The air quality in Delhi was in the “very poor” category on Thursday and is expected to worsen in some regions on Friday, PTI reported.

The overall Air Quality Index in Delhi stood at 353 at 7 pm on Thursday, according to the Central Pollution Control Board. The level of PM 2.5 (fine particulate matter in the air with a diameter of less than 2.5 micrometres) was 213, which is in the “poor” category and the level of PM 10 was 400, which falls under the “very poor” category.

The Centre-run System of Air Quality and Weather Forecasting, or SAFAR, said the air quality is likely to deteriorate further by Friday. “Some areas of Delhi, particularly the south-east and the north-west part of Delhi, are likely to slip into the ‘severe’ category, but the overall AQI for Delhi city will remain in the upper side of ‘very poor’,” it said.

Ghaziabad, Faridabad and Noida also recorded “very poor” air quality, according to SAFAR.

An Air Quality Index 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”.

The national capital is expected to experience a cold wave for three days starting Thursday, the India Meteorological Department had said on Tuesday.