New Delhi: Delhi’s air quality slipped back in to ‘very poor’ category Friday due to accumulation of pollutants owing to low wind speed, even as authorities warned further increase in pollution level on Saturday.

The overall air quality index was recorded at 337 which falls in the “very poor" category, according to Central Pollution Control Board (CPCB) data.

Five areas in Delhi -- Anand Vihar, Wazirpur, Bawana, Jahangirpuri and Mundka -- recorded severe air quality, according to the data. While as many as 23 localities recorded ‘very poor’ air quality and six ‘poor’, the data said.

The level of PM2.5 (or particulate matters 2.5 -- particles in the air with a diameter of less than 2.5 micrometres) was recorded at 205 and the PM10 level was recorded at 379, 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’.

According to the Centre-run System of Air Quality and Weather Forecasting (SAFAR), air quality index for the national capital is in very poor category and expected to increase further by tomorrow but will remain in very poor.

“Thereafter a slight improvement is predicted. At present, wind speed is slow and hence not good for air quality as it allows pollutants to get accumulated. Humidity continues to be high and temperature is likely to cool, both unfavourable," the SAFAR said in a report.

Delhi’s air quality showed a slight improvement on Thursday due to increased wind speed, favourable for dispersion of pollutants, and settled in the poor category.

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