On an unusually hot day in Los Angeles, Braddock Sandoval perused tables heaped with wigs, a Darth Vader mask, a black tutu, and various other garments.

Sandoval, a chemistry professor at Occidental College, had dropped by a campus clothing swap aimed at keeping unused Halloween items out of the trash. Looking to pick up a free outfit, he selected a Hawaiian flower necklace and a shark costume to try on.

"I think there’s too much cheap plastic waste in this world. So the idea that people are recycling costumes is fantastic, both for budgets, like me, but also just for the planet," Sandoval said with a laugh. Guided by his finds, he planned to attend an upcoming Halloween dance party dressed as a great white shark eating a tourist.

Each October, Halloween outfits take center stage, but millions are thrown into landfills after being worn just once. Most are manufactured from synthetic plastics that do not readily decompose, aggravating environmental pollution.…