Dhaka ranked second in the list of world cities with most poor air quality on Tuesday morning.

The metropolis’s air quality index (AQI) score was 221 at 9:30 am.

An AQI between 201 and 300 is said to be ‘very unhealthy’, while a reading of 301 to 400 is considered 'hazardous', posing serious health risks to residents.

India’s Delhi and Poland’s Krakow occupied the first and third spots in the list, with AQI scores of 244 and 211, respectively.

An AQI between 101 and 200 is considered 'unhealthy', particularly for sensitive groups while a reading of 201 and 300 is said to be 'poor'.

In Bangladesh, the AQI is based on five criteria pollutants -- Particulate Matter (PM10 and PM2.5), NO2, CO, SO2 and Ozone.

Read more: Dhaka keeps grapping with ‘very unhealthy' air

Dhaka has long been grappling with air pollution issues. Its air quality usually turns unhealthy in winter and improves during the monsoon.

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 a heart disease, chronic respiratory diseases, lung infections and cancer, according to several studies.

Read more: Dhaka's air 2nd most polluted in the world this morning

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.