Tin poisoning – wasn’t even grandma’s copper kitchenware tin-plated?

Dredging of old harbours has brought tributyltin (TBT) and other organic tin compounds into the headlines. Metallic tin is not poisonous unless it contains lead. Tin plating has in fact been used since antiquity to prevent toxic, especially stomach irritating copper compounds being released from copper kitchenware. Tin is also a common metal used for soldering milk cans and other steel plate containers. Some inorganic tin compounds are approved food additives (E512, tin chloride). Organic tin compounds are much more toxic than the metal itself or inorganic tin salts, but they are not nearly as toxic as organic mercury compounds.

Boat paints

Tributyl tin and triphenyl tin have been added to paints used on boats and ships as antifouling agents, since they are toxic to algae and barnacles, preventing their growth and attachment to the vessel’s hull. The aim is to achieve a similar kind of selective toxicity as antibiotics exert towards bacteria. Unfortunately tin compounds are rather toxic to fish as well, and therefore they are by no means ideal biocides.

Because of their environmental harm, the use of organic tin compounds in small boats was banned already in the 1990s, and the European Union prohibited their use in all ships in 2003. However, lots of tin compounds are still to be found in sea sediments in harbours and along busy shipping routes. The compounds accumulate to some extent in fish, especially their liver, but they do not bioaccumulate in the same sense as methyl mercury, to say nothing of the PCBs and dioxins.

Tin compounds have been used also in some consumer products such as plastics and textiles. Therefore they can be found in clothes and shoes. In some countries, they have been used as pesticides but only to a limited extent.

Effects

Organic tin compounds have caused concern lately because of their hormone disrupting potency. In some snails and other marine molluscs they cause what is called “imposex”, partial masculinisation of females. There is some information that imposex may occur in fish as well. There is no evidence of similar effects in mammals, but after very large doses, developmental effects have been reported in several species. At very high doses, organic tin compounds are also neurotoxic, but there is no indication this would be a problem in humans, e.g. as an occupational risk.

The most sensitive effect in mammals seems to be immunotoxicity. Tin compounds cause a shrinkage of the thymus gland, and because the thymus is such an important organ for the development of immune defence mechanisms, the risk assessment of tin compounds is based on these effects.

In rats, the highest level causing no adverse effects (NOAEL, no-observed-adverse-effect-level) has been shown to be 25 micrograms per kilogram per day. In a human being (60 kg) this would convert to 1500 micrograms per day, and if we wish to use a hundredfold safety margin, the tolerable daily intake (TDI) would be 15 micrograms per day.

Exposure levels

TBT concentrations in fish vary, but the highest concentrations are found in the liver, an organ that is not often consumed in most fish. However, in burbot liver, which is a well known delicacy, the levels may be high. In filets from fish caught in contaminated areas, the organic tin concentrations are in the range of 50–60 micrograms per kg, and maybe even higher. Therefore a fish meal may sometimes lead to an intake higher than the TDI.

The TDI value assumes a daily intake over the lifetime to guarantee safety in all conditions. Therefore an occasional higher intake by an angler is not of too much concern. The general recommendation to consume several species of fish rather than the same one kind all the time is all the advice one should heed in this respect. The safety margin of organic tin compounds is clearly better than that of organic mercury compounds. A thousand micrograms (1 mg) of methyl mercury are found in some fish per kg, and methyl mercury is clearly more toxic and more accumulating than tributyl tin.

The conclusion is that organic tin compounds are primarily an environmental rather than a health problem. On the other hand, the information is somewhat fragmentary, and both toxicity and exposure should be investigated further to settle the argument.

Tributyltin and triphenyltin are biocides that have been banned due to their environmental effects. There is every reason to investigate their levels in bottom sediments of harbours and shipping routes, but their toxic effects to humans are likely to be much less than those of methyl mercury.

Notes and references