New Delhi, Jan 1: Air quality in Delhi and its suburbs was severely toxic on the New Year day with pollution rising to "severe-plus or emergency" levels. The Air Quality Index (AQI) of Delhi was 400, considered severe, on Monday which turned out to be worst air quality on New Year day in the past two years with January 1, 2017 recording 344, and 2016 measuring 391 on a scale of 0 to 500. According to the Central Pollution Control Board (CPCB), with lower mixing height where effluents mix with air, slow wind speed and fog in the region will add up to the pollution levels. "The mixing height at present is around 175 meters, which is very low, and the wind speed was also less, about 0.9 mps. The humidity has gone up and effluents had mixed well in the air leading to increase the pollution levels," an official at CPCB said. The air quality drastically fell to "severe-plus or emergency" category with the average concentration of major pollutant PM2.5 or particles in air with diameter less than 2.5 micrometers, was found beyond 312 units at 6 p.m. at around 23 regions in NCR. With PM2.5 levels beyond 21 times the international standards, Vasundhara in Ghaziabad (PM2.5 544 units) and Anand Vihar (PM2.5 529 units) in east Delhi were the most polluted regions across NCR. Meanwhile, the AQI was 411 at Faridabad, 482 in Ghaziabad, 411 at Bhiwadi, 483 at Noida -- all considered "severe", while at Gurugram it was 378 which is "very poor". According to System of Air Quality and Weather Forecasting And Research (SAFAR), the air quality is set to deteriorate further on Tuesday. However, it will get better after three days.