Two broad categories of wood preservatives are used in industrial and commercial applications: oil-borne preservatives, which include pentachlorophenol (penta) and creosote, and the water-borne preservative chromated copper arsenate (CCA).

The level of protection required is determined by hazard exposure (i.e. climate conditions, whether the wood will be in ground contact or in a special environment such as salt water), what is expected of the installed product (service life and structural integrity) and the potential costs of replacing damaged wood.

Creosote

Creosote is the oldest and one of the most effective industrial preservatives for protecting wood from deterioration and decay caused by fungi, insects and marine organisms. It is virtually insoluble in water.

Creosote has been applied to a large variety of wood products for more than 150 years. It is used primarily for railway ties (where it is often blended with a heavy petroleum oil), utility poles, marine piling and timbers and highway construction.

Creosote is derived from coal tar which is formed during high temperature carbonization of bituminous coal. This carbonization process takes place during the making of coke by the steel industry. The coal tar is then distilled to produce creosote and other products.

Many factors affect the character and composition of creosote, including the characteristics of the coal which is used, the method of coal tar distillation and the temperature range in which the creosote fractions are collected. Over 150 chemical compounds have been identified.

Common sense dictates that the creosote preservative must be handled with care. Studies conducted on human workers exposed to creosote indicate there is no increased incidence of cancer or cancer-related deaths. Further studies corroborate that exposure to creosote fumes is not associated with any observed significant adverse health effects, including cancer.

Pressure-injected creosote does not exude or move out of the wood in a liquid form to any appreciable extent. According to several studies, this is true not only for wood in land use but also for properly treated piling and timbers in the marine environment. Furthermore, creosote in small concentrations is biodegradable. Tests indicate that creosote is low in toxicity to birds and moderately toxic to fish.

Initially, creosote treated wood is dark brown to black in colour and weathers to a light brown. It has distinct odours which diminish with age. Because creosote is oily, the treated wood is somewhat water repellant. This improves the wood's dimensional stability and reduces checking and splitting. Creosote treated wood is also more resistant to mechanical wear, which is of vital importance for such applications as railway ties and bridge decking.

Pentachlorophenol (Penta)

Pentachlorophenol (penta) is the most common oil-borne preservative used industrially for the long-term protection of wood against attack and destruction by fungi and insects. It has been used in Canada for almost half a century.

Oil-borne penta is used extensively for the treatment of utility poles. It is also used for railway ties, foundation piling, timbers in highway construction, construction timbers and poles, and fence posts.

Pure pentachlorophenol is a white, crystalline, aromatic compound. It is produced by reacting phenol with chlorine. Technical penta used in wood preservation usually contains about 10% related chlorophenols such as tetrachlorophenol and some trichlorophenol. It also contains some impurities such as octa-, hepta, and hexachlorodibenzo-p- dioxins and chlorodibenzofurans at levels of parts per million. It is important to note that the highly toxic tetrachlorodibenzo-p-dioxin (2,3,7 ,8-TCDD), which has received much attention in the media, has NOT been found in penta.

Pentachlorophenol manufacturers comply with all the requirements of the U.S. Environmental Protection Agency and impurities are below stated limits.

For wood preservation treatments, pentachlorophenol is dissolved in petroleum oil conforming to CSA 080.20 1 "Standard for Hydrocarbon Solvents for Preservatives". Penta treating solution concentrations range from 3% to 7% by weight, depending on the wood products and species to be treated.

If, through accidental exposure, pentachlorophenol is absorbed, inhaled or ingested, it does not accumulate in the body (the half-life is about 36 hours). Thus, after exposure, penta is released from the body relatively quickly in the urine. Studies carried out on treatment plant workers exposed to penta for up to 20 years show no long-term adverse health effects. Basic, common sense measures make penta safe for use.

While pentachlorophenol treated wood is safe for many uses, plants in direct contact with penta treated wood could be adversely affected. Therefore, it should not be used in greenhouses or next to house and food plants. Nor should it be used in home interiors or salt water applications.

However, pressure-impregnated penta does not evaporate or exude from wood to an appreciable extent. According to several studies, this is true not only for wood in ground contact, but also for properly treated piling and timbers in fresh water applications. It is also important to note that low concentrations of penta are biodegradable and subject to photodecomposition.

Penta treated wood is light to dark brown in color, depending on the oil used and the species treated, and usually weathers to a silvery gray over time. It is also somewhat water repellant. This improves the wood's dimensional stability and reduces checking and splitting. The wood is also more resistant to mechanical wear. The oil lubricates the wood, facilitating spur penetration when climbing poles.

Oil-borne penta also slows corrosion of metal fasteners. It is resistant to a variety of corrosive chemicals and is a suitable treatment for several types of chemical storage and conduit structures.

Chromated Copper Arsenate (CCA)

The base ingredients from which CCA treating solutions are made are hexavalent chromium, cupric oxide, arsenic pentoxide and water. Treatment solution concentrations usually range from 2% to 4% by weight depending on the species of wood to be treated and the product's end use.

It is important to note there are significant differences in the chemistry and toxicity of arsenic compounds. Inorganic pentavalent arsenate, one of the primary active ingredients in CCA, is a naturally-occurring trace element which is present in soil, water, air, plants and in the tissues of most living creatures – including humans. It should not be confused with trivalent arsenic compounds which are generally more toxic than pentavalent compounds and never used in the pressure treating process.

The pentavalent arsenate found in wood preservatives is also the most prevalent arsenic compound in nature. It is rapidly excreted by the kidneys and does not accumulate. Again, it is important to note that chronic inorganic arsenic intoxication is associated with trivalent arsenic, which is not present in CCA solutions.

The chemical reactions that fix the CCA components in the wood are complex. For example, in general terms, the pressure-injected chemicals in CCA are reduced to their metallic state and become bonded to the cellular structure of the wood. These highly insoluble precipitates are virtually immune to leaching (seeping from the wood) and will not vaporize or evaporate under normal conditions.

Many of the allegations about CCA wood preservative are anecdotal and groundless. To date, all evidence collected on the toxicity of CCA treated wood shows no health hazard, even by very exaggerated contact to treated wood. (The oral lethal dose of treated wood for a 70 kg human would be more than half a kg. Even then it might not prove fatal, because the digestive system would likely be unable to liberate all the arsenic from the wood.)

Studies show that properly impregnated wood retains its CCA preservatives in virtually undiminished quantities for decades. Environmental risks from properly processed CCA treated wood are negligible.

CCA treated wood is light green in colour and weathers to a driftwood grey over time. Seasoning after treatment leaves the wood dry, paintable, odourless and clean. This is preferred for applications where there is close human or animal contact. Water repellents, colour, mould inhibitors and additives to improve the climbability of poles can be applied during the treating process. While the oil carriers in penta and creosote impart water repellency to wood in the normal course of treating, CCA do not provide such weathering protection. Applying a water repellent additive during pressure treating slows the initial moisture loss of the wood and reduces shrinkage as the wood comes into equilibrium with the environment. Water repellents also help reduce effects of weathering on wood in use, thereby reducing checking and the rate at which the wood turns grey.

Similarly, colour pigments can be added to the preservative to mask the characteristic soft green of CCA, mould inhibitors can keep wood looking clean in excessive humidity conditions, and various additives can be used to "soften" poles so that they are more easily climbed.