The people of Tehran have long suffered from some of the dirtiest air in the world. It’s largely because Iran’s power plants burn an extraordinarily sulfur-heavy fuel known as “mazut.”
Mazut is essentially the bottom-of-the-barrel residue that’s left when everything else of value is refined out of oil. Much of the world bans burning it. Only a handful of other countries, like North Korea, rely on it so heavily.
Now, in the wake of the Israeli-led bombings of oil facilities across Iran, there are concerns that tanks of mazut are on fire at several locations known to store it, along with gasoline and diesel fuel, according to local news reports and analysts who track Iran’s energy sector. A spokesman for Iran’s foreign ministry, Esmaeil Baqaei, said military strikes were “releasing hazardous materials and toxic substances into the air,” the BBC reported.
Eoghan Darbyshire, a researcher at the Conflict and Environment Observatory, a London-based nonprofit that tracks the environmental costs of conflicts, said that fires like these were likely to produce “a cocktail of toxic gases that will continue to burn and smolder for days.”
Before the war, air quality in Tehran frequently hit levels considered “very unhealthy” by the air monitoring company IQAir. In 2025, the head of Iran’s Clean Air Scientific Association estimated that air pollution caused some 58,000 deaths annually, according to Iran’s semiofficial Mehr News Agency.
Iran’s increasing reliance on mazut stems from the country’s economic isolation and trade sanctions, as well as decades of mismanagement and the decay of critical infrastructure. Though the country has rich deposits of oil and natural gas, it has faced crippling fuel shortages at home.
Iran also sells much of its higher-value fossil fuel products overseas to raise foreign currency. That leaves mazut to fill in for a shortage at home.
In the past decade, the amount of mazut Iran has burned at its power stations and industrial sector has nearly tripled, according to a 2025 internal report prepared by the National Iranian Oil Company and reviewed by The New York Times. During colder months, Iran burns roughly 40 million to 50 million liters of mazut per day, the report shows.
The burning of mazut has contributed to political tensions in Iran, said Shirin Goli, a Nevada-based environmental consultant and former civil engineer at Tehran’s Sharif University of Technology.
It is just one part of an environmental breakdown in a country that also faces severe water shortages brought about by drought and groundwater depletion, she said. “There’s an environmental crisis. There’s a water crisis. The air people breathe is making everyday life so hard,” she said. “That’s one of the big reasons why people took to the streets, to demand healthy water and healthy air.”
Mazut is exceptionally polluting. When burned, it releases sulfur, which reacts with oxygen to form sulfur dioxide, a toxic gas.
Breathing sulfur dioxide can lead to severe respiratory problems. The gas also reacts with other compounds in the air to form fine particles small enough to enter the bloodstream through the lung. In the atmosphere, sulfur dioxide mixes with vapor and oxygen that then returns to the ground as acid rain, polluting water and endangering wildlife.
“The most important issue with burning mazut in Iran is that the sulfur content is very, very high,” said Dalga Khatinoglu, an Azerbaijan-based expert on Iran’s energy sector.
It is an irony that Iran, which holds the world’s second-largest natural gas reserves, needs to turn to mazut.
The sanction-hit Iranian regime has prioritized selling its gas overseas, including boosting deliveries to neighboring Turkey and Iraq, adding to the shortage at home. Iran “flares,” or intentionally burns off, about 23 billion cubic meters of gas per year, an exceptionally high amount that is equivalent to almost a third of Turkey’s annual total gas consumption, because Iran doesn’t have ample facilities to capture and sell it. Another estimated eight billion cubic meters a year is lost through leaks.
What Iran does have a lot of is mazut. And that has to do with the state of its refineries, which haven’t been significantly upgraded in decades. Six of Iran’s 10 refineries were built before the 1979 revolution.
While modern refineries are able to more efficiently convert crude oil into fuels like gasoline and diesel, refineries in Iran produce an unusually large share of mazut. According to the oil-company report, nearly 30 percent of all crude processed in Iran is turned into mazut, compared with less than 5 percent in many modern plants. At Iran’s biggest refinery at Abadan, built more than a century ago, that percentage is as high as 47 percent.
“Modernizing these refineries would require advanced technologies, financing, international partnerships — but all of those are restricted,” said Keyvan Hosseini, a research fellow at the University of Southampton.
Tehran residents have described the latest attacks and ensuing oil fires as turning “morning into night.” Several local media outlets reported that spills from an oil depot in Tehran had entered storm drains, igniting explosively.
There have also been accounts of acidic rain falling on the city. That could be from the combustion of sulfur-rich mazut.
Hiroko Tabuchi covers pollution and the environment for The Times. She has been a journalist for more than 20 years in Tokyo and New York.
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