Yes, pressure treated wood does leach chemicals into surrounding soil and water. The amount and type of chemicals depend on when the wood was manufactured, what preservatives were used, and environmental conditions like rainfall and soil type. Modern formulations leach far less toxic compounds than older versions, but some chemical migration is unavoidable with any treated lumber.
What’s in Pressure Treated Wood
The chemicals in your treated wood depend almost entirely on when it was made. From the 1970s through 2003, most outdoor residential lumber was treated with chromated copper arsenate, commonly called CCA. This preservative contained chromium, copper, and arsenic. At the end of 2003, CCA manufacturers voluntarily pulled the product from virtually all residential uses due to health concerns about arsenic exposure.
Today’s pressure treated wood uses copper-based alternatives. The most common are alkaline copper quaternary (ACQ) and copper azole, both water-based preservatives that protect against fungal decay and insect damage. ACQ combines copper oxide with quaternary ammonium compounds. Other options include copper naphthenate and copper-HDO, first registered in 2005. None of the ingredients in these modern formulations are classified as known or suspected carcinogens by the EPA.
How Much Actually Leaches Out
Research from the USDA Forest Products Laboratory measured the leaching from CCA-treated decking under simulated rainfall. Arsenic release ranged from 0.16% to 0.72% of the total arsenic content, depending on rainfall intensity. Slower, lighter rain actually produced more leaching per unit of rainfall than heavy downpours, likely because water had more contact time with the wood surface. Based on these rates, CCA-treated decking contributed roughly 0.5 to 1.2 parts per million of arsenic to the soil beneath a deck each year.
Modern ACQ and copper azole lumber also leaches, but the chemicals involved carry a very different risk profile. Small amounts of copper and quaternary ammonium compounds migrate into surrounding soil with rain. The EPA does not classify any of these ingredients as hazardous. Lab studies comparing sawdust from modern micronized copper azole wood and older chromated copper wood found that particles from modern treatments showed no toxicity to human lung cells at tested concentrations, while particles from older formulations reduced cell viability to 63% at the highest dose.
What Affects the Leaching Rate
Several factors determine how quickly chemicals leave the wood. The most important is age: leaching is fastest during the first few months after installation and then drops substantially as the most loosely bound preservative washes away. Wood with higher retention levels (more chemical per cubic foot) releases more, and surfaces with exposed end-grain lose preservative faster than face-grain surfaces.
Environmental conditions matter too. Acidic water and water containing dissolved organic acids pull preservatives out of wood more aggressively. Soil type controls how far those chemicals travel once they reach the ground. Sandy soils allow the most mobility, clay soils moderate it, and organic-rich soils bind preservative components most effectively, limiting their spread. High water flow, whether from irrigation or heavy rain, accelerates the process.
Raised Garden Beds and Food Safety
This is the question most people are really asking: is it safe to grow vegetables next to pressure treated wood? For modern ACQ and copper azole lumber, the answer is reassuring. The EPA considers nothing in these formulations hazardous, and there is no evidence that food safety is compromised by growing vegetables in beds made from current-generation treated wood.
Older CCA-treated wood is a different story, though the risk may be less about your food and more about direct contact. Research has shown that high levels of inorganic arsenic in soil will actually kill a plant before enough arsenic accumulates in its tissue to pose a dietary risk. The bigger concern with CCA lumber is skin contact, particularly for children, who are more vulnerable to arsenic exposure from touching contaminated surfaces and then putting their hands in their mouths. If your raised beds were built with pre-2004 lumber, replacing them with modern treated wood or naturally rot-resistant species like cedar is a reasonable step.
Sealants Reduce Leaching Significantly
Applying a stain or sealant to treated wood is one of the most effective ways to slow chemical migration. Research on coatings found that all stains tested reduced leaching of preservative components by about 60% on average. Even more notable, the reduced leaching persisted for over three years, even after the coatings showed visible film degradation starting around 12 months. The most effective coatings had enough flexibility to survive winter freezing without cracking and enough body to form a true barrier layer on the wood surface. Oil-based stains and penetrating sealants generally meet both criteria.
For wood near garden soil or play areas, sealing is a practical way to cut chemical release by more than half with a single afternoon of work. Reapplying every two to three years maintains protection.
Handling Treated Wood Safely
The greatest chemical exposure from treated lumber comes not from slow leaching into soil but from direct contact during construction. Sawdust from cutting or sanding contains concentrated preservative particles. When working with treated wood, wear gloves, long sleeves, long pants, and closed shoes. A dust mask or respirator helps reduce inhalation of treated sawdust, especially during prolonged cutting.
After handling, wash your hands thoroughly before eating or drinking. Clothing covered in sawdust should be laundered separately from other household laundry. Scraps and sawdust from treated wood should never be burned, as combustion releases concentrated preservative compounds into the air. Dispose of them with regular construction waste. Treated wood should also not be used for beehives, animal feeders, or any surface that contacts food directly.