Dhaka world's 'most polluted capital city'

Dhaka once again has topped a list of cities with the worst air quality in the world.

The capital's air quality index (AQI) at 9.30 am on Wednesday read 378, considered 'hazardous'.

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

Ulaanbaatar, the capital of Mongolia, and Hanoi of Vietnam occupied the second and third places in the list of cities with the worst air quality, with AQI scores of 356 and 278, respectively.

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 overall AQI is based on five criteria pollutants – Particulate Matter (PM10 and PM2.5), NO2, CO, SO2, and Ozone (O3).

Bangladesh has a subtropical monsoon climate characterised by wide seasonal variations in rainfall, high temperatures and humidity.

Generally, Dhaka's air starts getting fresh when monsoon rain begins from mid-June. The air remains mostly acceptable from June to October.

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.

Over 80 percent living in urban areas which monitor air pollution are exposed to air quality levels that exceed WHO guideline limits, with low- and middle-income countries most at risk.