LAHORE: Punjab has been significantly affected by smog, with Lahore topping the list of cities with the worst air quality. New Delhi remained the world’s second most polluted city, while Lahore remained the world’s most polluted city. Ulan Bator, Mongolia, remained third on the AQI, Kolkata fourth, and Dhaka fifth in terms of air pollution. Bangladesh’s Dhaka remained fifth most polluted city, Nepal’s Kathmandu 7th and Karachi 8th on the Index with worst air quality. According to the reports of international media and a study of the United States National Aeronautics and Space Administration (NASA), burning of crop residue and garbage in northern Indian states causes dangerous hike in environmental pollution in the region. AQI as high as 151-200 is considered unhealthy, while an AQI rating between 201 to 300 is more harmful and AQI over 300 is dubbed extremely hazardous. According to experts, hike in air pollution used to be recorded in the winter. A change in the wind speed, wind direction and sliding minimum temperature increases air pollution. The air becomes heavier in the winter as compared to summer, causing poisonous particles in the atmosphere to move downwards and making the atmosphere polluted. As a result, a layer of polluted particles, including large amounts of carbon and smoke, covers a city. The smoke produced by burning crop remnants, factories and burning coal, garbage, oil or tyres enters the atmosphere and the effects of this appear at the onset of winter and remain till the season’s end, experts said. As a result, air pollution has reached exceedingly dangerous levels, putting the quality of the air at risk. Ground-level ozone, particulate matter, carbon monoxide, sulphur dioxide, and nitrogen dioxide are the five types of pollution used to calculate the AQI.