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Specific Wood Preservatives and Components

Wood Preservatives

Acid copper chromate (ACC) contains hexavalent chromium (chromium (VI)) and copper. Chromium (VI) is highly toxic when eaten, inhaled, or if it gets on the skin. It is also a strong skin sensitizer. People who produce or handle ACC-treated wood are usually required to wear personal protective equipment. The U.S. Environmental Protection Agency (EPA) registers ACC wood preservatives for industrial and commercial uses only. There are no residential uses allowed.

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Alkaline Copper Quaternary (ACQ) wood preservatives (types A, B, C, and D) contain copper oxide and a quaternary ammonium compound. They have replaced more toxic wood preservatives that contain chemicals like arsenic or chromium. The U.S. Environmental Protection Agency (EPA) has registered ACQ wood treatments for both residential and industrial uses. These include lumber, fence posts, decking, landscape ties, utility poles, and marine pilings.

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Bis-(N-cyclohexyldiazeniumdioxy)-copper (Cu-HDO) wood preservative is made up of copper (Cu) and N-cyclohedyl-diazeniumdioxide (HDO). Wood treated with Cu-HDO can be used as decking, fencing, sill plates and framing, and gazebos. Cu-HDO is highly toxic to aquatic life. Wood treated with Cu-HDO is not for use in aquatic environments, nor for use in beehives. Cu-HDO is not a skin irritant or skin sensitizer, but it is corrosive to eye tissue.

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Borate wood preservative is made up of disodium octoborate tetrahydrate. Disodium octoborate tetrahydrate is a chemical compound that contains boron, sodium, oxygen, and hydrogen atoms. It is a white powder. Boron is a naturally occurring mineral. Borate wood treatment chemicals are used primarily on wood that will be protected from the weather. They have been used for construction elements such as sill plates and sheathing for over 70 years. Borate wood preservatives are low in toxicity if they are breathed in or eaten. Some forms of borate can be corrosive to eye tissue and some are moderately toxic if they get on skin.

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Chromated copper arsenate (CCA) wood preservatives contain chromium, copper, and arsenic. If you touch CCA-treated wood, residue of chromium and arsenic can get on your skin or clothes. This typically does not cause much exposure. However, people may swallow the residue by eating, smoking, or putting their hands in their mouths before washing them. Children are more likely than adults to put their hands in their mouths so have a higher risk of exposure. Long-term exposure to arsenic can increase the risk of cancer over a lifetime. Because of this risk, CCA-treated wood is no longer used in most residential settings.

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Copper azole wood preservatives can be either type A, type B, or type C. Type C, which is currently the most widely available, contains copper and two fungicides, tebuconazole and propiconazole. Type A (copper, boric acid, and tebuconazole) and Type B (copper and tebuconazole) are no longer produced. Copper azole is greenish brown in color and has little to no odor. It can be used above and below ground, and as decking in freshwater and marine environments.

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Copper naphthenate is used to pressure treat wood. It can also be brushed or sprayed on wood, or applied by dipping the wood into it. Copper naphthenate is an oil-type preservative that protects against insect damage. Products containing copper naphthenate protect wood that will touch the ground or water. Uses include utility poles, docks, posts, siding, fences, and other landscaping features. It is not meant to contact food or be used in vegetable gardens. It can cause skin and eye irritation. When freshly treated, the wood is bright green and has a strong odor. Over time, copper naphthenate weathers to a light brown color.

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Creosote wood preservative is a thick black mixture of distilled coal tar. Creosote-treated wood may only be used in commercial applications; there are no residential uses. Many railroad ties are treated with creosote or a different chemical, pentachlorophenol. Creosote-treated wood can leach chemicals that may dissolve in water, move through soil, and contaminate groundwater. It can also be taken up by plants and animals. Several agencies including the U.S. Environmental Protection Agency (EPA) consider creosote a probable human carcinogen. Learn more about pesticide movement in the environment.

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DCOI or DCOIT is one of several chemicals in the isothiazolinone chemical class that is used for wood treatment. These chemicals kill bacteria and fungi. DCOI is an oil-based preservative used in treating utility poles, crossarms, and railroad ties. It has replaced pentachlorophenol because of environmental concerns. It is not considered to be environmentally persistent.

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MCA and MCQ are very similar to alkaline copper quaternary (ACQ) and copper azole (CA) wood treatments. However, they use very small particles of solid copper, usually in the form of copper carbonate, rather than soluble copper. Micronized copper-treated wood releases less copper to the environment than copper azole, although both amounts are considered to have low environmental impact. The risk of exposure to copper is highest when the wood is handled soon after treatment. MCA is the most widely used waterborne wood preservative in the country.

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Pentachlorophenol (PCP) is a restricted-use pesticide that can only be used by professionals. PCP is considered a "probable human carcinogen" by the U.S. Environmental Protection Agency, and it can also cause other health effects. The U.S. EPA cancelled all uses because risks to workers are greater than the benefits of its use, and because there are other treatments available. However, existing stocks of PCP may be used to treat wood until 2027. PCP-treated wood is not allowed in indoor use, and it cannot contact food, feed, or drinking water. It is used for utility poles, railroad ties, and wharf pilings.

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Wood Components

Arsenic is a naturally occurring element that is widely distributed in soils and minerals. We are regularly exposed to small amounts of arsenic in the air we breathe, the water we drink, and the plants we eat. Old agricultural soils may contain high levels of arsenic from these former uses. Arsenic is now used to protect wood from rot and decay.

Most forms of arsenic tend to stick to soil or sediment particles, but some can dissolve in water. When it dissolves, arsenic may leach into lakes, rivers, or groundwater. Airborne arsenic particles can travel long distances. Burning, sawing, or sanding arsenic-treated wood can increase the chance of inhaling arsenic. Arsenic can leach to the surface of treated wood.

Most arsenic compounds are not well absorbed by your skin so only small amounts are likely to get into your body through your skin. Touching the wood and then touching your mouth, eating, or smoking can lead to the ingestion of arsenic. Long-term exposure to arsenic can increase the risk of cancer over a lifetime. Learn more about CCA-treated wood.

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Chromium is a naturally occurring element that exists in rocks, soil, plants, and animals. Chromium can exist in several different chemical forms. Although chromium (III) is an essential nutrient that helps our bodies use sugar, protein, and fat, the type of chromium used as a pesticide is chromium (IV), which is much more toxic. Chromium (IV) is used as a pesticide to preserve wood. Chromium compounds can be deposited onto soil and water from airborne particles. Chromium in the environment will readily change from one chemical form to another. Chromium (IV) compounds are known to cause cancer in humans who eat or breathe enough of it.

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Copper is a naturally occurring metal that is found in rock, soil, water, and air. Small amounts of copper are necessary for both plant and animal health, but too much copper can be toxic. Copper can bind tightly to organic material in soil. Soil type and amount of organic matter affect how readily copper will move in the soil. Dissolved copper compounds can also bind to organic particles suspended in water and settle out in the sediments of ponds, lakes, and rivers. Copper compounds are often used to control algae in water, to treat plant disease, and to preserve leather, wood, and fabrics.

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Cyproconazole is a water-based fungicide that is used to treat wood for uses above ground. It does not protect the wood from insects. Although cyproconazole is used as a fungicide on some food crops, its wood preservative formulations are typically not for use on wood that will contact food. Some cyproconazole wood preservatives also contain the antimicrobial didecyldimethylammonium chloride, or DDAC. Cyproconazole has been registered for use for either surface application or pressure treatment of wood for uses such as siding, plywood, millwork, shingles, and lumber.

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Propiconazole is a fungicide. It protects wood from rotting, but not from insect damage. It is used as a treatment for wood that will be used above ground. Propiconazole is used in products registered for turf and ornamental plants, food and feed crops, and as an antimicrobial preservative for other materials. Propiconazole and its breakdown products can persist in the environment. It is moderately mobile in soil and plants can take it up from soil. Propiconazole-treated wood is used for surface application or pressure-treating of siding, plywood, millwork, shingles, shakes, and above-ground structural lumber and timbers.

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If you have questions about this, or any pesticide-related topic, please call NPIC at 800-858-7378 or email us at npic@oregonstate.edu.

Last updated July 16, 2026