Bangladesh's capital Dhaka ranked fifth yesterday in the AirVisual Index of the world's cities with the worst air quality. In the real-time air quality ranking, Dhaka scored 164 yesterday and classified as "unhealthy", according to AirVisual.

On Tuesday, Dhaka remained on top of the air quality index for some time scoring 361 and was also classified as 'hazardous'. The United States Environmental experts develop the Air Quality Index (AQI) to rank air quality. The AQI is divided into six categories, indicating increasing levels harmful to one's health.

India's Mumbai topped the index with a score of 176 followed by Karachi of Pakistan and Kathmandu of Nepal. Bangladesh, one of the most densely-populated countries in the world, has been struggling with air pollution for a long time and Dhaka continuously ranks among the world's most polluted cities. Many environmental experts blamed the pollution for ongoing construction of Metro Rail. They also noted brick kilns, vehicles run on fuel containing higher levels sulphur, as well as construction work, have all been identified as major sources of air pollution.

On February 9, green activists, including Poribesh Bachao Andolan (Poba) and Nagorik Odhikar Sangrakkhan Forum, voiced deep concern that around 90 percent dwellers in Dhaka city are affected by serious dust pollution and urged the authorities concerned to take effective steps to check it. They also said the intensity of dust pollution rises usually during winter due to road digging and delayed disposal of wastes, causing the spread of various respiratory, allergy and skin diseases.

Following it, the High Court has ordered the Director General (DG) of Department of Environment (DoE) to take steps to conduct mobile court drives twice a week against those responsible for air pollution in the city.

However, a new study that uses data from the Global Burden of Disease Project has found that ambient air pollution shortens an average Bangladeshi's life by 1.87 years.

The study also suggested that that better air quality could lead to a significant extension of human lifespan around the world.

Currently, the World Health Organization (WHO) estimates that, worldwide, seven million people die every year from exposure to such pollution with most deaths occurring in low and middle-income countries, chiefly in Africa and Asia.

An AQI value over 300 represents hazardous air quality, and below 50 the air quality is good.

The index is based on the five criteria of pollutants regulated under the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide and nitrogen dioxide.

The AQI debuted in 1968, when the National Air Pollution Control Administration undertook an initiative to develop an air quality index, and to apply the methodology to Metropolitan Statistical Areas.

Worldwide, ambient particulate matter ranks as the sixth leading risk factor for premature death, according to the 2018 'State of Global Air' report.

These risks are acute in Dhaka, said the report. It also suggests partial pollution like PM2.5 microscopically slam at 2.5 micrometers in diameter or less and a byproduct of combustion which is inhaled by residents.