Cigarette butts are toxic to fish and should be labeled as toxic hazardous waste, U.S. researchers say.

Scientists at San Diego State University say that a single cigarette butt containing a small amount of unburnt tobacco is enough to contaminate a litre of water and kill half of the fish swimming in it.

"Based on this new research, we believe that cigarettes should be considered toxic waste and new requirements need to be established for how they are disposed," Tom Novotny, a public health professor at San Diego State University, said in a statement.

The researchers tested the toxicity of the tobacco on fresh and saltwater fish: fathead minnows and top smelt — two species that the U.S. Environmental Protection Agency typically uses in pollution studies.

The cigarette butts were left soaking in the water for a day before fish were placed in it for testing.

Whole cigarettes and cigarette butts with unburnt tobacco were found to be the most toxic, but even filters that had been smoked and that had no tobacco left on them were found to be toxic.

Cigarette filters are made of cellulose-acetate, which does not biodegrade.

The researchers presented their conclusion at a meeting of the American Public Health Association in Philadelphia earlier this month, and have submitted their study for publication in a peer-reviewed journal.

Cigarette butts are considered the most littered item in the world. Novotny recently estimated that 767 million kilograms of cigarette butts — or about 4.5 trillion butts — end up as litter every year.