Image source: Getty Images
BBC Verify has found significant differences between two Indian government datasets showing air pollution data during Diwali in Delhi. This indicates that the government data may not have revealed the complete picture of pollution.
On November 8, the sky of Delhi lit up with fireworks to celebrate Diwali. During this period, some monitoring stations recorded very high levels of PM2.5. PM2.5 are very small particles present in the air. These can damage people's lungs and cause other serious health problems.
But a comparison with Delhi Pollution Control Committee (DPCC) data revealed that all hourly PM2.5 concentrations above 1,000 micrograms per cubic meter were not included in the Central Pollution Control Board's (CPCB) widely used Air Quality Index (AQI).
Delhi is one of the most polluted cities in the world and the air quality here often remains dangerously poor.
Therefore, it is very important that people living in Delhi can trust the government data.
Firecrackers are not the only cause of pollution. Many reasons like smoke coming out from vehicles, dust from construction works, burning of stubble in fields and atmospheric conditions are also responsible for this. But firecrackers can further deteriorate the air quality. Especially in winter.
Image source, Reuters
CPCB, whose data is commonly used by the Indian media, has issued a statement.
It says readings above 1,000 micrograms are considered outside the measurement range of its instruments. These instruments have been designed to measure pollution up to a certain limit. Therefore such data is not included in the calculations.
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The statement further said, "In case of occasional extreme situations like burning of firecrackers on Diwali, this difference may be visible in the data of the concerned monitoring station. The data of such hours are not included in the 24-hour average of the concerned pollutant of that station."
Dr Manoj Kumar, a researcher at the Center for Research on Energy and Clean Air (CREA), told BBC Verify that removing high pollution readings could misrepresent the actual level of pollution.
This may also reduce people's trust in government statistics.
Our analysis found that the average of CPCB data available from the same 30 monitoring stations during the 24 hours on Diwali night was 240 micrograms per cubic metre.
This was less than half the average of 551 recorded in DPCC data.
What do pollution figures tell?
Image source, ANI
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On the night of Diwali between Sunday and Monday, as the celebrations reached their peak, the already poor pollution levels in Delhi increased rapidly.
BBC Verify examined the DPCC data. It was found that the hourly level of PM2.5 was recorded at more than 3,000 micrograms at three different pollution measuring stations.
Dr. Kumar said that exposure to high levels of pollution particles even for a short period of time can have adverse effects on health.
According to the air quality guidelines of the World Health Organization (WHO), the annual average amount of PM2.5 in the air should not exceed five micrograms per cubic meter.
Between 9 pm local time (15:30 GMT) and 7 am on November 9, the peak of Diwali celebrations, 217 hourly readings of PM2.5 were not available at CPCB's 59 monitoring stations in Delhi.
During this period, only 16 stations recorded complete data, while 43 stations had at least one reading missing.
A total of 116 readings of more than 1,000 micrograms were recorded at 30 monitoring stations of DPCC. Not a single one of these readings was included in the published records of CPCB, to which these stations send their data.
This is important because as per CPCB methodology, a station needs to have at least 16 hours of valid PM2.5 measurements to arrive at a daily average.
Therefore, stations with large numbers of missing or invalid readings will not be included in the calculation of official pollution figures.
There was such a monitoring station in RK Puram in South Delhi, where data was not available for several hours. The pollution level here had increased to about 2,500 micrograms.
Based on hourly PM2.5 readings at this station, BBC Verify averaged the 24 hours following 5pm on 8 November. It was about 945 micrograms.
But on taking the average of CPCB's published data of 14 hours out of these 24 hours, this level was much lower, i.e. 299 micrograms.
Data for the remaining 10 hours was not available. For this reason it was not even included in the calculation of daily average.
Missing data across the city meant that despite pollution levels rising rapidly at night at some monitoring stations, the CPCB recorded Delhi's AQI at 344 at 4 pm on November 9. A day before this at the same time it was 376.
Dr. Sierra Clark, an environmental epidemiologist at City St George's University of London, warned that missing readings with high levels of pollution could miss data on days that "contribute most to short-term health problems such as asthma attacks and heart attacks."
He told BBC Verify that for transparency, two types of situations can be shown in Delhi's pollution data.
In one, days with extremely high levels of pollution should be included and in the other, they should be removed.
Dr. Clark further said, "The second scenario will reveal the general level of pollution. It will show people's everyday experience in a better way. But it is also important to show the impact of those few days and their contribution to pollution."
Pollution in Delhi is not limited to PM2.5 only. Satellite images show a dense haze of pollution over the city on 8 and 9 November.
But instead of on the ground, satellite data measuring nitrogen dioxide, carbon monoxide and fire activity in the atmosphere recorded an increase on November 8.
However, their overall average for the Diwali week was not much higher than that of 2024 and 2025.
Graphics: Jess Carr and Tom Shiel
Published by Collective Newsroom for the BBC.
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