Why is black rain falling on Iran and how dangerous is it?

On March 8, the skies over northern Iran were clouded with smoke as the U.S. 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 the health of civilians in Iran and other countries.

What happened?

On the night of March 7-8, U.S.-Israeli strikes hit Iranian oil facilities for the first time since the war began just over a week ago, sparking large fires at four oil storage facilities and an oil transfer center in Tehran and neighboring Alborz province.

Flames hung over Tehran at night and black smoke billowed over the city during the day. Soot covered the streets and cars and filled people’s balconies. Most worryingly, thick black raindrops fell on the roofs and streets of the capital, which until recently suffered from a long drought.

Authorities warned of acid rain and local people complained of sore throats and burning eyes.

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

This could have serious environmental and health consequences, but scientists lack key details, starting with the chemical composition of the smoke, says Anna Hansell of the University of Leicester in the United Kingdom.

What’s in the black rain?

Unlike burning gasoline in your car, much of the oil would likely 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 into the air a wildly varied mix of burned and unburned particles, most of which would be harmful to humans if ingested in large enough quantities.

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

First, the smoke would have contained partially or fully burned carbon, or soot, as well as polyaromatic hydrocarbons. Oil also contains sulfur and nitrogen which, when burned, form oxides of sulfur and nitrogen. These can react with humidity in the air to form acid rain.

These substances likely create even thicker smog than that which blanketed London for much of the 20th century, most infamously in 1952. “It’s potentially several orders of magnitude larger than London’s smog,” says Hansell.

Since the missiles were hitting buildings, the smoke likely also carried tiny particles of materials like concrete, glass and plastic. Finally, explosions can throw oil droplets into the air which then rain down.

“I don’t know if the darkness is just caused by burning diesel, where you get this sort of black, oily smoke that’s carried in the raindrops, or if you also have very small oil droplets,” says Hansell.

Will this be harmful to people?

If black rain enters the water supply and people drink it, it could cause gastrointestinal symptoms, depending on its amount and chemical composition. People might experience stomach upset, heartburn, or diarrhea.

Even more worrying, if nitrogen and sulfur dioxide form acid rain, it could irritate the eyes and throat, as some residents have already reported.

But the bigger threat could be smoke rather than black rain. Simply inhaling large amounts of small particles can have serious health consequences, while the specific chemical composition is often a secondary concern.

“If you get raindrops on your skin, yes, there will be potentially carcinogenic compounds on your skin, but you can wash them away,” says Hansell. “If they get into your nose and mouth, they may linger longer, but very fine smoke particles in the air can penetrate deep into the lungs and potentially enter the bloodstream.”

High levels of particulate matter in the lungs can increase all-cause mortality and cause various conditions such as cardiovascular disease, lung cancer, chronic lung disease and diabetes.

Bioaccumulation of toxins in the environment could also contaminate fish, livestock and crops, potentially causing long-term health problems.

Could this threaten other countries?

Oil droplets and larger particles tend to fall from the atmosphere relatively quickly. But small particles can travel hundreds or even thousands of kilometers on the wind, like the Saharan dust particles currently reaching the UK. Particles projected by Iranian strikes could even potentially reach Washington DC, although they would then likely be in very low concentrations.

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

Iranians should minimize their exposure by staying indoors, advises Hansell. If they go out, they must wear a face mask and protective glasses to prevent acid rain from entering their eyes.

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

People abroad may be on the lookout for similar signs, but health authorities in other countries will likely issue an alert if winds carry large amounts of particles from Iran.

“Any large-scale environmental damage that you cause knows no boundaries, so what goes into the water system, what’s in the air, will be transported elsewhere,” Hansell says.