You might not think cigarette butts could be that bad for plants.

Cigarette butts are the world’s most commonly littered item. Around the world, cigarette butts are tossed. After all, what is a cigarette, really? It’s tobacco, which is a plant. It’s paper, which biodegrades. So what could be so harmful? Researchers from Anglia Ruskin University, in the UK, sought to find out what effects discarded cigarette butts have on plant life. 

Cigarette butts are tossed even by people who would never normally litter. It’s assumed street cleaners will get them, or that, at worst, they’re just harmless pieces of paper. Despite the increasing restriction of where smoking is allowed, cigarette butts are still on city streets. And you know what else is on city streets? (Hopefully?) Trees, plants, and grass.

The three major components of a cigarette are shredded tobacco, paper, and the filter. The tobacco, when still burning, isn’t great for plants, but it’s not a major issue. Neither is the paper. But the filter, the researchers found, has some problems. Cigarette filters are made from cellulose acetate, a type of plastic. Chunky plastic eyeglass frames are made from this material. But if you shred it, you end up with a sort of foamy fiber, and that’s what serves as the filter in most modern cigarettes.

The researchers placed cigarette filters, both used and unused, in planters growing clover and grass. They found that the filters dramatically reduced the health of these plants, which normally can grow just about anywhere. The butts reduced the stem length, root weight, and germination rates significantly.

Exactly why the cigarette butts have this effect is unknown. The researchers think the actual plastics in the butts are doing this, having found the same effect from both smoked and unsmoked filters. There’s also the possibility that some of the additives in the filters, which make them more flexible, could be leaching out.

More research is needed, but in the meantime—don’t litter your butts!