Delhi’s air quality index once again slipped to the ‘poor’ category on Wednesday, and is likely to deteriorate further to the ‘very poor’ category by Thursday.

The AQI on Wednesday was 232 with PM10 being the main pollutant, according to the Central Pollution Control Board (CPCB) bulletin.

So far this month, the national capital has recorded four ‘poor’ AQI days.

AQI in the 201 to 300 range is considered ‘poor’, while an index between 301 and 400 is categorised as ‘very poor’. AQI in the ‘poor’ range can cause “breathing discomfort to most people on prolonged exposure”, and ‘very poor’ AQI can lead to “respiratory illness on prolonged exposure”, according to the CPCB.

Going by the SAFAR forecasting system, the effective fire count in the northwest of Delhi is 656. The contribution of crop residue burning to PM2.5 levels in Delhi is 16%, the SAFAR forecast said on Wednesday. The wind direction is from the northwest, favouring the transport of pollutants, and local winds are calm making the dispersion of pollutants difficult.