The experts have advised citizens to wear masks to avoid hazardous air pollution in Karachi as air quality in city deteriorates with dropping mercury. Minimum temperature in the port city has dropped to 15.5 Celsius with blowing northern winds as the Met Office has predicted the temperature will further drop in the next few days. Karachi was ranked 9th most polluted city in the world on the Air Quality Index (AQI) on Monday morning. The metropolis was measured to have 156 reading of particulate matter on the air quality index. Earlier, health experts described Karachi’s air quality as “very unhealthy” and advised citizens to restrict their outdoor activities especially during peak pollution hours. It is to be mentioned here that AQI as high as 151-200 is considered unhealthy, while an AQI reading between 201 to 300 is more harmful and AQI reading over 300 mark is extremely hazardous. According to experts, the air becomes heavier in the winter, 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 an area. The smoke produced by factories or by burning coal, garbage, oil or tyres, enters the atmosphere and its impact appears at the onset of the winter and remains till the end of the season, experts said. Although southwestern winds blowing from the sea in Karachi, work as a filter for the air, these winds remain mostly suspended during the winter, experts said.