According to the World Health Organisation's guidelines, 25 is the maximum level of PM2.5 that a person can be safely exposed to in a 24-hour period.
The Central Pollution Control Board or CPCB has 17 monitoring stations in Delhi. Most of them reported Delhi air quality as "poor" today -- though PM2.5 level has gone down, it is still not under the safe limit.
The CPCB also put air quality in Noida, Ghaziabad and Gurgaon in the "poor" bracket.
Air quality has improved due to overnight rain, wind from the northwest and cyclonic formation in Rajasthan, said Mahesh Palwat, director of private weather agency Skymet.
"The western disturbance has moved away from hills and hill states that have seen rainfall and snowfall. So winds from northwest with speed of up to 15 kilometres per hour are bringing pollution level down. The situation will be better day-by-day," Mr Palawat told news agency IANS.
He said rains always have a washout effect on particulate matter.
According to SAFAR, an app made by the Ministry of Earth Science, air quality in Delhi improved to "moderate" today morning. The System of Air Quality and Weather Forecasting and Research records 24-hour rolling averages of PM10 and PM2.5 and uses this data to estimate the air quality index or AQI.
Thirty times smaller than the width of a human hair, the size of the PM2.5 pollutant makes it dangerous for the human body. The other pollutant, PM10, may get trapped in the throat, but PM2.5 is fine enough to pass these natural barriers and enter the lungs and the bloodstream, doctors say.
An official of the India Meteorological Department said Delhi recorded 1 millimetre rainfall till 8:30 am on Saturday.
(With inputs from IANS)