Delhi's air quality turns poor

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New Delhi, Jan 6 (PTI) Delhi's air quality turned 'poor' on Wednesday due to unfavourable meteorological conditions, weather department officials said.

The city's 24-hour average air quality index (AQI) was 226. It was 140 on Tuesday and 151 on Monday.

An AQI between zero 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 city's air quality remained in the 'moderate' category on Monday and Tuesday due to rain and strong winds, Kuldeep Srivastava, the head of the India Meteorological Department's regional forecasting centre, said.

However, the winds slowed down on Wednesday and the moisture in the air made the pollutants heavier, he said.

The maximum wind speed was 8 kmph on Wednesday.

The central government's Air Quality Early Warning System for Delhi said the city's ventilation index - a product of mixing depth and average wind speed – was 4,000 m2/s on Wednesday.

It is likely to be 2,500 m2/s on Thursday.

Mixing depth is the vertical height in which pollutants are suspended in the air. It reduces on cold days with calm wind speed.

A ventilation index lower than 6,000 sqm/second, with the average wind speed less than 10 kmph, is unfavourable for dispersal of pollutants. PTI GVS SNE SNE