The port city has been ranked among the world’s most polluted cities as the level of hazardous particles in air reached to 324 on Sunday morning. According to a recent reading of the Air Quality Index (AQI) this noon hazardous particulate matter in Karachi was recorded at 188. AQI as high as 151-200 is considered unhealthy, while an AQI between 201 to 300 is more harmful and an AQI over 300 is dubbed extremely hazardous. Air pollution of particulate matter in Lahore was recorded 171, Peshawar 414, Islamabad 171, New Delhi 321, Kolkata 188 and Dhaka 288, as per the AQI reading. An expert said that in winter an increase in air pollution witnessed, change in the wind speed, wind direction and sliding minimum temperature increases air pollution, he said. The smoke from factories and vehicular traffic in cities are among some factors adding to the air pollution, he added. According to experts, 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 the city. The smoke produced by 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.