Dhaka's air quality slipped into the 'unhealthy' zone on Sunday morning.

With an air quality index (AQI) score of 165 at 9 am, 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.

On the other hand, an AQI between 50 and 100 is considered ‘moderate’ with an acceptable air quality. However, there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.

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.

With the advent of winter, the city’s air quality starts deteriorating sharply due to the massive discharge of pollutant particles from construction works, rundown roads, brick kilns and other sources.

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.