The sulphuric stench that drifted over large parts of Johannesburg last week - described by residents as reeking of “rotten eggs”, “cat urine” and “burning sulphur” - has dissipated.
The sulphuric stench that drifted over large parts of Johannesburg last week – described by residents as reeking of “rotten eggs”, “cat urine” and “burning sulphur” – has dissipated. But the questions it has raised about the region’s air quality and the systems meant to protect it, linger.
For the second time in two months, large parts of the city and beyond were engulfed in a noxious odour traced to hydrogen sulphide (H₂S) and sulphur dioxide (SO₂).
While the immediate episode has passed, experts warn that it offers a glimpse into a deeper and persistent pollution problem affecting millions living downwind of the country’s industrial heartland.
At the centre of that concern is the cluster of coal-fired power stations and heavy industry to the east of the city in the Highveld Priority Area: an air pollution hotspot that is home to 12 of Eskom’s 15 coal-fired plants and the vast coal-to-liquids facility operated by Sasol in Secunda.
Lauri Myllyvirta, lead analyst at the Centre for Research on Energy and Clean Air, said the reported smells and spikes in H₂S concentrations “strongly implicate” the Sasol Secunda coal-to-liquids plant as a source of last week’s episode.
“The entire period of 9–11 March, when Johannesburg and surrounding areas experienced high pollution levels, had wind from the east,” he said. “Those conditions caused pollution from Eskom coal power plants and the Sasol Secunda coal-to-liquids plant to build up in the city.”
Secunda is the only major source of this pollutant large enough to send pollution levels “high enough that people can smell them over such a large area”.
Johannesburg, he said, faces an extraordinary situation, with a vast concentration of some of the most polluting coal-burning facilities in the world located within reach of the city. A distance of a hundred kilometres may seem long but it is only a few hours as the wind travels.
“I carried out atmospheric modeling that predicts the dispersion of the pollution from Secunda hour-by-hour and those modeling results show the emissions from the plant reaching the city in a concentrated plume at the time when the smell and measured spikes in (H₂S) concentrations were reported, which establishes a clear link to the facility,” he said.
A regional pollution problem
Government investigations have stopped short of naming a single source but broadly align with a regional picture.
“The emission plume as experienced last week was not local,” said Nthatisi Modingoane, the spokesperson for the City of Johannesburg. “It was an external source located outside of the city boundaries.” Residents in other municipalities also reported the smell further points to a regional pollution event rather than a localised incident, he added.
According to the department of forestry, fisheries and the environment, monitoring showed that while sulphur dioxide levels were elevated, they remained within national standards. Hydrogen sulphide readings recorded noticeable peaks at several monitoring stations, including Secunda, Springs, Irene, Midstream and Buccleuch.
South Africa does not yet have national ambient standards for hydrogen sulphide but concentrations were significantly higher than the World Health Organisation’s nuisance odour threshold of 5.02 parts per billion.
Prevailing south-easterly winds likely carried polluted air from Mpumalanga into Gauteng, affecting communities across Ekurhuleni, Johannesburg and Pretoria.
Invisible threat of PM2.5
The odour itself, while unpleasant, is only part of the story. Hydrogen sulphide is a toxic gas. At low concentrations, it can irritate the eyes, nose and throat and trigger breathing difficulties, particularly in people with asthma. At higher concentrations, it can cause respiratory distress, loss of consciousness and, in extreme cases, death.
Myllyvirta warned that the more serious risk lies in the chronic pollution underpinning hydrogen sulphide and sulphur dioxide. These emissions contribute to the formation of fine particulate matter — PM2.5 — the most dangerous air pollutant.
While authorities assured the public that pollutant concentrations remained low, he said this framing is “misleading” because PM2.5 levels were elevated. PM2.5 levels across Johannesburg were above 15 micrograms per cubic metre on 10–11 March, exceeding WHO guidelines, although not constituting a severe episode.
“The major public health concern is persistent elevated PM2.5. Chronic exposure over months and years leads to severe health impacts, including deaths from cardiovascular diseases, cancer and respiratory diseases, as well as adverse birth outcomes,” he said.
The massive concentration of coal-fired power and heavy industry contributes to Gauteng’s PM2.5 levels year-round. “It just tends to only be noticed when the smelly H₂S emissions hit the city.”
Research by the South African Medical Research Council supports this concern, showing that people living near coal-fired power stations, particularly in Mpumalanga, face significantly higher risks of disease and premature death.
These include increased mortality from heart disease, hypertension and respiratory infections, as well as higher rates of childhood pneumonia and certain birth defects.
Poor regulation
Myllyvirta pointed out that South Africa’s regulation of hydrogen sulphide remains limited, particularly in relation to coal gasification processes used at Sasol’s facilities.
He described the country’s large-scale hydrogen sulphide emissions as an “oddity”, noting that the only other country with a significant coal-to-oil industry, China, requires controls for the pollutant.
In most major coal-using economies, including China, the European Union, Japan and South Korea, flue gas desulphurisation systems are routinely installed to remove sulphur dioxide from emissions.
“The avoided health costs and other damages from sulphur dioxide emissions exceed the costs of installing and operating the technology,” he said. “But the government has been much more concerned about the cost to the polluters than the cost to society.”
Tighter controls
According to the department, following similar odour events in 2022, it investigated sources including Sasol’s Secunda operations. While the facility was found to be operating within legal emission limits, authorities concluded that those existing limits may not adequately protect public health, particularly for communities around Secunda.
The department said it is working to introduce hydrogen sulphide as a regulated “criteria pollutant”, which would allow national ambient standards to be set. Legislative changes are expected to be finalised this year.
“These regulatory changes are intended to provide long-term solutions to reduce hydrogen sulphide emissions and limit public exposure.”
Compliance does not necessarily mean safety, said Robyn Hugo, director of climate change engagement at Just Share. “Even if Sasol is complying with its air emission licence, hydrogen sulphide monitoring data show elevated peaks – well above WHO recommended thresholds. That makes it clear the licence allows emissions at unacceptably high levels.”
Sasol is the largest private emitter of toxic air pollutants in Africa and the “most obvious cause of the rotten egg odour,” she said, noting it has already received multiple government concessions regarding air pollution.
“Air pollution is the world’s largest environmental health risk and levels in the Highveld Priority Area cause ongoing sickness and early deaths, violating the environmental right,” she said, noting that it is a relief that the department is finally acting.
“They should fast-track ambient air quality standards for hydrogen sulphide and publish the proposed minimum emission standards amendments – then stand firm against Sasol’s predictable claims that compliance would cause economic disaster.”
Major emitters with financial incentives to maintain the status quo resist regulations that force them to internalise social and economic costs, often borne by the poorest and most vulnerable, she noted.
Sasol and others repeatedly argue that stricter rules are unwise and would have devastating consequences. The department’s role is to withstand this resistance and enforce regulations that serve the public interest.
Sasol’s response
Sasol, however, emphasises that the episode reflects regional conditions rather than a single facility.
The company said it has reviewed Secunda Operations’ plant status, point-source emissions, ambient air-quality data and meteorology for 9–11 March.
“Secunda Operations remained stable and no operational incidents or abnormal process conditions have been identified that would indicate an uncontrolled or atypical release,” it said.
The company acknowledged that under certain meteorological conditions, particularly sustained easterly to south‑easterly winds combined with atmospheric stability, emissions from multiple sources across the Mpumalanga Highveld – including Sasol Secunda – can be transported towards Gauteng. “Sasol is therefore assessing the prevailing regional meteorology as part of its ongoing review.
“Sasol acknowledges that Secunda Operations are a material source of sulphur‑bearing emissions, as coal gasification and downstream processing generate hydrogen sulphide streams that must be managed and treated.
“At the same time, independent academic research demonstrates that ambient hydrogen sulphide levels across the Highveld are influenced by multiple sources.”
The company said a peer‑reviewed source‑apportionment study conducted at the Elandsfontein atmospheric research station found that urban sources were the largest contributors to ambient hydrogen sulphide measured in excess of background levels, followed by the Johannesburg‑Pretoria conurbation.
“The petrochemical sector associated with Secunda contributed a smaller, though material, proportion of ambient hydrogen sulphide at that receptor.”
This confirms that while Sasol Secunda “can contribute to regional ambient hydrogen sulphide under certain meteorological conditions, regional odour episodes reflect the combined influence of several sources rather than a single facility acting in isolation”, it said.
“Episodic odour events are the result of unfavourable atmospheric conditions that intensify impacts, from various sources, by suppressing dispersion and promoting the accumulation of emissions within affected areas.”
The company said it continues to optimise sulphur-management systems, monitor emissions in accordance with its atmospheric emission licence and implement a sulphur dioxide reduction roadmap for the Secunda steam plants.
Since 2015, it has invested more than R10 billion in emission-reduction projects, aiming to cut overall sulphur dioxide emissions from the Secunda steam plants by roughly 30% by 2030. Sasol also highlighted its use of acid-gas removal systems, sulphur recovery units, wet sulphuric acid plants, gas-liquor treatment systems and thermal oxidisers, all supported by continuous monitoring.
“The sulphurous odours cause concern for communities and we remain committed to transparency, scientific rigour and continuous improvement in our air-quality performance,” the company said.
“We will continue to cooperate with authorities, advance emission-reduction initiatives and strengthen regional monitoring to support factual, evidence-based understanding of odour events.”