A thick cloud of black smoke and a toxic haze has blanketed Thessaloniki after a major overnight fire in the Oreokastro suburb, prompting authorities to urge residents to remain indoors.

A 112 emergency text alert issued at dawn advised people to keep doors and windows closed and follow instructions from the authorities as smoke and the smell of burning spread across the city.

Dimosthenis Sarigiannis, professor of environmental engineering at the Aristotle University of Thessaloniki, said the haze consists of fine particulate matter produced by incomplete combustion. He warned that the fire’s spread to industrial facilities containing recycling materials, detergents and oils had made the smoke more hazardous.

“The smoke contains volatile organic compounds that irritate the eyes and throat, as well as carcinogenic substances such as benzene, dioxins and furans,” he said.

Sarigiannis urged residents to wear masks if they must go outdoors and advised against using pressure washers to clean ash from balconies, warning this could re-release harmful particles into the air.