JOHANNESBURG - The Forestry, Fisheries and Environment Department has responded to the bad odour across parts of Gauteng.
The department says hydrogen sulphide gas is the likely culprit.
Residents in areas including Johannesburg and Ekurhuleni reported the odour earlier this week.
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Air quality monitoring shows elevated levels of the gas at several stations, including Secunda, Springs and Buccleuch.
Officials say weather conditions carried polluted air from Mpumalanga into Gauteng.
The department warns that exposure may irritate the eyes and respiratory system and can cause headaches, nausea and breathing difficulty.