The air quality in Delhi worsened on Friday just three days after being recorded as ‘satisfactory’. At 4 pm on Friday, the air quality index was 197, which falls under the ‘moderate’ category. By late evening on Friday though, it crossed 200, into the ‘poor’ category, reported The Times of India.

According to the System of Air Quality and Weather Forecasting And Research, levels of pollution in the Capital are likely to rise over the next three days.

The air quality in Delhi is the best during the monsoon when rains settle dust and strong winds prevent accumulation. Wind patterns have changed over the past 48 hours, Hindustan Times reported weather experts as saying.

“This happens during this time of the year when the weather transitions from monsoon to winter,” said former chief of Central Pollution Control Board’s air quality laboratory D Saha. “The wind becomes calmer and moisture declines. Loose soil on the surface is getting suspended in the air but is not getting dispersed. As a result local pollution is building up.”

On Friday, the primary pollutant was PM 10. At 8 pm on Tuesday, it was 45 micrograms per cubic metre, while at 8 pm on Friday, it was at 222 micrograms per cubic metre against the safe standard of 100, according to the Central Pollution Control Board.

PM, or particulate matter, is a fine mixture of solids and liquid droplets in the air, and PM 10 is a category of pollutants that are less than 10 micrometres in diameter. PM 2.5 pollutants are small enough to lodge themselves into human lungs.

PM 2.5 increased from 21.2 micrograms per cubic metre at 8 pm on Tuesday to 97.6 micrograms per cubic metre during the same time on Friday. A reading of up to 60 micrograms per cubic metre is considered satisfactory.