Russia's Norilsk smelter complex and a town in South Africa's eastern coal mining province have the highest sulphur dioxide (SO2) emissions in the world, according to satellite data from U.S. space agency NASA.
The NASA-compiled data published on Monday was commissioned by environmental group Greenpeace India and used the space authority's satellites to track anthropogenic sulphur dioxide emission hot spots around the world.
Scientists say that excessive exposure to SO2 particles causes long-term respiratory difficulties and stunted growth in infants among other problems.
Norilsk, 300km (186 miles) inside the Arctic Circle, has the largest individual SO2 emissions, followed by the South African town of Kriel, about 150km east of Johannesburg, Monday's report found.
The industrial city of Norilsk is home to Norilsk Nickel, the world's leading nickel and palladium producer.
The company is implementing a massive program to improve the ecology of the city and its surroundings.
The first stage of the program is complete. After the closure of an old nickel plant and reconstruction of nearby plants, emissions in the city's residential area fell by 30%, Nornickel told Reuters on Monday.
The second phase will involve a $2.5 billion sulphur project to help to reduce sulphur dioxide emissions in and around Norilsk by 75% from 2015 levels by 2023.
Under snow for up to nine months of the year and in darkness for more than a month in winter, Norilsk has acquired a reputation as one of the most polluted settlements in the world because of its metals plants. Prisoners from Joseph Stalin's labor camps built the first smelters there 80 years ago.
The South African town of Kriel, meanwhile, is home to state power utility Eskom's 2,850 megawatt Kriel Power Station, a short distance from two other coal-fired plants - Matla and Kendel - as well as Sasol's coal-to-liquid plants.
The town is part of a 31,000-sq-km area that covers three provinces and houses 12 coal power stations. It was declared a high-priority zone by the government in 2007 because of dangerously high pollution.
South Africa is Africa's worst polluter and one of the world's top 10 coal producers, with an estimated 3.5% of the world's coal resources, according to the International Energy Agency.
Environmental and community groups sued the government in June for failing to tackle high pollution in the Highveld Priority Area. The groups want the court to force the government to implement an air quality management plan that was published by the environmental affairs minister in 2012.
Environmental ministry spokesman Albi Modise said the NASA report was worrying and that national air quality plans needed to be reviewed urgently but that economic growth also had to be protected.
"You can't wake up and say you're closing all the coal power stations; imagine what will happen to electricity supply and the communities around them? We have to balance growth with protecting the environment."
Eskom, which provides 90% of the country's power, relies on a fleet of aging coal-fired plants to power Africa's industrialized economy and is struggling to meet its emissions target.
The utility implemented nationwide rolling power cuts this year because of capacity shortages.
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