Dhaka, the overcrowded capital city of Bangladesh, has ranked the sixth on the list of cities with the worst air quality with an AQI index of 153 at 9:20 am on Friday.

The air was classified as 'unhealthy', according to the air quality and pollution city ranking. Dhaka’s air on Friday morning turned into ‘unhealthy’ despite rainfall since Thursday. However, the air was in the ‘moderate’ category for the past three consecutive days.

Read: Dhaka's air quality ‘moderate’ for third consecutive day

When the AQI value for particle pollution is between 101 and 150, air quality is considered ‘unhealthy for sensitive groups’, between 150 and 200 is ‘unhealthy’, between 201 and 300 is said to be 'very unhealthy', while a reading of 301+ is considered 'hazardous', posing serious health risks to residents.

Pakistan’s Lahore, India’s Delhi and Indonesia’s Jakarta occupied the second, third and fourth spots in the list, with AQI scores of 236, 175 and 167, respectively.

The AQI, an index for reporting daily air quality, informs people how clean or polluted the air of a certain city is and what associated health effects might be a concern for them.

The AQI in Bangladesh is based on five pollutants: particulate matter (PM10 and PM2.5), NO2, CO, SO2, and ozone.

Read: Air quality 'moderate' this morning

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

As per World Health Organization (WHO), air pollution kills an estimated seven million people worldwide every year, mainly due to increased mortality from stroke, heart disease, chronic obstructive pulmonary disease, lung cancer, and acute respiratory infections.

Read more: Dhaka's air ‘unhealthy for sensitive groups’ for 3rd consecutive day