Delhi breathes its worst air at night as October grows dirtier

Delhi's fine-particle pollution has risen about a fifth since early October, and CPCB data shows its dirtiest hours come around midnight.

At 1 am on October 21, as most of Delhi slept, 45 of the city's 53 reporting air monitors recorded air quality that was Poor or worse in Central Pollution Control Board terms.

The city's average that hour was 191.2 micrograms of fine particles (PM2.5) in every cubic metre of air. That is Very Poor on the CPCB's scale, and it was the dirtiest hour of the week that began on October 14. India's limit for PM2.5, under the national air quality standards, is 60 micrograms averaged over a whole day.

Delhi's air is usually reported as a single number per day. Going just by that, this October has been calmer than last year. The CPCB's daily air quality index put Delhi at 179 on October 20 (Moderate). It averaged 154.6 between October 1 and 20 this year, against 178.7 over the same days in 2025.

Hour by hour, the readings are harsher. In the week to 1 am on October 21, nearly a quarter of all readings at Delhi's 59 monitors were Poor or worse for fine particles, according to India Today's analysis. A daily figure cannot show the worst hours, which fall when most people are asleep.

One week, hour by hour: The chart below shows the week, monitor by monitor. Two in three of the week's Very Poor or Severe readings came between 9 pm and 6 am. The pale band on October 15 is a windy afternoon — at 2 pm, with wind at 2.3 metres per second, the city's average fell to 9 micrograms, the cleanest hour that week.

The air is getting dirtier: On an average day in the first week of October, each cubic metre of Delhi's air held 59.3 micrograms of fine particles. In the seven days to October 20, it held 71.2, about 20 per cent more.

Nights are the worst: Since October 1, the air has been dirtiest around midnight, at 80.4 micrograms on average, and cleanest at 3 pm, at 44.4. From midnight to 6 am, the air held 1.6 times as many fine particles as from noon to 6 pm.

The hours line up with the wind. Wind across the city's monitors averaged 0.6 metres a second around midnight and 1 metre a second at 3 pm. In this data, dirtier hours go with weaker winds, but that alone does not show the wind causes them.

The fires have barely begun: Farmers in Punjab, Haryana, and Uttar Pradesh burn rice stubble each autumn, and the Indian Agricultural Research Institute counts the fires from satellites. It counted 506 such fires in the three states from October 1 to 20 this year: 345 in Punjab, 155 in Uttar Pradesh, and six in Haryana. Over the same days between 2018 and 2024, the count ranged from 2,031 to 9,722. Last year, it was 817.

The air turned dirtier this month while the three states still had few crop fires. Most of the burning comes later. Between 2018 and 2024, 83.6–94.5 per cent of a season's fires in these three states came after October 20.

Many sources: Smoke from the fields is only part of the mix. A study by The Energy and Resources Institute and the Automotive Research Association of India traced Delhi's winter PM2.5 in 2016–17 to vehicles (23 per cent), crop and household burning (22 per cent), road and construction dust (15 per cent), and industry (10 per cent), with 26 per cent formed in the air from other gases.

Place matters too. In October and November 2021, Delhi's own sources made up 34.4 per cent of its PM2.5 and the rest of the National Capital Region 31 per cent, while farm fires contributed 7.3 per cent on average, a model run by the Indian Institute of Tropical Meteorology found.

These readings show when and where the air was dirty; they cannot show why. The CPCB sets categories for a day's average; this story colours single hours with them, so an hour in the Poor band is not a Poor day. The monitors also miss areas without one — and the air indoors.

The official forecast is for worse air. The Air Quality Early Warning System run by IITM in Pune, in its October 20 bulletin, expected Delhi's air to be Poor from October 21 to 23 and to stay Poor for the six days after.

Beyond Delhi, the animation below shows how fine particles are forecast to move across India in the coming days, according to the European Union's Copernicus Atmosphere Monitoring Service.

Most of the season's crop fires are still ahead. If the past seven years are a guide, at least four of every five of this season's fires in the three states have yet to be lit.

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