Harjavalta, Finland has been a smelting town since the 1930s, with most of the local economy built around metal production. It’s also a favorite summer breeding spot for Pied Flycatchers. But nearly three decades ago, scientists noticed that the visiting birds weren’t having as many chicks as usual. They traced the culprit back to the town's sole copper smelter: Sulfur oxide and heavy-metal particulates from the factory were wreaking havoc on the female flycatchers’ metabolisms, causing them to produce thin-shelled eggs that wouldn’t hatch. Around the same time, a different team of scientists discovered smelter workers had higher incidences of respiratory and stomach cancers. Both birds and people were harmed by the noxious mix of pollutants in the air—but birds showed symptoms years earlier. Today, the Harjavalta copper smelter has cut toxic emissions by nearly 99 percent. Working conditions are much safer, and the Pied Flycatcher population has slowly rebounded as...