Dark, oily precipitation described by residents and officials as “black rain” has fallen in parts of Iran after strikes hit oil facilities, with the Associated Press reporting warnings that the toxic smoke could pose serious health threats to people who breathe it or come into contact with it.
Residents in Tehran complained last week of burning eyes and difficulty breathing when the dark and oily precipitation fell near the Iranian capital, following strikes on several fuel oil depots and a refinery, the report said. Plumes of dark smoke have also been seen across other parts of the region over two weeks of war as Iran retaliates against U.S.-Israeli airstrikes by firing drones and missiles at oil and natural gas facilities of its Persian Gulf neighbors.
Officials and experts described black rain as a phenomenon that can occur when soot, ash and toxic chemicals combine with water droplets in the atmosphere and then fall back to Earth during rain. It is commonly reported after oil refineries or oil fields catch fire, and can also be caused by wildfires, volcanic eruptions and industrial pollution.
In Iran, experts said microscopic soot formed when hydrocarbons in fuel oil burned incompletely. Peter Adams, a civil and environmental engineering professor at Carnegie Mellon University, said burning oil also forms compounds called polycyclic aromatic hydrocarbons, or PAHs, as well as toxic gases such as sulfur dioxide and nitrogen oxides that can contribute to acid rain.
Health officials and researchers warned that microscopic soot particles—about 40 times smaller than the width of a human hair—can lodge deep in the lungs and enter the bloodstream. They said such exposure can cause breathing and heart problems that can lead to premature death, and that exposure to PAHs can increase cancer risk.
In Iran, the World Health Organization and the country’s health and environmental officials advised people to stay indoors and wear masks, according to the report. They warned that the rainfall was highly acidic and could burn skin and cause lung damage.
V. Faye McNeill, a chemical engineering professor at Columbia University who specializes in atmospheric chemistry, said, “We can definitely expect acute health effects from an event like this,” and added that even regular air pollution events can trigger health problems and lead to more hospitalizations, especially among the elderly, children and people who already had health problems. She said, “But this is a higher level, so there likely are health problems going on right now because of it.”
The report said some Iranians also feared the polluted rain could contaminate drinking-water reservoirs and waterways, noting that it contains heavy metals. While experts said rain can wash hazardous chemicals out of the atmosphere in relatively short periods, they also warned that the overall picture depends on how long the burning continues and whether additional strikes occur.
Adams said it usually takes only hours for fuel tanks to burn out, but that oil fields can burn for months. For fires that burn out more quickly, he said, most soot and chemicals disperse on the wind and wash out of the atmosphere in about three to seven days, adding that if there are no more strikes, “at least what’s in the atmosphere is going to go away,” even if longer-term health risks remain uncertain. He cautioned that future strikes could trigger additional problems and that the situation is “a real mess for the people in the immediate vicinity.”