For smokers, the world has always been one big ashtray, with cigarettes flicked away pretty much anywhere. That’s especially true now, since smokers are increasingly forbidden to light up in restaurants, office buildings and even new no-smoking condos. In the great river of litter human beings create each year, so tiny a thing as a cigarette butt hardly seems to amount to much. But with the world’s smokers burning through a breathtaking 5.6 trillion cigarettes per year — 4.5 trillion of which are simply tossed away outside after they’re smoked — little things add up fast. That, as it turns out, can be especially dangerous for one type of nonhuman critter: fish.

About a third of all of the trash found on U.S. shorelines consists of cigarette butts. There’s no such thing as good litter, but butts may be among the worst, since they’re impregnated with concentrated quantities of the 4,000 chemicals — many of them highly toxic — that occur naturally in tobacco and are added in the cigarette-manufacturing process. In a new paper published in the journal Tobacco Control, a team of researchers headed by Eli Slaughter of San Diego State University’s Graduate School of Public Health sought to determine the kind of harm those poisons can do.

Slaughter and his team broke cigarette waste down into three categories: smoked filters with some scraps of tobacco left; smoked filters with all of the tobacco burned or washed away; and unsmoked filters, which themselves contain a whole stew of chemicals. They immersed samples of each type of butt in separate 2-liter (.5 gal) containers of water and allowed them to soak. In some of the vessels, 16 butts were added to the water, in some 8, in some 4, 2, 1 or just a half a butt. After 24 hours, the butts were removed and fish were added. (More on Time.com: See scenes from the tuna trade)

The two types of fish the researchers chose for their study were the topsmelt and fathead minnow, both common in U.S. waterways. All of the fish were 14 days old or younger. What Slaughter and his team were looking for was what’s known as the LC50 — the lethal concentration of cigarette butt leachate in water that would kill 50% of the sample.

Of the three types of cigarette butts used, they found, it was the filter with traces of tobacco still clinging to it that was the deadliest, with an LC50 of just one butt per liter. Smoked filters with no tobacco had an LC50 of 4.3. Unsmoked filters with no tobacco attached were not far behind, at 5.1. That figure surprised the researchers — but only a little. Even the most pristine cigarette filter is still made of 15,000 cellulose acetate fibers surrounded by paper or rayon and treated with glues, salts and other chemicals to hold it all together and help the cigarette burn evenly. Could any of that be good for you? There’s no telling what the deadliest chemicals in the smoked butts were, but high on the list have to be pesticides (sprayed on tobacco crops), acetone, formaldehyde, benzene, hydrogen cyanide and argon.

There are obvious flaws in the study,not the least being that toxins from cigarettes dropped in or near the ocean get diluted a whole lot more than those dropped in a tiny 2-liter vessel. What’s more, topsmelt and fathead minnows are hardly the only kinds of fish in the sea, and plenty of others may be affected by butt toxins differently. But Slaughter and his team did not intend their study to replicate what actually goes on in the real world; rather they simply wanted to establish toxicity thresholds that can be used as a baseline for further research. They acknowledge a 2002 study by the Royal Australian Society Chemical Institute concluding that littered cigarette butts pose a “low to moderate risk to aquatic organisms.” Low to moderate risk, however, is still not good and that doesn’t account for the “bioaccumulation” factor — the way long-term exposure to cigarette residue can cause toxins to collect in individual fish, and the way those poisons can get concentrated as big fish eat little fish and the chemicals move up the food chain. (More on Time.com: See photos of the Redneck Fishing Tournament)

The simplest fix, of course, would be for smokers to stop tossing their butts wherever they jolly well please. The better answer for any organism smart enough to dream up the idea of cigarettes in the first place is to quit smoking the things altogether and give everybody — on the land and in the seas — a break.

More on Time.com:

Special Report: Environmental Toxins

Photos: Your Doctor Wants You to Smoke

Special Report: The World’s Most Polluted Places