The area around coal-fired plants owned by South Africa's state power utility Eskom ranks as the world's second-largest hotspot of sulphur dioxide emissions, according to Greenpeace.
The level of sulphur dioxide emissions in the Kriel area in Mpumalanga province only lags the Norilsk Nickel metal complex in the Russian town of Norilsk, the environmental group said in a statement, citing 2018 data from NASA satellites.
The province is home to most of Eskom's 15 coal-fired power plants as well as coal-to-fuel plants owned by Sasol, South Africa's biggest company by revenue.
Government is under growing pressure to act against air pollution caused by the two companies as it's being sued by environmental activists over the violation of the constitutional right to clean air. A plan to double the amount of sulphur dioxide that coal-fired power plants and boilers can emit was put on hold in May after a legal challenge was filed against the environment ministry.
An earlier Greenpeace study for the third quarter of 2018 showed that Mpumalanga also had the worst nitrogen dioxide emissions from power plants of any area in the world.
"South Africa's air is absolutely filthy," said Melita Steele, a senior climate and energy campaign manager at Greenpeace Africa. "We simply cannot afford to waste any more time by delaying industry compliance with air-quality legislation or the transition to renewable energy."
Sasol will have to adhere to new sulphur dioxide emission limits by 2025, but has warned that it might not be able to afford the equipment needed to meet the standards. Eskom has filed for permission to delay complying with emission limits at some of its plants.
Sulphur dioxide is linked to lower respiratory infections and increased risk of stroke, as well as increased risk of death from diabetes.
Eskom accounts for 42% of South Africa's greenhouse gases, while Sasol emits 11% of the total, the companies have said.