India’s pollution board is reviewing its monitoring methodology after data gaps during Diwali. Thick smog reduced visibility on 9 November, with severe air quality in parts of Delhi and nearby cities. IQAir put fine-particle pollution at 879 micrograms per cubic meter in Mandir Marg after monitors resumed reporting.
India’s Central Pollution Control Board says it is reviewing its monitoring methodology after gaps emerged in Delhi’s official pollution data during Diwali. The board said its system excludes some readings above the range its analyzers are calibrated to measure, and rejected accusations that the gaps were an attempt to conceal pollution.
On 9 November, smog obscured roads and high-rises after overnight fireworks. Several areas of Delhi and surrounding National Capital Region cities recorded air quality in the severe category. Delhi’s air had already been at its worst level for a Diwali eve in a decade on 7 November.
Monitors resumed reporting around 8 hours after the fireworks peaked, CNA said. Afterward, monitoring organization IQAir put the concentration of PM2.5—fine particulate pollution—at 879 micrograms per cubic meter in central Delhi’s Mandir Marg, using data from a government station. The reading describes that locality, not the whole city, and does not establish the peak concentration.
The World Health Organization’s guideline is 15 micrograms per cubic meter averaged over 24 hours. The early-morning Mandir Marg reading covers a different period, so comparing the two does not establish a daily-average exceedance of the same magnitude. Fine particles can cause cancer, and severe air pollution poses significant health risks.
The Delhi Pollution Control Committee later released provisional hourly readings missing from central board records. In Mundka, the highest station reading reported was 3,744 micrograms of PM2.5 per cubic meter at 02:00 India time (00:30 Yerevan time) on 9 November, Hindustan Times said.
The air quality index (AQI) summarizes pollution as a single number. The central board’s published citywide figures did not include the missing readings; Hindustan Times estimated higher figures using the board’s calculation method.
The federal Stage II pollution controls restrict diesel generators and the burning of coal and firewood, while vehicle checks under Delhi’s separate winter plan have left commuters in queues for hours.
| Period or bulletin | Published by the board | Hindustan Times estimate |
|---|---|---|
| 24 hours ending at 09:00 India time (07:30 Yerevan time) | 370 | about 384 |
| Bulletin at 16:00 India time (14:30 Yerevan time) | 344 | about 360 |
The newspaper said the index stops at 500 for an hour with a concentration above about 420 micrograms per cubic meter, so much larger concentrations add little to the index.
Delhi’s recurring winter smog is fueled by vehicle emissions, industrial activity and crop-residue fires in neighboring states. Diwali fireworks add fine particles and soot to that pollution. Winter controls, including vehicle restrictions and construction bans, began on 1 November.
India’s Supreme Court had restricted conventional firecrackers in October, allowing only “green firecrackers” during three-hour windows on 7 November and 8. People violated the restriction during the celebrations, DW said.
In its November 10 statement, the central board attributed differences between official and private AQI values to measurement and methodology. It said its analyzers are calibrated to measure particulate concentrations up to 1,000 micrograms per cubic meter. Automated checks exclude hourly readings beyond that range from an affected station’s 24-hour pollutant average, which the board said can appear as a gap during extreme episodes.
The board said it is reviewing its methodology in light of advances in monitoring technology and international practice. Internal discussions and consultations are underway on processing readings beyond the system’s design range.
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The court limited fireworks use on 7–8 November to 06:00–07:00 and 20:00–22:00 local time, as Delhi’s air quality index reached 377 on Wednesday morning.
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