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, thick black raindrops 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.

That could have serious environmental and health impacts, but scientists are missing key details, starting with the chemical composition of the smoke, says Anna Hansell 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 that wash 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.