Happy Earth Day (2011)

Linfen, China

The pollution in this Shanxi province city -- situated in the heart of the country's enormous coal industry -- is said to be so bad that residents literally choke on coal dust in the evenings. As the Guardian reported in 2007, Linfen held the title of "world's most polluted city" for many years -- though is also said to have shown minor improvement as of late.

Mailuu-Suu, Kyrgyzstan

Mailuu-Suu has one of the largest concentrations of radioactive waste in all of former Soviet Central Asia. As NPR reports, Mailuu-Suu was once an elite Soviet industrial town, where uranium was mined and processed. Though the mining came to a stop in the 1960s, the radioactive waste remains to this day.

Norilsk, Russia

The Siberian city of Norilsk houses the world's largest heavy metals smelting complex. Approximately 134,000 people -- over half of the current population -- are reportedly affected by the severe air pollution in Norilsk, with the human life expectancy is at least 10 years below that averaged in other Russian cities.

Sukinda, India

Twelve Sukinda mines reportedly operate without environmental controls, while miners are constantly exposed to contaminated dust and water, and gastrointestinal bleeding, tuberculosis and asthma are common ailments. Infertility, birth defects, and stillbirths and have also resulted.

Dzerzhinsk, Russia

Until the end of the Cold War, Dzerzhinsk was among Russia's principal production sites of chemical weapons, and is still a significant center of Russian chemical manufacturing. Nearly 300,000 tons of chemical waste were improperly disposed of there between 1930 and 1998, and toxic residuals from those manufacturing sites are present in both ground and surface water. The Guinness Book of World Records once named it the most chemically polluted city in the world.

Chernobyl, Ukraine

The infamous site of the world's worst nuclear disaster still shows its scars 25 years later. There are reportedly no plans yet to breathe new life into the poisoned region, and the 19-mile (30.6-kilometer) exclusion zone around the plant remains uninhabitable.

Kabwe, Zambia

In 2006, Kabwe was ranked the world's fourth most polluted site by the Blacksmith Institute, a New York-based organization monitoring pollution in the developing world. Decades of lead and zinc mining have left a city poisoned by debilitating concentrations of lead dust in the soil and by metals in the water.

Sumgayit, Azerbaijan

Sumgayit was a major Soviet industrial center housing more than 40 factories manufacturing industrial and agricultural chemicals. While the factories remained operational, a reported 70-120,000 tons of harmful emissions were released into the air annually, and factory workers and city residents have been exposed to a combination of high-level occupational and environmental pollution problems for several decades. A continuing lack of pollution controls, dated technologies and the improper disposal and treatment of accumulated industrial waste are just some of the issues that plague the city.

Tianying, China

Located in China's Anhui province, Tianying is reportedly one of the largest lead production bases in the country, with an output accounting for half the nation's total production. Low-level technologies, illegal operation and the lack of any serious pollution control measures in the firms have caused several severe lead poisoning cases in the region.

La Oroya, Peru

Peru's Clean Air Act reportedly cites La Oroya in a list of Peruvian mining towns suffering from critical levels of air pollution, and adults and children have been exposed to the toxic emissions and wastes from a nearby poly-metallic smelter plant.