Toxic smoke released by U.S.-Israeli airstrikes on Iranian oil infrastructure has returned to the ground as so-called “black rain,” raising health concerns among international experts and residents in affected areas.

Residents near Tehran have reported irritation and breathing issues last week when the dark, oily precipitation fell following strikes on fuel oil depots and a refinery outside the Iranian capital, the Associated Press reported.

The rare rainfall occurs when dense plumes of smoke and soot rise into the atmosphere and mix with moisture in clouds. As rain forms, it can carry those pollutants back to the surface, creating dark-colored precipitation that may contain harmful particles and chemicals.

Large columns of smoke have been visible across parts of the region during roughly two weeks of escalating hostilities between Iran and forces from the United States and Israel. Iranian forces have responded to the airstrikes by launching drones and missiles at oil and natural-gas facilities belonging to neighboring states in the Persian Gulf.

Environmental and health specialists said exposure to the residue in “black rain” can irritate the eyes and skin and may aggravate respiratory conditions.

Health officials have advised residents in affected areas to limit outdoor exposure during such events, avoid direct contact with contaminated water, and seek medical attention if symptoms such as persistent coughing, burning eyes, or breathing difficulties develop.