Why does black rain fall on Iran and how dangerous is it?

The sky in northern Iran was dark with smoke on March 8 as US and Israeli bombing campaigns against the country continued, and black rain even fell over the capital, Tehran.

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

What happened?

On the night of March 7 and 8, US-Israeli strikes hit Iran’s oil facilities for the first time since the war began just over a week ago, igniting major fires at four oil storage facilities and an oil transfer center in Tehran and nearby Alborz province.

Flames appeared 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 alarmingly, thick black raindrops fell on roofs and streets in the capital, which until recently experienced a long drought.

Authorities warned of acid rain, and locals complained of sore throats and burning eyes.

The black rain was probably due to the smoke from the oil plant 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.

It could have serious environmental and health consequences, but researchers 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 gasoline in your car, much of the oil would probably be thicker and less refined, and the combustion process would be much less thorough. As a result, smoke from the fires could have carried up a hugely varied mixture of burnt and unburned particles, most of which would be harmful to humans if ingested in large enough quantities.

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

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

These substances likely create a smog that is even thicker than the smog that blanketed London for much of the 20th century, most notoriously in 1952. “This is potentially several orders of magnitude greater than the London smog,” says Hansell.

Because missiles hit buildings, the smoke is also likely to carry small particles of materials such as concrete, glass and plastic. Finally, the explosions can throw oil droplets into the air which then rain down.

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

Will it be harmful to humans?

If black rain enters the water supply and people drink it, it can cause gastrointestinal symptoms, depending on the amount and chemical composition. People may experience abdominal pain, heartburn or diarrhoea.

More worryingly, if nitrogen and sulfur dioxide form acid rain, it can irritate the eyes and throat, as 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 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 some potentially carcinogenic compounds on your skin, but you can wash that off,” says Hansell. “If they get into your nose and mouth, they can persist 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 a number of conditions such as cardiovascular disease, lung cancer, chronic lung disease and diabetes.

Bioaccumulation of toxins in the environment can also contaminate fish, livestock 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 tiny particles can travel hundreds or even thousands of kilometers with the wind, such as dust particles from the Sahara now reaching Britain. Particles lifted by the Iran attacks could even potentially reach Washington DC, although they would likely be in very low concentrations at the time.

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

People in Iran should minimize exposure by staying indoors, advises Hansell. If they go outside, they should wear some kind of face mask and goggles to avoid acid rain getting into their eyes.

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

People abroad may be on the lookout for similar signs, but health officials in other countries are likely to alert if the wind delivers particles from Iran in large quantities.

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