So says Dr. Tom Novatny, head of the Cigarette Butt Eradication Project. He and a team of researchers at San Diego State University found that just a single filtered cigarette butt could kill fish in a liter of water in as little as four days.

"The most important finding in this research is that it seems to be the filter, or rather what's in the left-over filter that is most dangerous to our water," said SDSU public health researcher Richard Gersberg. Cigarette filters are made of non-biodegradable cellulose-acetate plastic, but their intended function -- soaking up tar, smoke particles and noxious chemicals from a lit cigarette -- is what makes them so toxic. So all that stuff you don't want in your lungs leaches out into the water for the fish to ingest instead.

Cigarette butts are the most littered item in America -- it's estimated that billions of butts end up in the world's waterways each year. And if you think about it a step further, they're ending up in landfills in vast quantities, too, where they can leach their toxic contents into our water supply. Yet another really good reason to stop smoking.