The health consequences of smoking have been well-documented, but cigarettes can continue to do harm long after they've been stubbed out. As Business Insider reports, cigarette butts account for the largest source of trash in the world's oceans, outnumbering plastic items like straws and water bottles.

Even with cigarette sales sharply declining in recent years, the amount of litter they produce remains significant. It's difficult to recycle filters and often dangerous to throw them in the trash with flammable materials. With public ashtrays scarcer than they once were, many smokers opt to leave the remains on the ground.

Most filters contain cellulose acetate, a non-biodegradable type of plastic. So instead of breaking down over time, the waste washes into streams, rivers, and, when they're disposed of on beaches, directly into the sea.

The Ocean Conservancy estimates that roughly 60 million cigarette butts have been collected from the ocean since the 1980s. Unlike other waste items that pollute the ocean but often get more attention, including plastic bags and six-pack rings, cigarette filters can inflict serious damage on marine life. They contain many of the same chemicals as full cigarettes, including nicotine, lead, and arsenic.

Some unusual initiatives to clean up cigarette butts have been proposed over the years, including training crows to collect them and using them to pave roads, but real change needs to start with cigarette smokers. Instead of leaving them on the sidewalk, filters should be extinguished and safely stored until there are enough of them to send to a special recycling center that handles difficult-to-recycle materials. TerraCycle will even send you a special recycling receptacle designed for collecting cigarette ashes and filters.

[h/t Business Insider]