After attacks on Iran's oil facilities, toxic black rain endangers the public

Clouds of toxic smoke unleashed into the atmosphere by U.S.-Israeli airstrikes on Iranian oil facilities made a dangerous return to Earth in the form of “black rain,” prompting international health officials to warn of serious risks to the public.

Residents in Tehran complained last week of burning eyes and difficulty breathing when the dark and oily precipitation fell near the Iranian capital after several fuel oil depots and a refinery were struck.

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 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:

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.

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.

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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A man rides a bike as fires and plumes of smoke rise after debris from an intercepted Iranian drone struck an oil facility, according to authorities, in Fujairah, United Arab Emirates, Saturday, March 14, 2026. (AP Photo/Altaf Qadri)

Residents look on and take pictures as flames and smoke rise from an oil storage facility struck as attacks hit the city during the U.S.–Israeli military campaign in Tehran, Iran, Saturday, March 7, 2026. (Alireza Sotakbar/ISNA via AP)

FILE - A thick plume of smoke rises from an oil storage facility hit by a U.S.-Israeli strike in Tehran, Iran, March 8, 2026. (AP Photo/Vahid Salemi, file)

Clothes clips covered in soot from burned fuel are seen as smoke from a U.S.-Israeli strike on an oil facility late Saturday lingers in the cloudy sky over Tehran, Iran, Sunday, March 8, 2026. (AP Photo/Vahid Salemi)

Two women from the Iranian Red Crescent Society stand as a thick plume of smoke from a U.S.-Israeli strike on an oil storage facility late Saturday rises in the sky in Tehran, Iran, Sunday, March 8, 2026. (AP Photo/Vahid Salemi)

The prospect of a second round of talks was uncertain Tuesday after Iran’s chief negotiator said Iran would not negotiate in the face of threats while U.S. President Donald Trump offered mixed messages about the path ahead for the U.S. war against Iran, declaring that he was in no rush to end the conflict.

Mohammed Bagher Qalibaf, Iran’s chief negotiator and parliament speaker, wrote in a post on X early Tuesday that “We do not accept negotiations under the shadow of threats,” and the Islamic Republic has been preparing “to reveal new cards on the battlefield.”

Trump indicated that he still expects to dispatch his negotiating team, led by U.S. Vice President JD Vance, to Pakistan’s capital of Islamabad for talks, even as Iran insisted it would not take part until the U.S. leader dialed back his demands. Trump said he’s “highly unlikely” to renew the ceasefire before it expires Wednesday.

Since the war started, fighting has killed at least 3,375 people in Iran and more than 2,290 in Lebanon. Additionally, 23 people have died in Israel and more than a dozen in Gulf Arab states. Fifteen Israeli soldiers in Lebanon and 13 U.S. service members throughout the region have been killed.

Here is the latest:

Iran said Tuesday it hanged a man convicted over allegedly setting fire to a mosque in northern Tehran during nationwide protests in January.

The judiciary’s Mizan news agency identified the man as Amir Ali Mir Jafari.

There was no immediate information about Jafari among activists who follow Iran.

Iran has been accused of repeatedly holding closed-door trials against suspects who can’t challenge the evidence placed against them.

Iran already has hanged people from the January protests, something that U.S. President Donald Trump had described as a red line before the recent war.

Shares are mixed in Asia and oil prices have slipped following the latest rise of U.S.-Iran tensions.

The lackluster start to trading Tuesday followed a modest retreat on Wall Street.

On Monday, the S&P 500 slipped 0.2% from its all-time high and the Dow industrials edged less than 0.1% lower. The Nasdaq composite fell 0.3%.

The price for a barrel of Brent crude oil remains above $95.

Trump attacked critics after a second round of talks with Iran was thrown into doubt by the U.S. Navy’s seizure of an Iranian-flagged cargo ship.

Financial markets have had vicious swings, both up and down, since the war began because of uncertainty about how long it may last.

The fear is that a long-term disruption could keep so much oil and natural gas off global markets that it creates a punishing wave of inflation for the global economy.

President Donald Trump speaks in the Oval Office of the White House, Saturday, April 18, 2026, in Washington. (AP Photo/Julia Demaree Nikhinson)

Women share a moment as they look at a smartphone at the main gate of the Tehran University as a banner shows portraits of the late Supreme Leader Ayatollah Ali Khamenei, right, and the late revolutionary founder Ayatollah Khomeini in Tehran, Iran, Sunday, April 19, 2026. (AP Photo/Vahid Salemi)

A woman talks on her cellphone as she walks past a billboard showing Rais Ali Delvari, a national hero in an early 1900 uprising against British forces in southern Iran in the Persian Gulf, right, and the late Revolutionary Guard's navy chief Alireza Tangsiri, who was killed in the U.S.-Israeli strike in late March 2026, commanding the closure of the Strait of Hormuz, on a building at a square in downtown Tehran, Iran, Monday, April 20, 2026. (AP Photo/Vahid Salemi)

A man mourns over the coffin of a Hezbollah fighter who was killed in the war between Hezbollah and Israel during a mass funeral in Bazouriyeh village, south Lebanon, Monday, April 20, 2026. (AP Photo/Mohammed Zaatari)