Delhi’s air quality has rapidly worsened to “severe” on the air quality index (AQI) on Thursday morning, as residents woke up to an AQI of 452.

AQI between 401 and 500 is considered to be severe, which can impact healthy people and seriously affect those with existing health conditions, according to the Central Pollution Control Board (CPCB).

The AQI was in the “very poor” – between 301 and 400 — category till Wednesday afternoon, but worsened rapidly to “severe” due to a shift in meteorological conditions.

Wind speed gradually reduced and became calm, which left no room for dispersal of pollutants at night.

Also read | Change in wind pattern covers Delhi in a thick blanket of smoke

India Meteorological Department (IMD) scientists said a layer of smoke and pollution had settled closer to the surface on Wednesday, which blocked sunlight and reduced visibility through the day. The weather condition led to a dip in the maximum temperature to 28 degrees Celsius (C) — 2 degrees C below normal – on Wednesday.

“Smog-like conditions have developed because of air pollution, low wind speed and subsidence of cold air over this region. Air is subsiding over the northern plains, particularly Delhi-national capital region (NCR). As a result, dispersal of pollutants is not possible,” said Vijay Soni, scientist, air quality division, IMD.

The Air Quality Early Warning System under the Union Ministry of Earth Sciences (MoES) had said air quality has started worsening significantly after 10:00 am on Wednesday due to strong subsidence and accumulation of pollutants.

The AQI may reach the higher end of the very poor to severe category before improvement starts on Thursday, it has forecast.

“We are expecting wind speed to be low for the next couple of days. I don’t see these conditions improving immediately,” said Kuldeep Shrivastava, head, regional weather forecasting centre.

The minimum temperature on Thursday increased to 12.1 degrees C, as compared to 10.6 degrees C on the previous day.

“There is shallow fog on Thursday. The minimum temperature has increased because of the layer of pollution, which doesn’t allow the surface heat to radiate back at night,” Shrivastava added.

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