Dhaka's air quality slipped back to the 'unhealthy' zone on Friday morning.

With an air quality index (AQI) score of 187 at 8am, the metropolis ranked second in the list of world cities with the worst air quality.

An AQI between 101 and 200 is considered 'unhealthy', particularly for sensitive groups.

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Pakistan’s Lahore and China’s Beijing occupied the first and third spots in the list, with an AQI score of 195 and 182, respectively.

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.

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 in winter and improves during the monsoon.

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Air pollution consistently ranks among the top risk factors for death and disability worldwide. Breathing polluted air has long been recognized as increasing a person’s chances of developing a 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.