Delhi smog overwhelms pollution monitors, regulator says

Severe Diwali smog in New Delhi overwhelmed some air pollution monitors after PM2.5 concentrations exceeded their calibrated measurement range, India's Central Pollution Control Board said.

The board said particulate matter analysers used for the National Air Quality Index are calibrated to measure concentrations of up to 1,000 micrograms per cubic metre. The World Health Organization 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 board said automated validation treats hourly readings above the calibrated range as outside the range and excludes them from the index calculation. It said such readings can appear as a data gap during extreme episodes such as Diwali firecracker bursting.

Opposition politicians accused authorities of deliberately allowing official readings to go missing for several hours overnight from Sunday into Monday as pollution rose during Diwali fireworks. The Central Pollution Control Board rejected accusations of deliberate under-reporting, saying the gaps reflected measurement limits rather than an attempt to hide pollution.

Monitoring organisation IQAir said readings reached 879 micrograms per cubic metre in the Mandir Marg area after monitors resumed reporting, citing data from a city government station. That level is more than 58 times the WHO daily guideline.

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