What would you say is the most littered item on US roadways? I think of two things: gum and cigarette butts. But let’s focus on cigarettes for now. Cigarette filters are made from cellulose acetate, a plastic which is technically biodegradable. However, cigarette butts only degrade under conditions described by researchers as “severe biological circumstances,” such as when filters end up in sewage. Even under optimal conditions, it can take at least 9 months for a butt to degrade.