Although the monsoon season of 2026 witnessed a lot of rainfall in Delhi, it was unable to keep the air of the city clean. In a recent study conducted by the Centre for Science and Environment (CSE), it was found that there was only one day when the air quality was ‘Good’ in Delhi from June 1 to August 31, 2026.
The study was conducted keeping in view the weather and air quality of the monsoon season, which comprised 92 days. During this period, Delhi witnessed 576.9 mm of rainfall according to the data available for Safdarjung and Palam.
PM2.5 Fell During Rain But Returned Quickly
According to the findings from the study conducted by CSE, rainfall had a temporary effect on PM2.5, which is among the most important pollutants of the city’s air.
The average concentration of PM2.5 during rainfall is 32.7 micrograms per cubic meter. However, the effects are only temporary.
After 24 hours, PM2.5 concentration becomes 36.2 micrograms per cubic meter. In 48 hours, the concentration increases to 45.1 micrograms per cubic meter.
Consequently, there is an increase in PM2.5 by about 37.9% in 48 hours after rainfall.
Another comparison was made between pollution levels before, during and after six rainfall cases in the monsoon season. Pre-rain PM2.5 concentration was 40.6 micrograms per cubic meter, while during rainfall it was 32.7 micrograms, a decrease of about 19.5%.
However, after rainfall, the process of accumulation of pollutants starts again. At 48 hours, PM2.5 concentration becomes 45.1 micrograms per cubic meter, 11.1% higher than before rainfall.
Monsoon Air Was Better Than April-May, But Not Clean
Delhi’s overall air quality did improve during the monsoon compared with the hotter months immediately before it.
According to the CSE analysis, Delhi’s average PM2.5 concentration during April and May was 60.2 micrograms per cubic metre. During the June-August monsoon period, it dropped to 40.2 micrograms.
That represents a reduction of approximately 33.2%.
However, lower pollution levels did not mean consistently clean air.
Out of the 92 monsoon days:
- 1 day had ‘Good’ air quality
- 35 days had ‘Satisfactory’ air quality
- 53 days had ‘Moderate’ air quality
- 3 days had ‘Poor’ air quality
The only ‘Good’ air day was July 9, when the average rainfall recorded at Safdarjung and Palam was 67.75 mm.
Interestingly, Delhi’s highest single-day rainfall during the monsoon was recorded on August 8, at 101.05 mm. Even then, the city’s air quality was classified as only ‘Satisfactory’.
What Does ‘Moderate’ Air Quality Mean?
The CSE study points out that ‘Moderate’ does not mean that the air is completely clean.
Under the classification discussed in the study, 24-hour average PM2.5 levels in this category can range between 61 and 90 micrograms per cubic metre.
This is above India’s national PM2.5 standard of 60 micrograms per cubic metre.
For comparison, during April-May, Delhi had 38 Moderate days, 17 Poor days and six Satisfactory days. There was not a single ‘Good’ air day during those 61 days.
Rain Cannot Remove All Types Of Pollution
Rain is effective at bringing down particulate pollutants such as PM2.5 and PM10 by removing some particles from the air.
However, gaseous pollutants are a different problem.
The CSE study found that ground-level ozone, nitrogen dioxide and carbon monoxide remained important pollutants during several monsoon days.
Ground-level ozone was the leading pollutant on 20 monsoon days. It had also been a major pollutant on 49% of the days during the summer period analysed by CSE.
Nitrogen dioxide emerged as the leading pollutant on 10 monsoon days, while carbon monoxide was the major pollutant on 7% of monsoon days.
Vehicles And Industry Can Keep Pollution In The Air
According to the CSE analysis, emissions from vehicles, diesel generators and industries can continue contributing gaseous pollutants even when it rains.
When sunlight returns after cloudy or rainy conditions, chemical reactions involving these pollutants can contribute to the formation of ground-level ozone. This explains why heavy rain does not automatically translate into consistently clean air in Delhi.
The study therefore highlights that controlling only particulate pollution is not enough. Different sources and types of pollution need to be addressed throughout the year.
Pollution Could Rise Again Before Winter
The temporary improvement brought by the monsoon is expected to weaken as weather conditions change. As winter approaches, lower wind speeds and changes in the height of the atmospheric mixing layer can make it harder for pollutants to disperse.
CSE has therefore argued that Delhi-NCR cannot depend only on emergency measures during the winter pollution season.
The study calls for pollution-control measures throughout the year, particularly targeting emissions from vehicles, industries, construction activity, waste burning and diesel generators.