Dhaka, the densely populated capital of Bangladesh, ranked 21th among the world's most polluted cities on Tuesday morning, with an Air Quality Index (AQI) of 93 at 10:20 am.

The air was classified as 'Moderate', meaning it could pose health risks to people who are more vulnerable, according to the AQI scale.

India's Delhi, Pakistan's Lahore and Thailand's Chiang Mai were the three most polluted cities, with AQI scores of 199, 191 and 176.

An AQI between 101 and 150 is considered 'unhealthy for sensitive groups', while 151-200 is 'unhealthy', 201-300 is 'very unhealthy', and anything above 301 is 'hazardous' and can cause serious health problems.

The AQI measures daily air quality, showing how clean or polluted the air is and what health effects people might experience.

In Bangladesh, the AQI is based on five main pollutants: particulate matter (PM10 and PM2.5), nitrogen dioxide, carbon monoxide, sulphur dioxide and ozone.

Dhaka has long faced serious air pollution. The situation usually gets worse in winter and improves during the monsoon season.

The World Health Organization says air pollution causes around seven million deaths worldwide each year, mainly due to stroke, heart disease, chronic respiratory illness, lung cancer and acute infections.