Israeli airstrikes on oil facilities in Tehran on 7 March led to sulphur dioxide emissions equivalent to a small volcanic eruption, potentially exposing people as far away as China to acid rain and toxic air pollution.

As part of the US and Israeli campaign against Iran, warplanes several oil depots and a refinery that night, sparking that lit up the sky and smoke for days. Black rain containing soot and hydrocarbons fell on the Iranian capital, and residents reported eye and skin irritation and difficulty breathing.

Now, data from a new generation of Chinese satellites has shown that the plume of sulphur dioxide released by these explosions and fires covered 300,000 square kilometres, passing over Turkmenistan, Uzbekistan, Kyrgyzstan, Kazakhstan and China.

The brief attack prompted a days-long spike in emissions, injecting a total of 29,800 tonnes of sulphur dioxide, according to at Wuhan University in China and his colleagues. For comparison, Iceland’s Eyjafjallajökull volcano was emitting about  per day when its ash cloud shut down air travel in Europe in 2010.

The concentrations of sulphur dioxide measured by the satellites reached levels that could impair lung function, irritate the eyes and throat, and exacerbate asthma or bronchitis, especially among children and older people, says .

“Although the major emission event lasted only one to two days, the research notes that the potential impact on the regional atmosphere should not be neglected,” he says. Pollutants may have been rained out over water sources and agricultural land, potentially contaminating drinking water and food, he adds.

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Sulphur dioxide reacts with different compounds of hydrogen and oxygen in the air to form sulphuric acid, leading to smog and acid rain. During the Great Smog of 1952, sulphuric acid and other pollution from burning coal killed an .

The attack on Tehran released about sulphur dioxide than some coal-fired power plants in high-income countries emit in a year, although a coal plant in a nation that doesn’t require scrubbers on smokestacks can emit far more of it.

Besides sulphur dioxide, the burning oil facilities emitted soot and heavy metals. According to at the University of York, UK, the massive quantity of sulphur dioxide emitted suggests the plume held harmful quantities of even more dangerous pollution. This might include nitrogen oxides and polycyclic aromatic hydrocarbons, as well as unburned hydrocarbons like benzene, all of which to cancer.

“[Sulphur dioxide] would be emitted with a whole range of other things,” says Carpenter. “That amount in one single fire has huge implications for people’s health… over thousands of kilometres.”

These fine particles can stay aloft for days, travelling with the wind. It was impressive that the study was able to trace the evolution of the plume over such a wide area, she says.

The plume only lasted for about three days, which probably isn’t enough time to cause cancer. And the satellites measure sulphur-dioxide concentrations through the entire atmosphere, so the toxin concentration at ground level is unclear. But the pollution could potentially have triggered asthma attacks, strokes or even heart attacks in especially vulnerable people, according to Carpenter.

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The Fengyun 3 satellite constellation that the study drew upon provides atmospheric concentrations of sulphur dioxide and other major pollutants to the public within three hours, which could improve disaster response, says Yin. “Satellite data are useful for pollution assessment and early warning for downstream areas.”

The skies in northern Iran were dark with smoke on 8 March as the US and Israeli bombing campaign against the country continued, and black rain even fell on the capital Tehran.

The catastrophic scenes have raised concerns about threats to civilian health in Iran and other countries.

What happened?

Overnight on 7 and 8 March, US-Israeli strikes hit Iran’s oil facilities for the first time since the war started a little over a week ago, igniting large fires in four oil storage facilities and an oil transfer centre in Tehran and the nearby Alborz province.

Flames loomed over Tehran in the night, and black smoke billowed over the city during the day. Soot covered the streets and cars and filled up people’s balconies. Most alarmingly, fell onto roofs and streets in the capital, which until recently was experiencing a long drought.

The authorities warned of acid rain, and local people complained of their throats aching and their eyes burning.

The black rain was probably caused by the smoke from the oil facility fires. When precipitation falls through such polluted air, it can wash soot and other particles out of the smoke and carry them to the ground in the form of black raindrops.

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That could have serious environmental and health impacts, but scientists are missing key details, starting with the chemical composition of the smoke, says at the University of Leicester in the UK.

What’s in the black rain?

Unlike burning petrol in your car, much of the oil would probably have been thicker and less refined, and the combustion process would have been much less thorough. As a result, smoke from the fires could have carried aloft a hugely varied mix of burnt and unburnt particles, most of which would be harmful to humans if ingested in large enough amounts.

“It’s going to be quite a nasty toxic moisture,” says Hansell.

First of all, the smoke would have contained partially and fully burnt carbon, or soot, as well as polyaromatic hydrocarbons. Oil also contains sulphur and nitrogen, which, when combusted, forms sulphur and nitrogen oxides. Those can react with moisture in the air to form acid rain.

These substances are probably creating a smog that’s even thicker than the smog that blanketed London in much of the 20th century, most infamously in 1952. “This is potentially several orders of magnitude larger than the London smog,” says Hansell.

Because missiles were hitting buildings, the smoke is probably carrying tiny particles of materials like concrete, glass and plastics as well. Finally, the explosions may be throwing droplets of oil into the air that are then raining out.

“I’m not clear if the blackness is solely caused by burning diesel, where you get this sort of greasy black smoke that’s being carried in the raindrops, or whether you’ve actually got some very small droplets of oil as well,” says Hansell.

Will it be harmful to people?

If black rain gets into the water supply and people drink it, it could cause gastrointestinal symptoms, depending on its quantity and chemical make-up. People could experience stomachaches, heartburn or diarrhoea.

More worryingly, if nitrogen and sulphur dioxide are forming acid rain, that could irritate the eyes and throat, similar to what some residents have already reported.

But the biggest threat may be the smoke rather than the black rain. Simply inhaling large amounts of small particles can severely impact health, while the specific chemical composition is often a secondary concern.

“If you get raindrops on your skin, yes, there will be some potentially carcinogenic compounds on your skin, but you can wash that off,” says Hansell. “If they get into your nose and mouth, they might persist for longer, but very fine smoke particles in the air can penetrate deep into the lungs and potentially get into the bloodstream.”

High levels of particles in the lungs can raise all-cause mortality and cause a variety of conditions such as cardiovascular disease, lung cancer, chronic lung disease and diabetes.

The bioaccumulation of toxins in the environment could also contaminate fish, farm animals and crops, potentially causing long-term health problems.

Could it threaten other countries?

Oil droplets and larger particles tend to fall out of the atmosphere relatively quickly. But small particles can travel hundreds or even thousands of kilometres on the wind, such as dust particles from the Sahara that are currently reaching the UK. Particles lofted by the Iran strikes could even potentially reach Washington DC, although they would probably be in very low concentrations at that point.

But smoke from the fires is more likely to reach other parts of Iran and countries in the Middle East, depending on the wind and atmospheric conditions.

People in Iran should minimise their exposure by staying indoors, Hansell advises. If they do go outside, they should wear a face mask of some sort and goggles to keep acid rain from getting into their eyes.

They should find a different water source, such as bottled water, if they detect a funny taste or black particles in their drinking water.

People abroad can be on the lookout for similar signs, but health authorities in other countries are likely to issue an alert if winds are delivering particles from Iran in large quantities.

“Any large-scale environmental damage that you do like this, it doesn’t recognise borders, so what’s going into the water system, what’s going into the air, it’s going to be carried elsewhere,” says Hansell.

Trifluoroacetic acid (TFA), a potentially toxic “forever chemical”, has more than tripled in the global environment in two decades due to refrigerants that are helping to close the hole in the ozone layer.

The amount of TFA falling out of the atmosphere via wind and rain has risen from 6800 tonnes per year in 2000 to 21,800 tonnes in 2022. Although that is below known safe thresholds, TFA’s effects on human health haven’t been studied in detail and its accumulation in the environment is expected to accelerate.

TFA eye deformities in most of the rabbit fetuses exposed to it in one trial. The European Union has classified it as harmful to aquatic life and is considering whether to deem it toxic to human reproduction.

“It is shocking that we’re emitting large amounts of a chemical into the environment that we have a very poor understanding of its impacts, and it’s irreversible basically,” says at Lancaster University, UK, who led the new research.

Humans and animals will be exposed to TFA in soil and surface water until it flows out to sea and is trapped in ocean sediments after decades or centuries.

Chlorofluorocarbons (CFCs) were once found in refrigerators, aerosol sprays, fire extinguishers and other items. But they were banned in 1989 after it was discovered they were creating a hole in Earth’s protective ozone layer. They were largely substituted by hydrofluorocarbons (HFCs), which react with hydroxyl radicals in the atmosphere to form compounds including TFA.

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While HFCs are now being phased out, they are often replaced by hydrofluoroolefins (HFOs), which break down into TFA at a far higher rate. HFO-1234yf, which is now in air conditioners in hundreds of millions of cars, produces 10 times more TFA than the HFC it is gradually replacing. Pesticides, pharmaceuticals and industrial chemicals are also sources of TFA.

Cores from ice caps in northern Canada and Svalbard have revealed that TFA concentrations have been increasing there since the 1970s. Based on long-running atmospheric measurements of nine CFC replacement gases, Hart and her colleagues modelled the rate at which TFA was produced and deposited around the world, which revealed a 3.5-fold global increase.

That rate could as much as double by 2050 based only on HFCs, which can last for several decades in the atmosphere. Other research found that HFO-1234yf could boost TFA production by more than 20 times by 2050.

While the world can’t go back to CFCs and should continue moving away from HFCs, which have a significant global warming effect, the replacements for these chemicals need further scrutiny, says at the University of York, UK.

Ammonia already cools many food warehouses and industrial processes, and could also be used in home refrigerators and air conditioners. Carbon dioxide is another natural refrigerant.

“We need to take a serious look at whether there are better alternatives to HFO-1234yf,” Carpenter says. “TFA has increased and is going to further increase… It’s been found in all kinds of food products now that it never used to be found in. It’s everywhere.”

A 2020 discovered high concentrations of TFA in the blood of 90 per cent of people in China, which is a hotspot for the chemical because of industrial pollution and warm, wet weather in many places.

The EU, which is developing proposals for a ban on forever chemicals, predicted that freshwater TFA concentrations would eventually reach a toxic level. But it has come under fire for a consultancy that has also lobbied for chemical manufacturers and disputed this expected increase in TFA.

The new findings are a call to action to study both HFOs and their possible replacements, so countries can break the cycle of adopting chemicals with unexpected consequences, says Hart. Unlike HFCs, HFOs break down in days, giving us greater control. “If we stop emitting them, you’ll stop TFA production very quickly,” she says.