NEW DELHI: Delhi's pollution levels shot up today with most of the monitoring stations recording the city's air quality in 'severe' category, although the 24-hour average remained 'poor' as PM 2.5 and PM 10 were nearly three times above the safe limit.The spurt has been caused by calm wind movement and increased humidity. The polluting particles, of microscopic sizes, are not getting dispersed due to the atmospheric phenomenon, an IMD official explained.Although visibility averaged above a kilometer, it will start coming down once the temperature drops to single digit which will be followed by formation of fog and subsequently smog, he said.The real time readings of Delhi Pollution Control Committee ( DPCC ) stations had PM 2.5 and PM 10 at 381 and 647 micrograms per cubic metre in Anand Vihar while the same were at 339 and 498 in Mandir Marg, the two most polluted spots of the city.The national air quality index of Central Pollution Committee Board ( CPCB ) had 'severe' readings of the Punjabi Bagh, Mandir Marg and R K Puram stations, where PM 2.5 was the most prominent pollutant.System of Air Quality and Weather Forecasting And Research ( SAFAR ) stations in Delhi University , IGI Airport, Dhirpur also had 'severe' Air Quality Index (AQI), bordering 450 in most cases. The 24-hour average of PM 2.5 and PM 10 were 197 and 330 micrograms per cubic metre respectively.'Severe' is declared when PM 2.5 and PM 10 cross 253, 421 micro gram per cubic metres respectively. 'Very poor' signifies PM 10 and PM 2.5 levels between 351 and 420, and 211 to 252 micro gram per cubic metre, respectively.Permissible levels of PM (particulate matter) 2.5 and PM 10 are 60 and 100 micrograms per cubic metre respectively and consistent exposure to anything beyond that can harm the respiratory system as the particles embed themselves deep inside the lungs.