Over-exposed: TBTO Review

Infants in homes treated with tributyltin oxide (TBTO) containing wood preservatives could be exposed to 25 times as much of the chemical as the operator who applies it, according to a Health and Safety Executive (HSE) evaluation. TBTO has a very high acute toxicity to mammals and is classified by the World Health Organization as highly hazardous. It is an eye and respiratory tract irritant and is also on the UK Department of the Environment ‘Red List’ of water polluting chemicals. It is also known to have an adverse effect on marine life and concentrations as low as 2 parts per billion in sea water have been shown to cause dog whelks to change their sex. TBTO has been widely used over many years in the construction industry to protect timber from fungal attack, and in marine environments to prevent the fouling of ship hulls.

Restrictions already imposed

In 1987 its use as an antifouling paint was restricted in the UK to ships over 25 metres in length and its concentration in antifouling paints still in use was limited to 1%.

    In its evaluation the Advisory Committee on Pesticides (ACP) now recommends that professional use of liquid TBTO be allowed to continue provisionally but only where it is used in the pretreatment of timber and is applied in closed vacuum/pressure immersion tanks. The maximum working strength of TBTO is also to be restricted to 1.5% by weight, and use of professional TBTO-containing  paste products and joinery lining formulations is to be allowed to continue.

    The current review focused on occupational exposure to TBTO and is based on monitoring experiments carried out by a group of companies. Although the participating companies are not identified, the implication is that this is a rare case of data sharing between companies which have an interest in the continued use of TBTO.

Exposure in the home

Calculations of potential operator and consumer exposure to TBTO used in timber treatment suggest that people and in particular children, living in homes where timber is treated with TBTO containing pastes, can absorb more of the chemical than the operators who apply it. According to the calculations in the ACP evaluation, an adult consumer is exposed to twice the dose to which the person who applied the chemical is exposed. An infant in the same home could be exposed to 25 times as much TBTO as an operator who would be wearing protective equipment. 

    Asked for their opinion on this potentially worrying anomaly, the Health and Safety Executive said that: “The independent Advisory Committee on Pesticides has examined the assessments of exposure from the use of approved paste type and joinery lining type TBTO containing wood preservatives in the home, and determined that provided they are used in accordance with their approval conditions there is no cause for concern regarding exposure levels”.

    In its evaluation, the ACP concluded that the non-enclosed use of liquid formulations should be discontinued over the coming three years. The amateur and professional use of liquid TBTO-containing products for on-site timber treatment was revoked in 1990.(MD)

Advisory Committee on Pesticides, Tributyltin oxide use in wood preservation, Health and Safety Executive Pesticides Registration Section, 1993.

[This article first appeared in Pesticides News No.21,September 1993, page 19]