Discarded cigarette butts, which end up in waterways, are harmful to fish, indicates research published today in a special supplement of Tobacco Control.

Cigarette butts are the most common form of environmental litter in the world, with around 5.6 trillion cigarettes smoked every year. Cigarette waste accounts for almost a third of the total amount of litter found on US shorelines alone.

A wide range of chemicals are used during tobacco cultivation and cigarette manufacture, residues of which often remain in the final product, say the authors.

They therefore tested whether discarded cigarette butts are harmful to a representative sample of marine fish, the topsmelt (Atherinops affinis) and a freshwater species, the fathead minnow (Pimephales promelas).

They looked at smoked cigarette butts (smoked filter plus tobacco remnant); smoked filters (no tobacco); and unsmoked cigarette filters (no tobacco) in a bid to find out if the harmful effects were concentrated in the filter or remnant tobacco.

Both artificially and naturally smoked cigarettes were used, with the various types of butts submerged and left to soak in diluted mineral water (freshwater) and natural seawater for 24 hours to allow the contents to leach into the water (leachate).

The results were then turned into a "stock" diluted to six different strengths, each of which was divided into four, and five fish introduced, so that there were 20 fish at each strength dilution. The fish were left for four days (96 hours).

The findings showed that leachate from all three types of butt was harmful, with the most toxic being smoked butts with tobacco remnant.

These butts were highly toxic to both the topsmelt and the fathead minnow, with an LC50 - the lethal concentration that kills 50% of the sample - of 1 cigarette butt per litre of water.

Leachate from smoked filters with no tobacco remnant was also very toxic, with an LC50 of 1.8 butts per litre of water for the topsmelt and an LC50 of 4.3 butts per litre of water for fathead minnows.

Somewhat surprisingly, although the least harmful of the three, unsmoked filters with no tobacco were still toxic to both species, with an LC50 of 5.1 butts per litre of water for the marine fish and of 13.5 butts per litre of water for the freshwater fish.

This might be explained by the fact that the filter of a filter tipped cigarette comprises 15,000 cellulose acetate fibres. These are treated with chemicals to bind them together on top of which they are surrounded by paper and/or rayon wrapping, which also contain chemicals, such as glues and salts to maintain burning while the cigarette is being smoked, say the authors.

Overall, fish were less sensitive to cigarette butt leachate than invertebrate organisms, but they displayed similar levels of sensitivity as marine bacteria, say the authors.

The authors say their findings add to the accumulating research on the toxicity of cigarette butt leachate to various acquatic organisms.

British Medical Journal