The eastern Ukrainian port city of Mariupol is less than 10 miles from the front line of the country’s long-simmering conflict with Russia, and in the years since 2014, its residents have suffered shelling, rocket fire, and the relentless anxiety of life in a war zone. But some are worried about a less tangible threat that may be even more dangerous: pollution from the two ancient, hulking iron and steel plants that drive the city’s economy.

Mariupol is one of the most polluted cities in a nation whose air is among the dirtiest in Europe. Mired in war with its mighty neighbor, beset by corruption and dominated by a small number of super-rich oligarchs who wield outsize