Wood used for construction, telephone poles, railroad ties, and garden beds may be treated with pesticides to prevent rotting. Wood treatments are not meant to be water resistant. These products are used to extend the life of wood by limiting damage from insects and mold. Treating wood also reduces the waste of forest resources by delaying the need for replacement due to decay.

The two main groups of treated wood are water-based and oil-based. Water-based preservatives are applied to wood with a water solution. Oil-based products are applied to wood in an oil-based solution. Surfaces of oil-based treated wood may not be paintable and odors may be stronger.¹

Manufacturing methods for treating wood include pressure treatment, dipping, or soaking. Some paint-on products are also available.² Each type of wood preservative has a different mixture of ingredients. Only a few wood preservatives are discussed in this fact sheet. For more information, visit our webpage about the different types of wood preservatives.

Treated wood should have an end tag attached to the end of the board. End tags may look similar to Figure 1. Proper use sites are written on the tag as a "Use Category." It will also state if the wood is intended for use indoors or outdoors, and for water, ground, or above ground contact. The end tag will also list the type of preservative used on the wood. Using the proper treated wood in the right situation will help to limit the risks of exposure to the pesticide.

Figure 1. Example of Treated Wood End Tag

When exposed to any pesticide, people may react in different ways. Some may be more sensitive than others. If someone were to touch the treated wood or inhale dust from the wood, there may be different levels of risk or toxicity. The risks may increase if the wood is cut or burned.

The risk from any pesticide depends on both the toxicity of the product and exposure. Tables 1 and 2 can be used to compare the toxicity levels of some wood preservatives.

Treated wood selection may depend on use location, cost, and other factors. The American Wood Protection Association (AWPA) sets voluntary standards for how to reduce wood decay through different treatments. These standards are based on wood durability, not health risk. Local building codes may specify the AWPA category or type of treated wood for a project. An Evaluation Report from the International Code Council-Evaluation Service (ICC-ES) may confirm that a certain type of treated wood complies with building codes.²

• Consider contacting your local County Extension Service or Master Gardeners for possible guidance with wood selection.
• Most treated wood is not for indoor use. Incorrect use could pose health risks.
• An infographic guide from AWPA can help to determine which type of treatment may be right for a project. Home use may include a fence, deck, or garden bed.
• Call the National Pesticide Information Center to compare risks between treated wood types, 8:00am–12:00pm PT Monday through Friday, or email at npic@ace.orst.edu.

Risks related to treated wood depend on the use site and type of treated wood. Using the correct type of treated wood for your project may help keep the risks low. Whether wood is used in a garden bed or another structure, keep these tips in mind:

• There may be substitutes if you choose to avoid treated wood. Consider untreated hardwoods, concrete, or plastics.
• Treated wood may be covered with heavy plastic or coated to limit leaching. It may take multiple coats of a sealant.¹⁶
• Small amounts of treated wood ingredients in play structures, decks, or picnic tables may transfer to skin or clothing from contact. Less copper came off on wipes after copper azole (CA) and micronized copper azole (MCA) treated boards were weathered for about a year.¹⁷ Consider using a tablecloth on picnic tables made from treated wood to reduce skin and food contact.
• Re-use of treated wood is not regulated by the EPA. Therefore, the risks of using recycled wood have not been evaluated by the agency.
• Adults and children may consider washing hands after touching treated wood before eating, drinking, or using the restroom.
• Do not burn treated wood and do not use ash or compost from treated wood in edible gardening.

• If cutting or sanding treated wood, use a dust mask to limit the chance of inhaling the treated sawdust. Consider wearing goggles and gloves to reduce exposure to eyes and skin.
• Dusts can build up in closed spaces. Work outdoors or in a ventilated space.
• If preservatives or sawdust have contaminated clothes, launder prior to wearing them again. Wash them separately from other clothes.
• Collect and dispose of sawdust from treated wood. Avoid contamination of nearby soils and water.

Certain types of treated wood may be used in backyard projects. Compounds like chromated copper arsenate (CCA), creosote, and pentachlorophenol are not registered to treat wood for consumers, including use in garden beds.^14,15,18 The risks of using recycled wood treated with CCA, creosote, and pentachlorophenol have not been evaluated by the EPA. However, it is expected that the amount of leaching from used wood is less than from freshly treated wood.^8,14,15

If compounds leach from treated wood, they may be available for uptake by garden plants. However, it is difficult to predict the amount of plant uptake. Materials used in treated wood that leach into soil may be bound to soil. They also may change into different forms that plants cannot take in through their roots. Information about plant uptake is limited.

Plant uptake may depend on many factors, including plant type, soil type, and distance to the treated wood. For some components of treated wood, levels may be higher in the roots and fibrous plant parts.¹⁹ Other studies suggest accumulation in the leaves.²⁰ The amount of root uptake can also vary by plant or crop, even when grown in the same soil.¹⁶

• Generally, preservatives tend to move less in organic-rich soils.²¹ In one study, adding compost to soil reduced the amount of arsenic taken up by carrot and lettuce plants.²²
• Consider planting edible crops away from the edges of garden beds. Soils next to treated wood are likely to have more chemicals from the wood.
• Peel root vegetables and wash soil from foods grown in a garden bed with treated wood. Soil can have higher levels of preservatives than the foods themselves.²³
• The amount of uptake of compounds from treated wood by plants is highly variable.

Many factors can affect whether a preservative can leach out of treated wood. Studies show that leaching can be highly variable. The type of preservative or wood and the manufacturing process can affect leachability.⁴ Moisture, soil type, soil contact, and soil acidity can also affect leaching.^{4,12,24,25,26} Metals from CCA, ACQ, and CA were least mobile in organic (compost-rich) soils.²⁷ Most leaching occurs in the first few months of use.^4,8,21,28

Once an ingredient leaches from treated wood, it may stick or bind to soil, making it less mobile. The distance a preservative may move could depend on the soil, age of wood, and type of preservative. Metals introduced into soil do not readily move downward following leaching, as most bind to soil. Introduction into groundwater may be limited.^15,26

Rules about the disposal of treated wood may differ in each state. Treated wood that has not been fully weathered (new boards or poles) may be considered hazardous waste. If treated wood is not considered to be hazardous waste, it may be disposed of at a landfill or through municipal trash collection. Contact a hazardous waste program in your state for specific regulations about treated wood.

Do not burn treated wood. Chemicals in treated wood may become more harmful if they are burned and inhaled. Treated wood should never be used as compost or as mulch.

Only specific pesticides defined by the U.S. Department of Agriculture (USDA) may be used to produce organic foods. Lumber that contacts food, animals, or soil may only be treated with substances on the National List of Allowed and Prohibited Substances. Certain ingredients, such as arsenic, are not allowed for use in organic systems. Different rules may apply if treated wood will not contact soil, animals, or food. Consider contacting a certifying agent for details on how to comply.