Walk down almost any city street, beach, or park, and you’ll see them: cigarette butts scattered along the curb, tucked into sidewalk cracks, or washed up along shorelines — 4.5 trillion of them. They’re now so common they’ve become nearly invisible. But that ubiquity masks a growing environmental crisis—one that has only intensified as nicotine products evolve. For decades, cigarette butts have been the most littered item in the world.