Uniform rainfall across the city in the past two days led to the city recording its cleanest air day on Monday since June 2015, when air quality monitoring and forecasting began in Mumbai.

The air quality index (AQI) — pollutant measuring indicator — was 35 for PM2.5 — particles smaller than 2.5 micron size that can easily enter the respiratory system and cause health ailments — according to the System of Air Quality Weather Forecasting and Research (SAFAR). AQI levels fell in the ‘good’ category and the health advisory read, “Air quality is satisfactory, and air pollution poses little or no risk.”

Researchers from SAFAR said it was a unique phenomenon because there was uniform rainfall across the Mumbai Metropolitan Region and surrounding areas, which had not been witnessed through the monsoon season so far. “There has been uniform rainfall within Mumbai and its surrounding areas like Thane, Navi Mumbai and even as far as Lonavla. As a result, all pollutants have been dispersed uniformly and completely washed out, resulting in the cleanest air recorded by us over the past two years. In the past, even if there was good air quality in Mumbai, pollution in the surrounding industrial areas affected the overall air quality of the city,” said Gufran Beig, project director, SAFAR. “We can expect similar AQI levels for till Wednesday and thereafter a rise in pollution levels is expected.”

Pollution levels in Mumbai were much better than London, which recorded PM2.5 AQI levels at 77, Singapore recorded 30 and New York recorded 5. Meanwhile, Delhi’s AQI was at the borderline between ‘poor’ and ‘very poor’ levels at 300.

The city could expect similar air quality on Tuesday as SAFAR predicted an AQI of 39 for PM2.5. On Monday, the AQI level for PM10 was 37 (good); ozone (O3) was 36 (good). An AQI level between 0-100 is good, 101-200 is moderate, 201-300 is poor and 301 and above is very poor.