LAHORE: Lahore once again on Thursday topped the list of the world’s most polluted cities with the worst air quality. According to the Air Quality Index (AQI), Lahore has been ranked the most polluted city with air quality ranking of 265 followed by India’s capital city New Delhi with 243 and Dhaka with 240. Poland’s Kraków city and India’s Calcutta were ranked fourth and fifth in the list with 222 and 179 AQI, respectively. AQI as high as 151-200 is considered unhealthy, while an AQI ranking between 201 to 300 is more harmful and AQI over 300 is termed extremely toxic. Experts say 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.