Delhi’s September air quality was moderate for 18 days straight — longest streak since 2016. Here’s why

An analysis of Central Pollution Control Board (CPCB) records underscores the growing role of meteorological conditions in governing the Capital’s pollution levels.

What does the data indicate?

For 18 straight days this month, from September 12 to 29, the city’s air quality index (AQI) stayed above 100 without respite. Day after day, residents breathed what CPCB classifies as “moderate” air (AQI 101-200) — a level that can cause discomfort for children, the elderly and those with asthma or heart trouble.

It was Delhi’s longest such run in September since 2016, when 29 of 30 days crossed the 100 mark.

This does not mean September 2025 was more polluted overall than other years. CPCB data shows the monthly mean AQI stood at 104.7, which was almost identical to 2024 (105.4) and 2023 (107.8), and only slightly higher than 2022 (104.4).

What set this year apart was the lack of variation: the city’s air hovered in the moderate band for nearly three weeks straight.

What caused the streak of moderate AQI days?

The streak had less to do with sudden pollution spikes and more with shifts in weather patterns.

For most of the monsoon, showers wash out dust and emissions. By the third week of September, rainfall weakens, winds shift, and skies clear, allowing pollutants to linger.

This year, the change came even earlier: the India Meteorological Department (IMD) declared the withdrawal of the monsoon on September 24, marking the earliest retreat since 2002. That left the city exposed to dry days when even routine emissions were enough to push AQI readings above 100.

However, CPCB records also reveal that the third half of September is consistently more polluted than the first. On average, since 2016, about two-thirds of days between September 21 and 30 have breached the 100 AQI mark, compared with about half in the first 10 days.

This seasonality explains why the longest September streaks of moderate pollution occur late in the month, even when the overall averages do not look severe.

The weather compounded the effect this year. As the monsoon ebbed, Delhi saw soaring temperatures. The maximum temperature rose to 38.1°C on September 28, making it the hottest September day in two years, followed by 37.5°C the next day.

Clear skies and solar exposure fuelled photochemical reactions that turned nitrogen oxides from traffic emissions into ground-level ozone, which CPCB bulletins have flagged as the most significant pollutant so far this month.

What can residents expect?

In all, 19 days this September have crossed the 100 AQI mark, fewer than the 29 logged in 2016 or the 22 in 2020. But the absence of a single break in the second half of the month set this year apart.

For residents, this has meant the cumulative effect of continuous moderate air.

With IMD forecasts pointing to drizzle and cloudy skies in the coming days, southeasterly winds and fresh showers are likely to wash out pollutants and bring much-needed relief after nearly three weeks of stalled air.

As per forecasts, Delhi could see satisfactory air quality (AQI 51-100) in the coming days.

The Indian Institute of Tropical Meteorology (IITM), in its air quality bulletin on Tuesday, said, “The air quality is likely to be in the ‘Satisfactory’ category from 01.10.2025 to 02.10.2025. The air quality is likely to be in the ‘Moderate’ category on 03.10.2025. The outlook for the subsequent 6 Days: The air quality is likely to be in the Moderate to Satisfactory category.”

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