Plague Maker

July Fourth: New York City

Hundreds of thousands line the banks of the East and Hudson Rivers awaiting the nation’s largest fireworks display. Soon the sky will explode in cascading showers of silver and gold. Everywhere, faces will turn skyward in wide-eyed wonder.

Then the sky will grow dark again—but it will not be empty. The air will be filled with clouds of smoke and specks of debris will rain down everywhere. Some will pick bits of paper from their children’s hair. Some will brush away ...