readings, to calculate an AQI value.

Can home monitors be trusted?

Low cost monitors such as the PurpleAir's project a laser beam into the atmosphere and use instruments to detect the light scattered by the particles. If the humidity is high, the particles will absorb water and increase in size, reflecting more light and the sensor will report higher levels of particulate matter than are actually present. A study conducted by researchers in 2018-2020 at the US embassy in New Delhi showed that in the humid post-monsoon and winter period, the sensors over-estimated PM2.5 levels when not calibrated. The team reported in 2023 in Atmospheric Measurement Techniques that when humidity was taken into consideration, season by season, they measured within approximately 25% of the reference instrument.

In an earlier study (2018) by the researchers of Duke University and IIT Kanpur, it was found that the readings of this common type of sensor ceased to increase with actual concentrations when PM2.5 levels exceeded approximately 125 µg/m³.

IQAir and AQI.in neither specify on their websites how they correct the readings of the low-cost sensors nor do they indicate whether or not users should apply a correction themselves.

A sensor has to measure only the air around it and if it is put on a balcony above a lane of firecrackers, then only this lane is recorded.

What kind of comparison is needed to be done between the numbers?

Most apps will display the concentration in micrograms per cubic meter (µg/m³) on the same line as the index, allowing for easy comparison of concentrations of various pollutants by source and country.

The WHO guidelines for 24-hour exposure to PM2.5 particles do not allow particles to exceed 15 micrograms per cubic metre (µg/m³).

However, when comparing two numbers, make sure to understand the scales of both numbers and the length of the series they span. Setting two sources side by side is not as effective as following one source over several days to see if the air is getting better.

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