To celebrate the World Environment Day on 5 June, a day designated by the UN to raise awareness of environmental issues, IBTimes UK looks at some of the countries with the highest rates of pollution.
According to World Health Organisation (WHO) air pollution - both indoor and outdoor – is the leading cause of death worldwide.
The main causes of pollution are: uncontrolled emissions from motor vehicles, dust, industrial waste, garbage, brick kilns, cooking stoves, burning of wood, coal and bio-mass.
WHO examined the concentration of fine particulates - tiny pieces of solid or liquid matter suspended in the atmosphere - of 2.5 micrometres or less in diameter (PM2.5).
These particulates are small enough to pass into the bloodstream and cause diseases such as emphysema and cancer.
PM 2.5 is the best indicator of assessing health impacts from air pollution.
The concentration of air pollution is measured in micrograms per cubic metre of air (ug/m3).
The levels of air pollution in Delhi were measured at 163 ug/m3 - the worst air conditions in the world.
This concentration of air pollution is far greater than what is usually considered safe.
"Too many urban [centres] today are so enveloped in dirty air that their skylines are invisible," Flavia Bustreo, WHO Assistant Director-General for Family, Children and Women's Health, said.
Half of the top 20 cities in the world with the highest levels of PM2.5 are in India, according to WHO.
The Bangladeshi city with the highest air pollution rate was Narayonganj with a rate of 89 ug/m3, followed by Gazipur (87 ug/m3) and Dhaka (86 ug/m3).
An estimated 15,000 premature deaths, as well as several million cases of pulmonary, respiratory and neurological illness are attributed to poor air quality in Dhaka, according to the Air Quality Management Project (AQMP), funded by the government and the World Bank.
The Pakistani city with the highest pollution levels was Karachi, which is the fifth city with the worst level of air pollution (117 ug/m3). Peshawar (111 ug/m3) and Rawalpindi (107 ug/m3) came in at 6th and 7th respectively.
"Pakistan has one of the highest childhood death burdens in the world, and pneumonia is the main single cause of death. As a contributor to the pneumonia burden, the country has a significant indoor air pollution (IAP) problem," WHO said.
Iran city with highest level of air pollution was Khoramabad with a PM2.5 average of 102 ug/m3. It was ranked 8th in a list of cities with high air pollution.
According to city officials, some 270 people die each day from blood cancer, heart and respiratory diseases, and other pollution-related illnesses.
The air in the country's capital, Doha, was ranked 12th most polluted, with a PM2.5 of 93 ug/m3. Al Wakrah came was 25th with 85 ug/m3.
According to a 2013 research by Qatar University, the level of fine particles in the air in Qatar is almost six times above the permissible limit.