10. Kuwait
Pollution level: 123 ug/m3
Kuwait is one of four oil-rich Middle Eastern nations to make the list. It is also the fourth largest exporter of oil among OPEC countries, with the petroleum industry accounting for half of Kuwait's GDP.
Kuwait made headlines during the first Gulf War in 1990 when Iraqi troops set fire to its oil fields, creating massive air pollution and ground contamination. That led to a decades long environmental clean up.
Today, pollution is largely caused by local oil refineries and industrial plants. Last year, 15,000 students protested against pollution but the government has maintained that levels of air pollution are within environmental standards. Some plants though have been temporarily closed to improve air quality.
A 2010 global survey by consulting firm Gallup found that 57 percent of Kuwaitis were dissatisfied with the air quality in the area they lived in. Local residents are reported to suffer from high rates of respiratory diseases such as asthma, cancer and skin conditions.
9. Nigeria
Pollution level: 124 ug/m3
Nigeria is the most populous country in Africa and the eighth most populous in the world with over 155 million people.
Rapid urbanization and economic development have led to haphazard industrial planning in cities like Lagos that are struggling with waste management and high levels of pollution.
The country is also Africa's largest oil producer, accounting for 2.3 million barrels of crude per day, according to the International Energy Agency. The Niger Delta region, where the world's biggest energy companies operate, has experienced some of the worst oil spills in history. In August, a United Nations report said 50 years of oil pollution in the Ogoniland area may require the world's biggest and longest cleanup. The 14-month study showed deeper pollution than previously thought in an area that is home to about one million people. The report said it could take 25 to 30 years to clean up the contaminated drinking water, land and ecosystems.
Pipeline vandalism to feed a black-market in oil is common in the region and often contributes to oil spills. In September, Shell said it would shut production of 25,000 barrels of crude per day in the area due to the recent upsurge of oil thefts.
8. Iran
Pollution level: 124 ug/m3
Iran is home to the world's most polluted city — Ahvaz, which has three-times the average amount of pollution in the country. Ahvaz, known for its oil fields, is a heavily industrialized desert city of 1.3 million people.
Iran has the world's third-largest oil reserves and the second largest natural gas reserves. Locally produced, low-quality gasoline has been blamed for the country's extreme air pollution. The high-octane fuel -- much lauded by the country's leaders -- is manufactured in petrochemical plants rather than refineries.
Iran's capital Tehran made headlines last December, when it was blanketed by smog, forcing the government to declare "pollution holidays" for several days, shuttering offices, businesses and schools. Tehran marked nearly a month of continuously high levels of pollution with hospitals reporting a spike in patients with breathing problems. The state's English language television channel Press TV said more than 80 percent of the city's air pollution was attributed to the 3.5 million vehicles on the roads.
7. United Arab Emirates
Pollution level: 132 ug/m3
The United Arab Emirates is the world's fourth biggest oil exporter and one of the most developed economies in the Middle East.
The country has experienced rapid economic growth over the past 30 years. UAE's oil and gas industries and the transportation sector are the main contributors of air pollution. Dubai, the emirate's most populous city, is among the world's worst traffic polluters. Earlier this year, the city's municipal government said about 42 percent of air pollution in the city comes from vehicles.
The UAE is also notorious for being one of the world's biggest producers of waste. Average household waste in its capital city Abu Dhabi and regional hub Dubai is 30 percent higher than the annual average waste produced by countries of the Organization for Economic Cooperation and Development (OECD). The UAE's water usage is also among the world's highest. In 2007, the World Wildlife Fund said the emirate has the world's largest per capita ecological footprint, meaning the country puts the more demand on the environment than any other.
In an effort to curb environmental damage, the government introduced a series of measures in the past decade. In 2008, the government launched the largest solar lighting project in the Middle East and North Africa in Dubai, called the Green Community. It's expected to save 351 tons of carbon dioxide pollution a year. The country is also on track to become free of plastic bags by 2013. It currently consumes 20 billion bags out of a global figure of 500 billion.
6. Egypt
Pollution level: 138 ug/m3
Home to one of the world's oldest civilizations, Egypt is one of four African nations to make the list of the most polluted countries.
Egypt's biggest source of pollution are factories notorious for polluting the air with lead, gases and cement dust. In addition to that, vehicle exhaust fumes are a major problem in big cities like Cairo, which had about 2.1 million vehicles on the roads as of 2007. About 95 percent of Egypt's 730,000 motorbikes are two-stroke models that emit hydrocarbons equivalent to emissions from 10 to 15 gasoline-operated cars. In Greater Cairo alone, about 300,000 motorbikes release 150,000 tons of air pollution a year.
The country's struggles with air pollution were further exacerbated this February when thick clouds of tear gas and toxic fumes filled the sky during an 18-day revolt to overthrow president Hosni Mubarak. Egypt's environmentalists have also been battling to control pollution caused every autumn when farmers burn rice chaff and other crop remnants. The pollution, known as "Black Cloud," occurs after the harvest and is responsible for over 40 percent of the air pollution in Cairo during this period.
5. Saudi Arabia
Pollution level: 143 ug/m3
Saudi Arabia is the world's largest producer and exporter of petroleum. It also the second largest oil producer and holds one-fifth of the world's oil reserves.
Over the past few decades, as Saudi Arabia's oil industry has boomed, environmental pollution along its 2,175-mile coastline has also grown. Oil, power, desalination and other industrial activities near the coast have led to sewage outflows, and oil and chemical plumes. The country also struggles with high levels of vehicle exhausts, which account for 50 percent of hydrocarbon pollution in the air. That, added to its dry desert climate and windstorms has led to widespread outbreaks of respiratory diseases.
The government has stepped up efforts to clean up the country's air. Earlier this month, French engineering group Alstom announced a multi-million dollar deal to supply machinery to reduce carbon emissions from power plants in Saudi Arabia's eastern Ras Tanura city. The country also launched its first solar power plant this month, which is expected to save 28,000 barrels of diesel fuel annually.
4. Senegal
Pollution level: 145 ug/m3
Inadequate urban planning amid rapid economic development over the past few decades has landed Senegal in the top five most polluted countries in the world.
The West African nation's pollution comes from mining, vehicle emissions and combustion of fuels for domestic energy needs. More than a decade ago, the World Bank estimated that children exposed to smoky stoves in Senegal's Gambia region, were six times more likely to develop acute respiratory infections.
Senegal also has high levels of auto pollution because 85 percent of all vehicles in its largest city Dakar are old and imported ones. The average age for cars is 15 years; for buses, 20 years old. About 40 percent of these vehicles have diesel engines. A World Bank transport study of Dakar, showed that health costs associated with air pollution were equivalent to about 5 percent of its GDP. Like most countries in West Africa, Senegal has introduced regulations to try and control air pollution, but a lack of resources makes enforcing emissions standards difficult.
The country has also been struggling to find a solution for the high concentrations of natural fluoride in its ground water, which ranges between five and 15 times the acceptable standard of 0.7 milligrams per liter. High levels of fluoride can lead to tooth and bone decay. The government is piloting a program of solar-powered water filtration systems in some villages to reduce fluoride exposure.
3. Pakistan
Pollution level: 198 ug/m3
Pakistan's air pollution is nearly ten times higher than levels considered dangerous by the WHO.
Political instability, corruption and a lack of government measures to curb carbon emissions have led to a cloak of thick smoke over major cities like Karachi, Lahore and Islamabad. In 2008, a government study revealed that every car in Pakistan, regardless of its age, generates 25 percent more carbon than one in the U.S.
Karachi, Pakistan's biggest city has a capacity for only 700 vehicles per hour, but according to the study, there are more than 11,000 vehicles per hour on the roads. The city, which already has more than half the country's 3.5 million vehicles, adds up to 400 new cars a day.
Water pollution from raw sewage and industrial waste has also become a major problem in the country. In addition, Pakistan's water supply is evaporating so quickly that it will become a "water-famine" country by the end of this decade, according to the country's Centre for Research and Security Studies.
2. Botswana
Pollution level: 216 ug/m3
It might seem strange that a country with a population of only 2 million people, and the largest proportion of land under conservation in the world, is the second most polluted nation in the world. But, that is the case of Botswana, 80 percent of which is covered by the Kalahari Desert.
The country was one of the poorest nations in Africa at the time of its independence from Britain in 1966. Today, Botswana is one of the world's fastest growing economies, and the largest producer of diamonds. The country has transformed itself into a middle-income economy with an annual average growth rate of about 9 percent, according to The World Bank. Mineral revenues account for about 40 percent of government revenues.
With growing wealth, the southern African nation has experienced widespread environmental damage. Wild fires and pollution from the mineral industry are the main sources of the country's poor air quality. Copper smelting for example has been blamed for sulfur dioxide and nickel emissions.
1. Mongolia
Pollution level: 279 ug/m3
Mongolia is the world's most polluted country and also home to one of the world's most polluted cities — Ulaanbaatar.
The city of 1.2 million accounts for about 45 percent of Mongolia's population. During the coldest months of the year -- December to February -- Ulaanbaatar's horizon is completely hidden behind a thick grey-brown smoky haze.
The country's main sources of pollution are its traditional coal-fuelled stoves and boilers used for heating and cooking, as well as congested traffic and old cars. Heating is essential for the survival of its people for about eight months of year. The country uses everything from coal, wood to refuse, such as black tar-dipped bricks and old car tires to fuel stoves and boilers. Ulaanbaatar's dry climate and severe windstorms further worsen its dangerous levels of airborne dust.
Rapid urbanization has also been a major factor behind the country's pollution problem. Ulaanbaatar's population has expanded by 70 percent over the last 20 years with the city's infrastructure unable to keep up with growth. With an air pollution level 14-times higher than the WHO's standard threat level, the number of premature deaths, chronic bronchitis and respiratory related hospital admissions are on a rapid rise. The government has been trying to mitigate the problems by introducing measures such as cleaner coal-based fuel and modern stoves to address its pollution crisis.
