When residents in Iran capital Tehran woke up on Sunday, the sun had disappeared. Thick black smoke from burning oil depots blotted out the daylight, forcing people to turn on lights in the middle of the morning. Then came the rain – black, oily and toxic. "I can't believe it, I'm seeing black rain," a 44-year-old engineer told Time magazine.

An overnight US-Israeli airstrike had hit multiple oil facilities. The fires burned for hours, releasing a toxic cocktail into the air. These pollutants bind with atmospheric moisture, forming acidic droplets. Normal rain has a pH of around 5.6, while acid rain is typically defined as having a pH below 5.2.

By morning, Iran's Red Crescent warned the rainfall may have registered as low as pH 4.0, which is "highly dangerous and acidic," with a potential of causing chemical skin burns and severe lung damage.

When the sky turns toxic

The immediate health effects were undeniable. Residents reported severe shortness of breath, burning eyes and headaches. The smoke contained fine particles that enter the bloodstream and are linked to cancer and heart disease. For pregnant women, this rain could leave lifelong consequences for their unborn children.

The environmental stakes go deeper. Severe acid rain can drop to pH 4.0 or lower. When water becomes that acidic, most fish die and water bodies become biologically dead. For a city already facing a water crisis, acid rain contaminating reservoirs could push the situation from critical to catastrophic. With nearly 10 million residents, even a 10% rate of acute respiratory cases could overwhelm hospitals.

A warning from history, a reality in Tehran

History offers some context. During the 1991 Gulf War, retreating Iraqi forces set fire to hundreds of Kuwaiti oil wells. The smoke was immense, yet the worst predictions – including a "nuclear winter" effect spreading globally – never materialized.

The critical difference today is location and intent. Kuwait's oil fields were in sparsely populated desert while Tehran is a megacity. Kuwait's fires were set during retreat; Iran's depots were bombed during active combat. And unlike 1991, the current situation in Iran shows no sign of ending.

If strikes continue, black rain could become Tehran's new normal. For the mother watching her child cough through the night, for the elderly man with no mask to wear, for millions trapped under a sky that burns – this is not "collateral damage." This is a war fought with the air they breathe.