The Department of Environment, Forestry and Fisheries will on Thursday visit the Sasol plant in Secunda which is suspected to be behind a sulphuric smell that is enveloping parts of Gauteng and Mpumalanga since the weekend.

"While there are many sources in the area that could be contributing to these elevated levels, the [Gert Sibande] District Municipality's air quality officer has been informed that the Sasol Secunda operation is experiencing stressed conditions as the facility started ramping up operations," said department spokesperson Albi Modise.

"The stench is likely a combination of elevated levels of sulphur dioxide and hydrogen sulphide."

Residents in both provinces took to social media complaining about the unbearable smell.

Modise said ambient air quality monitoring on the SA Air Quality Information System showed although sulphur dioxide levels were elevated, they remained compliant with ambient standards. "The pollutant causes an odour nuisance even at concentrations far below those that cause health hazards. People who are sensitive to unpleasant odours are likely to experience headaches and nausea."

"As part of the investigations, authorities are in the process of requesting the operations facility to account in terms of Section 30 of National Environmental Air Quality Management Act and to put mitigation measures in place, should they be found to be the source of the smell," he added.

Meanwhile, in a statement, Sasol said: "Sasol can confirm that recently our Secunda operations in Mpumalanga have been stable with no operational incidents that have/could have resulted in an increase in sulphur emissions.

"High-resolution analysers [on point sources], which monitor our emissions, report levels below the limits prescribed by our atmospheric emissions licences under which we operate.

"Given that we operate with due care and regard to the impact we may have on surrounding communities and the environment, we have put structures in place to engage with the community and encourage the public to use these forums to discuss all environmental issues, including air quality."