Air pollution still remains one of the top most challenges for Bangladesh -- its capital has been ranked the third most polluted city in the world. On Tuesday, the capital's air quality index (AQI) was recorded at 161 at 10.40am. The air was classified as ‘unhealthy’. Chile's Santiago and Pakistan’s Lahore occupied the first and second spots in the list with AQI scores of 164 and 162, respectively. Read: Global pollution kills 9 million people a year, study finds An AQI between 101 and 200 is considered 'unhealthy', particularly for sensitive groups. Similarly, an AQI between 201 and 300 is said to be 'poor', while a reading of 301 to 400 is considered 'hazardous', posing serious health risks to residents. AQI, an index for reporting daily air quality, is used by government agencies to inform people how clean or polluted the air of a certain city is, and what associated health effects might be a concern for them. In Bangladesh, the AQI is based on five criteria pollutants -- Particulate Matter (PM10 and PM2.5), NO2, CO, SO2 and Ozone. Dhaka has long been grappling with air pollution issues. Its air quality usually turns unhealthy during winter and improves during monsoon. Read: Light pollution threatens birds in cities Air pollution consistently ranks among the top risk factors for death and disability worldwide. Breathing polluted air has long been recognised as increasing a person’s chances of developing heart disease, chronic respiratory diseases, lung infections and cancer, according to several studies. As per the World Health Organization (WHO), air pollution kills an estimated seven million people worldwide every year, largely as a result of increased mortality from stroke, heart disease, chronic obstructive pulmonary disease, lung cancer and acute respiratory infections.