Delhi's air quality worsened further and slipped to 'severe' category Monday as heavy stagnant air prevented dispersion of pollutants, according to authorities. The Central Pollution Control Board or CPCB recorded an overall air quality index (AQI) of 403, which falls in 'severe' category. Neighbouring Ghaziabad, Noida and Faridabad also recorded 'severe' air quality. Noida's air quality was worst at a AQI of 452, CPCB data showed. An AQI between 201 and 300 is considered 'poor', 301 and 400 'very poor' and 401 and 500 'severe'. Nineteen areas in Delhi recorded 'severe' air quality and ten areas recorded 'very poor' air quality, according to the CPCB. Under the 'severe' category, even healthy people find it harder to breathe and doctors advise physical activity be kept at a minimum. Authorities said they are closely monitoring the situation. The overall PM2.5 level - fine particulate matter in the air with a diameter of less than 2.5 micrometre - was recorded at 257 and the PM10 level at 445, it said. The Centre-run System of Air Quality and Weather Forecasting (SAFAR) said the overall air quality of Delhi will further deteriorate over the next two days. "Values are likely to remain in same range until tomorrow which is likely to decline by Wednesday. Calm surface winds are not allowing pollutants to disperse," it said. "Winds are calm and dispersion is low. Western disturbances influence may impact Delhi's air quality by introducing the moisture and making air heavy. The expected fall in temperature and moderate fog is likely to increase pollution over coming two days is expected to the upper level of very poor," the SAFAR said. The SAFAR said that the air quality might improve Wednesday if sufficient amount of rainfall occur, which is expected. "However, marginal shower often deteriorates air quality because high moisture content overshadows the wash-out effect. In all probability, the AQI will remain within the limit of very poor and will not touch 'severe'," it said. "Levels of gaseous pollutants, NOx and CO are forecast to be enhanced - up to moderate range - after a long time due to a fall in boundary layer height and reduced vertical mixing," the SAFAR said.