The UN health agency, which has an office in Iran and works with authorities on health emergencies, said it has received multiple reports of oil-laden rain this week. Tehran was choked in black smoke on Monday after an oil refinery was hit, in an escalation in strikes on Iran's domestic energy supplies as part of the US-Israeli campaign.

"The black rain and the acidic rain coming with it is indeed a danger for the population, respiratory mainly," WHO spokesperson Christian Lindmeier told a press briefing in Geneva on Tuesday, adding that Iran had advised people to stay indoors.

People should protect themselves

Asked whether the WHO backed that advice, Lindmeier said: "Given what is at risk right now, the oil storage facilities, the refineries that have been struck, triggering fires, bringing serious air quality concerns, that is definitely a good idea."

He said the strikes had caused "the massive release of toxic hydrocarbons, sulfur oxides and nitrogen compounds, into the air".

Scientists said inhaling or touching the smoke or particles could cause headaches, skin and eye irritation, and difficulty breathing. Longer-term exposure to some of the compounds increases the risk of some cancers, they added.

One video sent to Reuters by a WHO employee showed another staff member mopping up black liquid at its office entrance in Tehran on March 8. Reuters was not able to independently verify the footage.

Air quality

Scientists said that the black rain was the result of a weather pattern coming into the area, bringing rain that combined with the particles in the air.

"The rain was an eye-opener to people," said Akshay Deoras, a research scientist at the University of Reading. But he said that long-term exposure to toxic air was likely more of a health risk, which people could protect themselves from by staying indoors, or wearing masks and covering their skin outdoors.

He said data was lacking about the air quality in the area, but stressed that weather forecasts suggested dry conditions were likely for the rest of the week. The air quality is also likely to improve over time, he added.

"The risk of exposure goes down provided we don't see fresh strikes – if we get fresh strikes, that is going to be problematic," he said.

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