Treated wood exposed to sun, wind, rain, soil, and water may release chemicals onto the wood’s surface. This process is called leaching. The chemical residues may transfer to the skin and clothing of people who touch the wood. The chemicals may also get into soil or water. There are a number of factors that influence leaching and environmental behavior.

The following factors affect the risk of leaching:

• Type of treatment applied to the wood
• Size and shape of the wood (its surface area)
• Soil type in contact with the wood
• Soil moisture and water movement

Metals used in treated wood such as copper and arsenic cannot be broken down. However, they may change their chemical form. This is called speciation. Environmental factors such as pH, temperature, soil moisture, and soil type can affect whether and how speciation occurs. The final form can affect chemical behavior and toxicity. For example, some forms of arsenic are more likely to move in soil and contaminate water than other forms.

You can reduce environmental risks when you need to use treated wood. Consider the following tips:

• Select the appropriate type of treated wood for the job and the setting. Not all types of wood preservatives can be used in the same types of environments.
• Store treated wood on supports to avoid ground contact and cover it until use to avoid leaching with rainfall.
• Contain sawdust and other construction debris. Use a tarp to capture sawdust and wood scraps or saw and drill over hard surfaces that can be swept.
• If the wood is already in place, consider using a tarp to catch sawdust or shavings.
• Avoid hard scrubbing or sanding of the wood surface. This may increase the release of preservatives onto the wood surface and create dust.
• Using sealants can reduce leaching of chemicals from the wood surface.
• Dispose of sawdust and scraps as instructed by your local waste management agency. Never compost them, use them as mulch, or burn them.

Additional Resources:

• Guide for minimizing the effect of preservative-treated wood on sensitive environments - USDA Forest Service
• Environmental impact of preservative-treated wood in a wetland boardwalk - USDA Forest Service
• Methods for Mitigating the Environmental Risks Associated with Wood Preservatives - USDA Forest Service
• Use of preservative-treated wood and alternative materials for building overwater and waterfront structures - California Coastal Commission
• Method for Estimating Wood Preservative Leaching - USDA Forest Service

If you have questions about this, or any pesticide-related topic, please call NPIC at 800-858-7378 (8:00am - 12:00pm PST), or email at npic@ace.orst.edu.