After a brief respite, air quality in the national capital deteriorated again on Tuesday due to calm winds and increased impact of farm fires, official said.

According to the Central Pollution Control Board (CPCB), the city's overall air quality index (AQI) read 242 at 4 pm on Tuesday, up from 214 at the corresponding time the previous day.

With a AQI of 330, Ghaziabad's air quality dropped to "very poor" category and was the most-polluted city in the country on Tuesday, the CPCB data revealed. Greater Noida (272) and Noida (256) recorded their AQI in the "poor" category.

An AQI between 201 and 300 is considered "poor", 301-400 "very poor" and 401-500 "severe".
Kuldeep Srivastava, a senior scientist at the India Meteorological Department (IMD), said the maximum wind speed dropped from 18 kilometers per hour on Monday to 10 kmph on Tuesday.

Low wind speed is leading to accumulation of pollutants. The speed will reduce further to 6-8 kmph on Wednesday. The temperature is also expected to dip by a few notches starting Wednesday which will affect air quality. There will be shallow to moderate fog in the morning.

The Ministry of Earth Sciences' air quality monitor SAFAR said as stubble transport-level winds are favourable, stubble burning is expected to account for 14 per cent of Delhi's pollution on Wednesday. It was 13 per cent on Tuesday.

Calm winds and low ventilation index are predicted for the next two days and the condition is conducive for the sudden accumulation of pollutants in the Delhi region, the System of Air Quality and Weather Forecasting And Research (SAFAR) said.

The air quality is likely to deteriorate to the higher end of the "very poor" category by early Wednesday and to the lower end of the "severe" category by November 21, it warned, adding from November 23, a fresh western disturbance is expected to increase the wind speed and improve ventilation.

The Supreme Court-mandated anti-pollution authority, EPCA, asked Delhi and NCR states on Monday to step up surveillance and enforcement to check local sources of pollution and crop burning in view of the predicted dip in air quality.

It had warned that the situation will worsen if there is light rain, which would be inadequate to wash away the pollutants and add to the moisture instead. EPCA had said hot mix plants, ready mix plants, and stone crushers will continue to remain closed in Delhi-NCR.

The Supreme Court on November 4 banned construction and demolition activities in the region till further notice.