Mumbai AQI Puzzle: Why IQAir Says 'Unhealthy' But CPCB Rates Air 'Satisfactory'

News india Mumbai AQI Puzzle: Why IQAir Says 'Unhealthy' But CPCB Rates Air 'Satisfactory'

Last Updated:October 26, 2026, 11:06 IST

The monitoring network explains only part of the discrepancy. The other major factor is the formula used to convert pollutant concentrations into an AQI number

This October, Mumbai’s air quality has presented a confusing picture. While international air-quality platform IQAir has repeatedly ranked the city among the world’s most polluted, India’s official pollution monitoring system has often classified its air as ‘satisfactory’. The discrepancy has raised questions about how air pollution is measured, which readings residents should trust and what the numbers actually mean.

The difference was particularly striking on October 15, when IQAir ranked Mumbai fifth among 122 cities worldwide and first among Indian cities, placing it ahead of Delhi. On the same day, the Central Pollution Control Board (CPCB) described Mumbai’s air quality as ‘satisfactory’.

The divergence is not necessarily evidence that one system is right and the other wrong. The two platforms use different monitoring networks, averaging methods and air quality index (AQI) scales. Mumbai’s coastal geography, weather conditions, and sharply varying pollution levels across neighbourhoods add to the complexity.

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Here is how the two systems work, why their readings differ and what residents should know when checking the air they breathe.

Why Are IQAir And CPCB Reporting Different AQI Levels?

The difference became particularly visible after the monsoon withdrew and Mumbai’s air quality deteriorated during the first weeks of October.

Between October 1 and 23, the CPCB dashboard classified Mumbai’s air as unhealthy on 11 days, while IQAir placed it in the unhealthy category on all 23 days. IQAir recorded its highest AQI reading of 188 on October 17, while the CPCB’s peak reading during the period was 127 on October 18.

The two systems also produced markedly different readings on October 21. IQAir recorded an AQI of 165 for Mumbai, placing it fourth among the world’s most polluted cities at the time. The CPCB recorded an AQI of 113 for the city, with individual monitoring stations reporting higher readings in some locations.

How Is Mumbai’s Air Quality Measured?

The Maharashtra Pollution Control Board (MPCB) monitors Mumbai through 26 Continuous Ambient Air Quality Monitoring Stations (CAAQMS). These stations collect outdoor air samples and measure concentrations of major pollutants, including:

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• PM2.5: Fine particulate matter small enough to penetrate deep into the lungs.

• PM10: Larger inhalable particles associated with road dust, construction and other sources.

• Nitrogen dioxide (NO₂): A gas associated with vehicle exhaust and fuel combustion.

• Sulphur dioxide (SO₂): A pollutant produced by burning sulphur-containing fuels and certain industrial processes.

• Ozone (O₃): Ground-level ozone formed through chemical reactions involving pollutants in sunlight.

• Carbon monoxide (CO): A gas produced by incomplete combustion.

The monitoring network also measures other pollutants covered by India’s National Air Quality Index, including ammonia and lead, where data are available.

The CPCB uses readings from monitoring stations to calculate pollutant-specific sub-indices. The highest relevant sub-index determines the overall AQI, subject to data-availability requirements.

India’s AQI has six categories: Good (0–50), Satisfactory (51–100), Moderate (101–200), Poor (201–300), Very Poor (301–400) and Severe (401–500). The reported city-level figure summarises air quality across the monitoring network. However, it does not mean that every neighbourhood is experiencing identical pollution levels.

How Does IQAir’s Monitoring System Differ?

Private air-quality platforms such as IQAir use a combination of government-operated reference stations and low-cost sensors. These sensors can be installed in more locations and help identify local pollution hotspots, including areas near busy roads or construction activity.

According to an Indian Express report, IQAir had six monitoring stations in Mumbai, including one government-approved continuous monitoring station and five low-cost sensors.

IQAir says it applies correction factors to low-cost sensor readings to account for measurement bias, including the effects of high humidity. Its city-level figures are calculated using the median of hourly aggregated pollutant concentrations from stations within the city.

This approach can capture local and short-term changes in pollution. However, readings from a sensor near a traffic junction may differ considerably from those recorded at a government station elsewhere.

Brihanmumbai Municipal Corporation officials have highlighted this distinction: government monitoring stations are designed to assess ambient air quality around their locations, while low-cost sensors can help identify more localised conditions.

Neither type of monitoring is automatically invalid. The location of a sensor, the pollutants it measures and the way its readings are aggregated all affect the final result.

The Biggest Difference: India’s AQI Versus The US AQI

The monitoring network explains only part of the discrepancy. The other major factor is the formula used to convert pollutant concentrations into an AQI number.

India uses its National Air Quality Index, based on national ambient air quality standards. IQAir told the Indian Express that it follows the AQI methodology of the United States Environmental Protection Agency (US EPA).

The two systems use different concentration thresholds and category breakpoints. Consequently, the same concentration of a pollutant can produce different AQI numbers and different labels.

Consider PM2.5, one of the main pollutants of concern in Mumbai. Under India’s AQI framework, a PM2.5 concentration of up to 30 micrograms per cubic metre falls within the ‘Good’ category. Under the US AQI scale cited by IQAir, the ‘Good’ category extends only to a PM2.5 concentration of 9 micrograms per cubic metre. This means a PM2.5 concentration of 20 micrograms per cubic metre could fall within the ‘Good’ category under India’s framework but the ‘Moderate’ category under the US system.

The concentration of pollution has not changed in this example; the benchmark used to classify it has.

The World Health Organization (WHO) has separate air-quality guidelines. Its 2021 guidelines recommend a 24-hour average PM2.5 concentration of no more than 15 micrograms per cubic metre, compared with India’s 24-hour national ambient air quality standard of 60 micrograms per cubic metre.

These benchmarks serve different purposes and should not be confused with AQI readings themselves. An AQI is an index derived from pollutant measurements, not a direct measurement of the quantity of pollution in the air. To compare two AQI figures meaningfully, it is important to establish which scale, pollutant and averaging period each figure represents.

Why Does Mumbai’s Geography Make The Problem More Complicated?

Mumbai’s geography creates considerable variation in air pollution across the city.

The city is a peninsula surrounded by the Arabian Sea on three sides, while hills and ridges influence local wind movement and the dispersion of pollutants. Dense traffic, construction activity and population concentrations can produce pollution hotspots that are not necessarily reflected uniformly across the city.

The location of a monitoring station therefore matters. A reading near a busy road may be substantially higher than one recorded at a station in a less congested area.

Weather conditions can also affect pollution levels and the readings recorded by certain sensors. High humidity can cause airborne particles to absorb moisture and grow in size, potentially influencing measurements. IQAir says it applies model-specific correction factors to low-cost sensor data to address known measurement biases, including those associated with humidity.

Mumbai’s prolonged dry spell and unusually high temperatures this October have also contributed to the deterioration in air quality. Atmospheric scientist Gufran Beig told the Indian Express that weak winds had restricted the dispersal of pollutants, allowing them to accumulate closer to the ground.

The early withdrawal of the monsoon, deficient rainfall and the broader El Niño-influenced weather pattern were among the factors identified in the report as contributing to the conditions.

Which AQI Reading Should Mumbai Residents Trust?

The answer depends partly on what residents want to understand, but neither figure should be treated as a complete picture of every location in the city.

The CPCB’s AQI provides an official assessment based on India’s national framework and its regulatory monitoring network. IQAir offers a different assessment based on its monitoring network and the US AQI scale, which applies different thresholds to pollutant concentrations.

For someone deciding whether to exercise outdoors, commute by road or limit a child’s outdoor activity, the most useful approach is to check recent readings from monitoring stations near their location, along with the pollutant responsible for the AQI and the time period covered by the measurement.

Residents should also consider health advisories and avoid interpreting a ‘satisfactory’ label as a guarantee that the air is harmless to everyone.

Under India’s AQI framework, the ‘Satisfactory’ category itself notes that sensitive individuals may experience minor breathing discomfort. Higher categories indicate increasing health concerns, particularly for people with respiratory or cardiovascular conditions, children and older adults.

Ultimately, the apparent contradiction between Mumbai’s AQI readings reflects differences in measurement, monitoring coverage and classification.

About the Author

Apoorva Misra

Apoorva Misra is an Associate Editor at News18.com with a keen interest in politics and current affairs. She loves uncovering fresh angles and telling stories through long-form features and explainers...Read More

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