Located at a horseshoe bend of the Monongahela River in Washington County—the heart of today’s Rust Belt—what would become Donora, Pa.was a farming community of 12 people in 1900. Within one year, it exploded to 4,000—as hordes of workers built and manned numerous factories. Its population topped 14,000 by 1920, but today numbers only 5,500. …