Despite heavy monsoon rains in Delhi, the air has not remained clean for long periods. According to a new study by the Centre for Science and Environment (CSE), during the period from June 1 to August 31, 2026, the air quality in Delhi was 'good' on only one out of 92 days.
During this period, the total rainfall registered in Safdarjung and Palme was 576.9 mm. Nevertheless, for 53 days, which accounts for 57.6 percent, the air quality was in the 'moderate' category.
The study showed that although PM2.5 levels decreased during the rain, they began to rise again as soon as the rain stopped. The average PM2.5 level during precipitation was 32.7 micrograms per cubic meter. However, 24 hours after the rain stopped, it reached 36.2, and after 48 hours, it reached 45.1 micrograms per cubic meter, indicating a 37.9 percent increase in PM2.5 levels over 48 hours.
CSE compared the air quality in Delhi in April-May and in June-August 2026. In April-May, the average PM2.5 level in Delhi was 60.2 micrograms per cubic meter, whereas during the monsoon season, it dropped to 40.2 micrograms, representing a decrease of approximately 33.2 percent. Analyzing six individual rain events during the monsoon season, CSE noted that the average PM2.5 level before the rain started was 40.6 micrograms. During the precipitation, it decreased to 32.7 micrograms, which is a reduction of approximately 19.5 percent. But after the rain stopped, PM2.5 started rising again: after 24 hours, it reached 36.2 micrograms, and after 48 hours, it reached 45.1 micrograms, which was 11.1 percent higher than the 40.6 micrograms recorded before the rain started.
For the entire monsoon period of 92 days, the air in Delhi was 'good' only on July 9. On this day, the average rainfall in Safdarjung and Palme was 67.75 mm. Furthermore, August 8 saw the highest daily rainfall of the monsoon season—101.05 mm—but on that day, the air quality was only 'satisfactory'.
Overall, during the monsoon season, 53 days had 'moderate' quality, 35 days had 'satisfactory' quality, and three days had 'poor' quality. Compared to April-May, where 38 out of 61 days were 'moderate', 17 were 'poor', and six were 'satisfactory', the monsoon season showed a higher proportion of 'moderate' conditions.
According to CSE, the 'moderate' category does not mean completely clean air; in this category, the 24-hour average PM2.5 can reach between 61 and 90 micrograms per cubic meter, exceeding the national standard of 60 micrograms.
Rain helps settle fine particles such as PM2.5 and PM10 present in the air. However, some gaseous pollutants do not disappear under the influence of precipitation. Therefore, during many rainy days, major air quality degrading pollutants included ozone, nitrogen dioxide, and carbon monoxide.
Ground-level ozone was the main pollutant on 20 days during the monsoon season, and also on 49 percent of days during the summer period. Nitrogen dioxide was the dominant pollutant on 10 monsoon days, compared to only 3 percent in the summer period. Carbon monoxide was the main pollutant on 7 percent of monsoon days.
CSE explains that some gases emitted by vehicles, diesel generators, and industry remain in the air even during rain. When sunlight breaks through the clouds, these gases can undergo chemical reactions, leading to the formation of ground-level ozone.
The natural help provided by weather conditions will weaken. After this, due to reduced wind speed in winter and the sinking of upper air layers, pollution may increase. CSE emphasizes that Delhi-NCR cannot rely solely on emergency measures during the winter; year-round pollution reduction efforts are necessary. To achieve this, it is critically important to reduce emissions from transport, industry, construction, waste burning, and diesel generators. Furthermore, coordinated action is required across the entire NCR.
