Air pollution levels in Delhi were the lowest this winter since 2018, a report by the Centre for Science and Environment has said.

The concentration of pollutants in the national capital’s air stood at 160 micrograms per cubic meter between October and January.

“The PM2.5 level, computed by averaging monitoring data from 36 Continuous Ambient Air Quality Monitoring Stations located in the city was 17% lower compared to the seasonal average of the 2018-19 winter,” the report said. “Based on the subset of the 10 oldest stations, there is an improvement of almost 20%”.

Particulate matter smaller than 2.5 microns (or about a ten-thousandth of an inch) is particularly dangerous to human health. Such particles are small enough to travel deep into the respiratory system, potentially impairing lung function.

The National Ambient Air Quality Standards require PM2.5 concentration to be less than 60 micrograms per cubic metre of air in any given 24-hour period to be considered safe.

As per the report, the number of days with severe or severe-plus air quality also remained the lowest in the last five years.

“This winter, 10 days had city-wide average in “severe” or worse AQI category, which is much lower compared to 24 such days in the previous winter and 33 in 2018-19 winter,” it said.

An air quality index between zero and 50 is considered “good”, 51 and 100 “satisfactory”, 101 and 200 “moderate”, 201 and 300 “poor”, 301 and 400 “very poor” and 401 and 500 “severe”. The AQI in the “very poor” category can lead to respiratory illness on prolonged exposure.

However, despite the decline in pollution levels, the air quality in Delhi was the worst among the areas in the National Capital Region.

“Delhi was the most polluted city in the NCR followed by Greater Noida, this winter,” it said. “But Dharuhera and Baghpat, much smaller towns, were next on this worst polluted list, placed above much larger cities of Faridabad, Gurugram and Ghaziabad.”

Centre for Science and Environment’s Executive Director-Research and Advocacy Anumita Roychowdhury said that the improvement in the air pollution level was a combined effect of meteorology and emergency action based on pollution forecasting.

“There was heavy and extended rainfall in the early phases of the season that prevented smog episodes from building up and also lowered the seasonal average,” Roychowdhury said. “This downward trend will have to be sustained with much stronger action on vehicles, industry, waste burning, construction, solid fuel and biomass burning to meet the clean air standard.”