Over decades, the factory, Titan, formed a gigantic effluent pond of sulfur waste that has acidified. Russian authorities say the pond is the source, while Ukrainians point to current operations at the factory.

Residents who remain in the area close their windows and stay indoors.

Armyansk, a city on the Russian-controlled side with a population of about 22,000, was an eerie ghost town when Abdureshyt Dzepparov drove through in early September. “It was unnatural,” he said. “There were few people in the town. The leaves had fallen off the trees, though it was very early fall, and the grass in the fields was black.”

Before the Russian annexation of Crimea, a canal diverting water from the Dnieper River in mainland Ukraine had fed the pond during dry summers. Ukraine, which along with the United States and European nations has refused to recognize the annexation, has since cut this water supply, drying up the pond, concentrating the acid and leaving a residue that is now blowing about.

Residents on both sides of the border have complained of coughing, allergies and a burning sensation in the nose and throat.