Is impregnated wood safe in my garden or children’s playground?
Construction materials may pose risks to people, if harmful chemicals are released into the indoor air or if there is skin contact. The chemicals that can cause these problems include wood impregnating chemicals, paints, varnishes, solvents, and some components of construction materials such as formaldehyde, or the materials themselves such as asbestos.
Impregnation of wood
There are three major groups of chemicals used to impregnate wood to prevent it being attacked by fungi and other microbes. The most effective method is pressure impregnation, where vacuum and/or pressure are applied in closed cylinders. This helps the chemicals to penetrate deeper into wood. The use of preservatives extends the lifetime of wood products by a factor of five to ten. Some electrical poles and railway sleepers are impregnated with creosote oils. These are tar-like brownish mixtures made by distilling coke oven tar. However, most pressure-impregnated poles or board are treated with water soluble salts of chromium, copper and arsenic. There are also a number of synthetic organic compounds used; pentachlorophenol is the compound with the longest history.
Tars and pitches have probably been the earliest chemicals used to preserve wood. In the Book of Genesis, Noah received the following instructions: "Make thee an ark of gopher wood; rooms shalt thou make in the ark, and shalt pitch it within and without with pitch." As long as ships were made of wood (and sailors of iron, it is said), pitch and wood tar were highly important both in making the vessel waterproof and also conferring resistance towards microbial growth, increasing their durability by many years.
Today creosote is distilled from coal tar, but it can be used only professionally by firms licensed for pressure impregnation. Due to the smell and the risk of smearing clothes, creosote is only used for poles and railway sleepers etc, not for materials in direct contact with humans. Old railway sleepers have been recycled into garden steps and terrace constructions. This practice has been banned in some countries, because there are carcinogenic polycyclic hydrocarbons present in tar and creosote. This sounds overprotective, because after 30 to 40 years in the primary use, there will be very little evaporation of polycyclic hydrocarbons from the impregnated wood. One should note that the asphalt used to cover streets and roads contains a similar mixture of hydrocarbons and is much more likely to cause exposure. On the other hand, creosote treated wood must not be burned as logs in a fireplace; any energy plant using them as fuel must have appropriate burning temperatures and emission controls.
Another traditional way of preserving timber with pressure impregnation has been to use a mixture of copper, arsenic and chromium salts. These confer a greenish tint to the wood. If the process is carried out correctly, relatively small amounts of salts are left on the surface. Thus, skin contact is the most likely route of exposure.
Recently there have been studies suggesting that there may be more leaching to soil than previously thought. It is also possible that some of these salts may end up in the hands of children, and because toddlers tend to eat considerable amounts of sand and dirt, this raises some safety concerns. In fact, some increases in body burden have been measured; maximally the arsenic intake was increased by one third. It has been estimated that children between 2 and 6 years put theirs hands in their mouths around 8 to 10 times every hour. Therefore washing hands of the children is also a very effective means of decreasing intake.
Due to the potential risks, some restrictions to the use of salt-impregnated wood have been introduced in different countries. The European Union restricted the marketing and use of arsenic, and the mixture of copper-arsenic-chromium is no longer permitted for residential or domestic purposes, although it can still be used for poles, bridges etc. Australia does not allow the use, if the product would be in close human contact, such as in playgrounds or furniture. The Environmental Protection Agency of the USA also advised against using these products for similar purposes. This has also lead to the development of new mixtures that do not include arsenic salts. They may not be quite as effective as the original mixture, but can be used when there is reasonable risk of arsenic exposure. The problem with arsenic is that since it is an inorganic substance it will not disintegrate, and gradually accumulates in the soil.
Organic synthetic compounds
The best known of older fungicides is pentachlorophenol which has been in use since the 1930s for many purposes including wood impregnation. Some technical products have contained mainly of tetrachlorophenol. However the synthesis of chlorophenol can also give rise to the creation of dibenzo-p-dioxins and dibenzofuranes, and therefore this group fell into disfavour in the early 1980s. Chlorophenols were mainly used by spraying, brushing or dipping the wood, and to some extent also in pressure impregnation. The typical use was to dip the newly sawed board or plank into a container with the pentachlorophenol solution to prevent blue-stain fungus from damaging the quality of the product. Therefore much of the "normal" board used for homes contained traces of chlorophenols. Since chlorophenols evaporate (though the dioxin impurities do not), some exposure was possible by breathing indoor air.
It is still uncertain which posed the greater risk, the main chemical chlorophenols, or the minor dioxin impurities. Some occupational studies have associated certain types of cancer with these compounds, but because their exposures always go hand-in-hand, it is not possible to determine from occupational studies alone which factor is more important. Some studies on the normal population have suggested that dioxins do not associate with cancer risk; therefore the risk seen in occupational studies is likely to be attributable to the chlorophenols.
In the European Union, chlorophenols are no longer being used. In the U.S. they are only used for utility poles and railway sleepers. Due to the possible adverse effects, other solutions have been actively sought. One alternative is to use naturally weather-resistant wood and here some tree species are better than others. However, the prohibitively high price limits their more widespread use. Their resistance is also based on similar resins to those present in tars, and therefore the jury is still out with respect to their better safety. Another possibility is heating wood to such a high temperature that it decreases its water penetration properties and possibly preserves the wood from rotting. Modification of wood by acetylation of the hydroxyl groups on cellulose makes it less prone to absorb water and therefore more durable. This treatment leaves no foreign chemicals in the wood, so there is no deterioration in its safety.
Wood impregnation is very important and saves timber resources and work especially in permanent structures having to face outdoor conditions. In house construction, the use of impregnated wood is also advisable in some critical sites where it will be subject to moisture. This should not be used as an excuse to be careless with the basic rule of keeping structures dry, since only a dry building is resistant to moulds. Most of the chemicals used are toxic, and therefore care is needed in their use. On the other hand, there is a relatively minimal risk of chemicals leaching out to cause problems with most types of use. In playgrounds and other environments where children are directly exposed, the most toxic compounds, such as those with arsenic, should be avoided.
Wood impregnation is a typical activity where one needs to undertake a benefit-risk assessment. Indiscriminate use can cause harm, but well advised use can be considered to be part of sustainable development.
Notes and references
- Genesis 6:14
- See the chapter "Where do the dioxins come from?"
One level up: A favourite of children - and some adults too
Previous chapter: How does the environment affect the child in the womb?
Next chapter: Can you catch toxoplasmosis from cat litter?