Delhi recorded its most polluted Diwali in five years, but the severity of the pollution was likely worse than official figures suggested, as several monitoring stations went offline on Sunday night during the peak period of firecracker use in the Capital, raising questions over the reliability of the air quality monitoring exercise.
AQI (air quality index) readings published by private agencies painted a considerably worse picture than official figures, with most parts of Delhi-NCR reporting extremely hazardous pollution levels, prompting experts to question the government's data.
Experts tracking the figures in real time said key monitoring stations stopped reporting readings during the night, when pollution levels surged as Supreme Court-mandated restrictions on the use of fireworks were flouted. They also flagged significant discrepancies between readings reported by government and private monitoring agencies.
Expressing concern over the absence of credible official data during the worst period of pollution, Anumita Roychowdhury, executive director of the Centre for Science and Environment (CSE), said the monitoring network's failure to provide real-time readings was inexplicable despite substantial public investment in its establishment and maintenance.
"When substantial public funds have been invested in establishing Delhi-NCR's air quality monitoring network — the densest in the country — backed by active operation and maintenance contracts, a data eclipse precisely during peak Diwali pollution is inexplicable," she said.
She said real-time data should have driven targeted public health advisories and calibrated emergency responses.
"While the severity of exposure is evident on the ground, credible official data is needed for institutional accountability, public awareness and mitigation. Losing data does not reduce the public health emergency," Roychowdhury said.
Responding to a question on the issue at a press conference, Delhi cabinet minister Kapil Mishra asked reporters to share details of AQI monitoring stations that were reportedly non-functional, adding that he would seek action on the matter.
More than 20 monitoring stations were not reporting data between midnight and 2 am during peak smog hours, according to an analysis by the Centre for Research on Energy and Clean Air (CREA).
Data from Delhi's 59 air quality monitoring stations showed that the number of stations with missing readings rose sharply as Diwali night progressed, peaking in the early hours of Monday.
While only one station had missing data at 7 pm on Sunday, the number rose to seven by 10 pm and 23 by 11 pm. By midnight, 29 stations were not reporting readings, with the number rising further to 36 at 1 am on Monday. This meant that only 23 of the city's 59 monitoring stations were reporting data at that time.
The number remained high at 2 am, when 35 stations had missing data, before gradually declining. At 3 am, 30 stations were not reporting readings, while the number fell to 20 by 4 am, 19 by 5 am and 16 by 6 am.
The situation gradually improved through the morning, with 13 stations reporting missing data at 7 am and seven at 8 am. By 9 am, only two stations were not reporting readings, while one continued to have missing data until 11 am.
Delhi's average AQI fell to 344 by 4 pm on Monday, compared with 376 at the same time on Diwali day. However, only 47 monitoring stations were recording data at 4 pm, of which 40 were in the very poor category and seven in the poor category. Bawana recorded the highest AQI at 396, followed by Burari at 393.
The Opposition criticised the authorities, with senior Aam Aadmi Party (AAP) leader Saurabh Bharadwaj accusing the Delhi government of "data fraud" and alleging that it was manipulating air quality readings to understate pollution levels, putting people's lives at risk.
The concerns came amid wide variations in air quality readings reported by different agencies on Monday, a day after Diwali.
While official data put Delhi's AQI at 370 at 8 am on Monday, private monitoring platforms reported considerably different figures.
Swiss air quality monitoring platform IQAir recorded an AQI of 245 on the US scale in the 'very unhealthy' category at 8.30 am, with PM2.5 concentrations at 170 micrograms per cubic metre (µg/m³), around 34 times the World Health Organisation's annual guideline value.
Meanwhile, AQI.in recorded an air quality reading of 637 in Delhi on Monday morning, classifying it as "hazardous". The platform also reported that the city's AQI had peaked at 2,283 after Diwali.
The differences in readings brought the monitoring network, data availability and variations in measurement parameters and averaging methods used by government and private agencies into focus, although this was not the first such controversy.
A similar row erupted after Diwali last year, when the Aam Aadmi Party accused the BJP-led Delhi government of deliberately shutting down air quality monitoring stations to conceal rising pollution levels.
The AAP had alleged that dozens of monitoring stations operated by the Central Pollution Control Board (CPCB), Delhi Pollution Control Committee (DPCC), India Meteorological Department (IMD) and Indian Institute of Tropical Meteorology (IITM) had gone offline as pollution levels rose on Diwali night and resumed functioning the following morning. Experts tracking the data in real time corroborated the allegations.
Commenting on Sunday's data gap, Radhey Shyam Sharma, head of the Department of Environmental Studies at Delhi University, said transparency in air quality figures was essential, particularly during an emergency when people needed reliable information about their exposure to pollution.
"Transparency in data should be there. There is also the question of accountability. We are talking about an emergency situation, a severe emergency," he said.
Sharma said the missing readings could create confusion and panic, stressing that the absence of data was not a solution to the Capital's annual air pollution crisis. Reliable information was important for policy formulation and assessing the severity of the situation, he added.
He also pointed to differences in how government and private agencies report air quality, including variations in averaging methods.
"Private agencies do not necessarily average the readings in the same manner. The official data should clearly specify the location and time of the readings. The exposure levels need to be clear so that there is a better picture of the situation," he said, adding that average AQI readings were bound to differ from peak pollution levels, when exposure to the associated health risks was at its highest.
Sharma said the monitoring system and its management needed to be improved rather than allowing data to go missing, adding that the focus should remain on people exposed to pollution and the accountability of those managing the network.
According to official data, the national capital recorded its most polluted Diwali in five years, with an average AQI of 376. The morning after was only marginally better, with an AQI of 371 recorded at 10 am on Monday, as a blanket of smog and fog covered the city.
The post-Diwali morning reading of 371 was also the worst since 2021.
The data gap followed an earlier controversy over the use of large water sprinklers near monitoring stations, purportedly to bring down pollution readings.
"They are constantly sprinkling water near the Jahangirpuri AQI station so that the average comes down. Similarly, the government switched off AQI monitoring stations on Diwali night and committed data fraud. How do you trust such a government?" Bharadwaj said.
Comparing AQI monitoring stations to blood sugar monitoring devices, he said, "If anyone has diabetes or a blood pressure problem, they check that every day because their medication depends on the levels.
"People check their diabetes levels regularly because what they eat and when they take medicines depends on it. If their machine is tampered with and shows normal levels, they will not take their medicines; they will eat sweets and die. This is what the government is doing right now."
He said the government had installed 20 new AQI monitoring stations in open and green areas with less traffic and lower population density to bring down the overall average and understate the extent of the crisis.
(With inputs from PTI)