Visibility dropped and the quality of air worsened as a blanket of smog enveloped the National Capital Region on Thursday. At least 30 trains were delayed and 15 were cancelled because of poor visibility and other operational reasons, ANI reported.

The Central Pollution Control Board said that at 11 am the Air Quality Index was at 453, which falls under the severe category. Such air quality is likely to affect healthy people and seriously impact those with existing diseases, the agency said. The air quality index at the United States Embassy read a hazardous 442 at noon.

Weather monitoring agency Skymet Weather reported that pollution levels were likely to increase because of light winds.

“This is fog and along with local emissions it became smog,” senior Meteorological Department scientist Kuldeep Srivastava told Hindustan Times. “Calm conditions are not allowing the pollutants to get dispersed. The wind speed, which was 15-16 km/hour a couple of days back, is now 5-6 km/hour.” The condition is likely to improve in a day or two as wind speed is expected to pick up, he added.