The short version
- Official air quality data from the Central Pollution Control Board omitted hourly PM2.5 readings exceeding 1,000 micrograms per cubic metre during Diwali.
- Independent analysis shows these exclusions resulted in significantly lower average pollution figures compared to data from the Delhi Pollution Control Commission.
- Experts warn that omitting extreme pollution episodes may misrepresent health risks and undermine public trust in environmental monitoring systems.
An investigation into air quality data from New Delhi during the recent Diwali festival has uncovered substantial inconsistencies between two major government datasets. These discrepancies suggest that official reports may have failed to capture the full severity of pollution levels experienced by residents. The findings emerge as concerns grow regarding the reliability of environmental monitoring in one of the world's most polluted cities.
During the peak of celebrations on November 8, fireworks contributed to a sharp rise in particulate matter across the capital. Monitoring stations recorded dangerous spikes in PM2.5, a fine pollutant known to cause serious respiratory and cardiovascular issues. However, a comparison of data from the Central Pollution Control Board (CPCB) and the Delhi Pollution Control Commission (DPCC) revealed that every hourly reading above 1,000 micrograms per cubic metre was absent from the CPCB's widely cited Air Quality Index.
The CPCB has stated that its equipment is calibrated for a specific detection range, and readings exceeding 1,000 micrograms are treated as outside this scope. Consequently, these extreme values are excluded from calculations. The agency noted that such exclusions can create apparent data gaps during occasional extreme episodes like Diwali, meaning those hours do not contribute to the 24-hour average for affected stations.
This methodology has significant implications for reported pollution levels. According to CPCB guidelines, a station requires at least 16 hours of valid PM2.5 measurements to calculate a daily average. If too many readings are missing or deemed invalid, the station's data is excluded from official calculations. During the peak celebration hours between November 8 and 9, 217 hourly observations were missing across 59 CPCB stations in Delhi. Only 16 stations reported complete data during this period.
The impact of these exclusions is evident in the divergence between the two datasets. Across 30 shared monitoring stations over a 24-hour period, available CPCB readings averaged 240 micrograms per cubic metre. In contrast, DPCC data showed an average of 551 micrograms per cubic metre for the same timeframe. This means the official figures presented less than half the pollution level recorded by the local commission.
Specific examples highlight the extent of the discrepancy. At a station in RK Puram, south Delhi, pollution peaked at nearly 2,500 micrograms per cubic metre. An analysis of hourly readings from this location calculated an average concentration of approximately 945 micrograms over the relevant 24-hour period. However, using only the CPCB-published data for the available hours produced a much lower average of 299 micrograms. The missing ten hours of data meant this station did not contribute to the official daily averages.
Despite sharp overnight rises in pollution at various monitoring points, the CPCB reported Delhi's Air Quality Index at 344 on November 9, a decrease from 376 the previous day. This downward trend in official figures contrasts with the severe conditions observed on the ground and in alternative datasets. Satellite imagery also confirmed a thick haze over the city during these dates, indicating high levels of pollutants such as nitrogen dioxide and carbon monoxide.
Health experts have raised concerns about the practice of excluding extreme readings. Dr. Manoj Kumar from the Centre for Research on Energy and Clean Air warned that omitting high values risks misrepresenting actual pollution levels and could erode public trust in official data. He emphasized that even short-term exposure to extreme particulate matter can have adverse health effects.
Dr. Sierra Clark, an environmental epidemiologist, noted that classifying high pollution readings as missing data leaves out days that contribute most to acute health events like asthma and heart attacks. She suggested that for transparency, reporting could present two scenarios: one including extreme episodes and another excluding them. This approach would provide a clearer picture of both typical daily conditions and the impact of severe pollution events.
While fireworks are a significant factor during Diwali, Delhi's air quality issues stem from multiple sources, including vehicle emissions, construction dust, agricultural fires, and atmospheric conditions. The controversy over data reporting underscores the need for robust and transparent monitoring systems to ensure public safety and informed decision-making regarding environmental health.
Sources behind this briefing
Go to the original reporting
- BBC News↗Was Delhi pollution data during Diwali misleading?
