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Plumes of dark smoke have also been seen across other parts of the region over two weeks of war, as retaliates against U.S.-Israeli airstrikes by firing drones and missiles at the oil and natural gas facilities of its Persian Gulf neighbors.

Rain washes hazardous chemicals out of the atmosphere in a relatively short period of time, experts said, but people exposed to black rain should take precautions to avoid short- and long-term health risks.

Here's what to know:

What is black rain?

It occurs when soot, ash and toxic chemicals combine with water droplets in the atmosphere, then fall back to Earth when it rains. It's common after oil refineries or oil fields catch fire, and also can be caused by wildfires, volcanic eruptions and industrial pollution.

In Iran, microscopic soot formed when hydrocarbons in the fuel oil burned incompletely, experts said. Burning oil also forms compounds called polycyclic aromatic hydrocarbons, or PAHs, as well as toxic gases such as sulfur dioxide and nitrogen oxides, which cause acid rain, said Peter Adams, a civil and environmental engineering professor at Carnegie Mellon University.

What are the health risks of exposure?

Microscopic soot, whose particles are about 40 times smaller than the width of a human hair, can lodge deep in the lungs and enter the bloodstream, causing breathing and heart problems that can lead to premature death, experts said. 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. They warned that the rainfall was highly acidic and could burn the skin and cause lung damage.

“We can definitely expect acute health effects from an event like this,� said V. Faye McNeill, a chemical engineering professor at Columbia University who specializes in atmospheric chemistry.

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.�

Some Iranians fear the polluted rain, which also contains heavy metals, could contaminate drinking-water reservoirs and waterways.

How long do the chemicals last in the atmosphere?

It usually takes only hours for fuel tanks to burn out. But — as happened in Kuwait during the Gulf War 25 years ago — oil fields can burn for months, said Adams.

For fires that burn out more quickly, most of the soot and chemicals will disperse on the wind and wash out of the atmosphere in about three to seven days, Adams said.

“So if we don’t create more problems, at least what’s in the atmosphere is going to go away,� even if long-term health risks haven't, Adams said.

“But we don’t know what’s going to happen with future strikes and whether other tanks will be struck or whether oil fields will be struck,� he added. “I'm less concerned about longer-term or regional stuff, but it's a real mess for the people in the immediate vicinity.�

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