New Delhi: Delhi experienced one of its most polluted Diwali days in a decade, with the national capital recording its fourth-highest daily average PM2.5 concentration of the year on November 8, 2026, according to an analysis of air quality data covering the period around Diwali from 2016 to 2026.The analysis conducted by Centre for Research on Energy and Clean Air (CREA) found that Delhi’s average PM2.5 concentration stood at 257 micrograms per cubic metre (µg/m³) on Diwali, making November 8 the fourth most polluted day of 2026 so far. It was also the fourth most polluted Diwali recorded in the national capital since 2016..Diwali pollution among the worst in a decadeA comparison of daily PM2.5 concentrations on and around Diwali between 2016 and 2026 ranked Diwali 2026 as the fourth most polluted Diwali day in the period.The annual rankings show considerable variation over the years. Diwali in 2020 ranked third among the most polluted days of that year, while in 2021 the festival ranked tenth. In 2023, Diwali ranked 98th, and in 2025 it ranked 32nd. The 2026 ranking of fourth place in this year’s festival is among the worst recorded in the period.The analysis also shows that pollution levels remained elevated in the days leading to Diwali. November 5, 6 and 7 ranked sixth, seventh and eighth, respectively, among the most polluted days of 2026. This means pollution levels were already high in Delhi even before Diwali celebrations began.Monitoring gaps raise concerns over peak pollution readingsThe analysis also flagged significant gaps in the city’s air quality monitoring data during the period of peak pollution.At 1 a.m. on November 9, when pollution levels peaked amid stagnant atmospheric conditions, 36 monitoring stations were missing hourly PM2.5 readings. The absence of data from these stations raises questions about how monitoring networks captured the extent of pollution during some of the most critical hours if they were offline.Another concern is the upper limit displayed on the Central Pollution Control Board’s Continuous Ambient Air Quality Monitoring (CPCB-CCR) public portal. Siri Fort recorded a PM2.5 concentration of 1,000 µg/m³, the maximum reading displayed on the portal.The World Health Organisation (WHO) recommends that levels of PM 2.5 should be no more than 15 µg/m³ for a 24-hour period and 5 µg/m³ annually.Compared to this, India’s standards are lax.India’s National Ambient Air Quality Standards (NAAQS) prescribes a 24-hour permissible PM2.5 limit of 60µg/m³ and an annual average of 40 µg/m³.As per the analysis, the CPCB portal does not display readings above 1,000 µg/m³. So how are extremely high pollution concentrations being fully represented in publicly available data, particularly during severe pollution episodes?The findings add to questions surrounding the availability and transparency of Delhi’s air quality data, an issue also highlighted in an earlier report by The Wire, which reported concerns over the functioning and placement of air quality monitoring stations.