Delhi's air pollution worsens every winter, with Diwali firecrackers compounding the problem.
But this year, questions have emerged over whether the capital's official pollution figures captured the full picture.
An investigation by BBC Verify has found discrepancies between two government datasets recording Delhi's air quality during the festival.
So, did the official numbers reflect the pollution levels residents were actually exposed to?
What Exactly Was Missing From The Data?
The discrepancy centres on PM2.5, fine particles released by sources such as vehicles, industries and firecrackers.
BBC Verify examined 24 hours of pollution data from 30 monitoring stations during Diwali. It found that all 116 hourly PM2.5 readings above 1,000 micrograms per cubic metre (µg/m³) in the DPCC dataset were missing from the CPCB's published records.
Three stations had recorded hourly concentrations exceeding 3,000 µg/m³.
The difference was also reflected in the averages. Across the same 30 stations, available CPCB readings averaged 240 µg/m³, compared with 551 µg/m³ in DPCC data. This meant the CPCB average was less than half the DPCC figure.
Why Were The Readings Left Out?
The CPCB said readings above 1,000 µg/m³ exceed the detection range for which its monitoring equipment is calibrated. These are excluded from calculations, creating data gaps during extreme pollution episodes.
Under CPCB methodology, stations need at least 16 hours of valid readings to calculate a pollutant sub-index. Without sufficient data, they may be excluded from daily calculations.
BBC Verify found 217 missing hourly PM2.5 observations across Delhi's 59 stations between 9 pm on 8 November and 7 am on 9 November. Only 16 stations had complete data, while 43 had at least one missing reading.
At RK Puram, DPCC data showed a 24-hour average of approximately 945 µg/m³ from 5 pm on 8 November. CPCB's available 14 hours averaged 299 µg/m³. The 10 missing hours meant the station did not qualify for the daily average calculation.
So, Did Delhi's Air Actually Improve?
At 4 pm on 9 November, the CPCB reported Delhi's AQI at 344, down from 376 at the same time on Diwali day. However, missing PM2.5 readings meant the figures may not have captured the full overnight pollution spike.
AQI is an index based on multiple pollutants, while PM2.5 measures the concentration of fine particles. The WHO's annual PM2.5 guideline is 5 µg/m³.
Dr Sierra Clark, an environmental epidemiologist at City St George's, University of London, told BBC Verify that excluding extreme readings could leave out days linked to short-term health events, including asthma and heart attacks.
She suggested publishing two pollution scenarios, with and without extreme readings, to show both typical pollution levels and the impact of severe episodes.
Why Do Accurate Pollution Figures Matter?
Firecrackers can worsen Delhi's already poor air quality, particularly during winter. BBC Verify's satellite analysis also showed increases in nitrogen dioxide, carbon monoxide and atmospheric fire activity on 8 November.
But the bigger concern is whether official pollution figures capture the full picture.
Residents rely on these numbers to understand health risks and make decisions about stepping outdoors. If extreme readings are excluded, the reported AQI could give a misleading impression of pollution levels and how quickly the air is improving.
This raises a bigger question: if the data misses the worst pollution episodes, how can residents trust the numbers used to assess the air they breathe?
TL;DR | News At A Glance
Why is Delhi's Diwali pollution data under scrutiny?
BBC Verify found significant differences between CPCB and DPCC pollution records, raising questions about whether official figures captured the full extent of Delhi's Diwali pollution.
What was missing from the official records?
All 116 hourly PM2.5 readings above 1,000 µg/m³ from 30 DPCC-managed stations were absent from the CPCB's published records.
How different were the two datasets?
Available CPCB readings averaged 240 µg/m³, while DPCC readings averaged 551 µg/m³ across the same 30 stations over 24 hours.
Why did the CPCB exclude these readings?
The CPCB said readings above 1,000 µg/m³ were outside the detection range for which its monitoring equipment had been calibrated.
How much pollution data was missing?
Between 9 pm on 8 November and 7 am on 9 November, 217 hourly PM2.5 observations were missing across 59 monitoring stations.
Did Delhi's AQI improve after Diwali?
The CPCB reported an AQI of 344 at 4 pm on 9 November, down from 376 at the same time on Diwali. However, missing observations raised questions about how representative the figures were.
What are the other causes of Delhi's pollution?
Vehicle emissions, construction dust, industrial activity, agricultural fires and weather conditions also contribute to the city's poor air quality.
