India’s air pollution monitors were overwhelmed this week by toxic smog so severe that concentrations exceeded their upper measurement limit, equivalent to 66 times the World Health Organisation’s recommended daily maximum, regulators said.

The admission comes after opposition politicians accused authorities of deliberately allowing gaps in official readings for several hours overnight Sunday into Monday, just as pollution surged with a barrage of fireworks for Diwali, the Hindu festival of lights.

New Delhi is one of the world’s most polluted capitals, where winter smog from vehicle emissions, farm fires and industry already chokes millions.

But the missing readings meant official data failed to capture the full extent of the pollution at its worst.

“The government which shuts down air quality monitoring stations on the night of Diwali, the government that does data fraud, how will you trust such a government?” Aam Aadmi Party (AAP) Delhi president Saurabh Bharadwaj said in a video posted on X.

“If Diwali night’s PM2.5 (toxic particulates) is not in the record, who will answer for that air?” senior Congress party lawmaker Ajay Maken said in a social media post.

The Central Pollution Control Board (CPCB) rejected accusations of deliberate under-reporting, saying the gaps reflected “differences in measurement of pollutant concentration” rather than an attempt to conceal pollution.

‘Extreme episodes’

Particulate matter analysers, installed when the National Air Quality Index (NAQI) was introduced in 2015, were calibrated to measure concentrations of up to 1,000 micrograms per cubic metre.

The WHO recommends a daily maximum of 15 micrograms per cubic metre for PM2.5, fine particles small enough to penetrate deep into the lungs and linked to cancer.

“The designed system of automated data validation treats hourly readings beyond the calibrated range as outside range, and such readings are not used in the computation,” the Central Pollution Control Board (CPCB) said in a statement late Tuesday.

“During extreme episodes, which are occasional like Diwali firecracker bursting, this may appear as a data gap at the particular station concerned”, it added.

Although choking clouds filled the streets early Monday, the scale of the pollution was only captured when monitors resumed reporting, around eight hours after the fireworks peaked.

Readings then hit 879 micrograms per cubic metre in the Mandir Marg locality, according to monitoring organisation IQAir, which cited data from a city government station. That is more than 58 times the WHO daily limits.

“There is one thing the people of Delhi can be certain of: the fact that the data has gone missing doesn’t mean the pollution has disappeared too,” Congress party lawmaker Pawan Khera added.

“The poisonous smoke we have been inhaling… will continue to choke our lungs, threaten our lives and shorten our life expectancy”.