Delhi's air quality remained 'very poor' on Monday due to slow wind speed and high humidity, authorities said.According to the Centre-run System of Air Quality and Weather Forecasting (SAFAR), the overall air quality index (AQI) was recorded at 326, which comes under 'very poor' category.The Central Pollution Control Board ( CPCB ) data said 20 areas in Delhi recorded 'very poor' air quality and 13 recorded 'poor' air quality.On Monday, the PM2.5 - particles in the air with a diameter of less than 2.5 micrometres - level was recorded at 174, while the PM10 level was recorded at 320, the CPCB data said.The air quality in Ghaziabad, Faridabad, Noida and Greater Noida was 'poor' while Gurgaon recorded 'moderate' air quality, it said.An AQI between 0 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'.Air quality is very poor and expected to increase gradually in next two two to three days but well within 'very poor' limit, the SAFAR said in a report."At present, winds are unfavourable for dispersion but it may slow down any time with fall in temperature which is expected. Humidity is still high which is unfavourable. Fire counts from stubble burning declined and will have marginal impact," it said.