Cigarette butts are the most common form of litter worldwide. Trillions are discarded every year in cities, parks, beaches, along railway tracks and roadside environments. Despite their small size, these remnants of smoked cigarettes represent a persistent form of pollution because their filters are made primarily of cellulose acetate—a plastic polymer derived from natural cellulose and highly resistant to environmental degradation and produced as tightly packed microscopic fibers.

A long-term study has now reconstructed what happens to cigarette filters once they enter the environment. By tracking their transformation over an entire decade, the research reveals that cigarette butts undergo a complex sequence of physical, chemical and biological changes—but they do not fully disappear. Instead, they slowly transform and persist in soils as microplastic-like residues.

The results, published in Environmental Pollution, offer one of the most comprehensive pictures so far of the environmental fate of cigarette filters and highlight the long-term nature of this type of pollution.