Air quality data from several government monitoring stations in Delhi went missing for hours during the severe pollution spike following Deepavali celebrations on November 8, raising concerns that the official air quality index (AQI) may have understated the actual level of pollution in the national capital.
Experts have warned that gaps in hourly particulate matter readings can distort AQI calculations, potentially making the air appear cleaner than it really is. The missing data were reported at several locations, including Anand Vihar, Patparganj and Bawana.
According to Manoj Kumar an analyst at the Centre for Research on Energy and Clean Air (CREA) 227 hourly PM2.5 observations were missing across 59 Central Pollution Control Board (CPCB) monitoring stations between midnight on November 8 and 5 am on November 9.
Only 14 stations reported complete data, while the remaining 45 recorded at least one missing hourly observation.
Kumar said the absence of these readings raised concerns about the accuracy of Delhi’s air quality assessment as missing PM2.5 measurements could artificially lower the calculated AQI.
PM2.5 refers to fine airborne particles measuring 2.5 micrometres or less in diameter. These particles can penetrate deep into the lungs and enter the bloodstream, increasing the risk of respiratory and other health problems.
Dr D. Saha, former head of the CPCB’s air quality laboratory, explained that the AQI is calculated using a running average of hourly concentrations of eight pollutants, including PM2.5 and PM10 over the preceding 24 hours.
He said missing hourly readings could affect the final AQI calculation, producing a value lower than the actual pollution level.
Saha also pointed out that PM2.5 analysers used by the CPCB have a measurement range of 0–1,000 micrograms per cubic metre. He argued that when concentrations exceed this limit, the readings should at least be displayed as 1,000 rather than left blank.
A former Delhi Pollution Control Committee scientist also questioned the decision not to capture higher pollution concentrations, arguing that even if measurement accuracy declines slightly at extreme levels, recording the peak values remains important for an accurate assessment.
CPCB data showed that PM2.5 concentrations at several locations had reached extremely high levels before the readings went missing.
At Anand Vihar, PM2.5 levels climbed to 767 micrograms per cubic metre at 11 pm on November 8. This was more than 51 times the World Health Organization’s 24 hour guideline of 15 micrograms per cubic metre. However, readings were unavailable for the following three hours, while PM10 data were missing for five hours.
At Patparganj, PM2.5 concentrations peaked at 930 micrograms per cubic metre at 10 pm, approximately 62 times the WHO guideline. The readings then disappeared for three hours, while PM10 data remained unavailable for seven hours.
Siri Fort in south Delhi recorded a PM2.5 concentration of 1,000 micrograms per cubic metre the upper limit displayed on the CPCB portal. Analysts raised concerns that concentrations exceeding this threshold were not being adequately represented.
Environmental activist Bhavreen Kandhari alleged that the missing readings concealed the actual scale of Delhi’s pollution crisis and called attention to the need for greater transparency in air quality reporting.
Delhi Labour Minister Kapil Mishra said the government would look into the issue when questioned about the missing data. The CPCB Member Secretary had not responded to requests for comment at the time of publication.
Similar gaps in monitoring data were also reported during Deepavali celebrations in the previous year.
The latest incident has renewed questions about the reliability of pollution monitoring during peak emission periods. Experts say uninterrupted data collection accurate measurement of extreme pollution levels and transparent reporting are essential for assessing public health risks and ensuring effective action against Delhi’s worsening air pollution.