Pressure-treated wood contains preservative chemicals forced deep into the lumber to protect it from rot, fungi, and insects. The specific chemicals depend on when the wood was made and what it’s designed for. Most treated lumber sold today for residential projects like decks and fences uses copper-based preservatives, while older treated wood (pre-2004) often contains arsenic. Here’s what’s actually in it and why it matters.
The Old Standard: CCA-Treated Wood
For decades, the most common treatment was chromated copper arsenate, or CCA. This formulation combined three heavy metals: chromium, copper, and arsenic. In the treating solution, chromium and arsenic were present at roughly a 2:1 ratio, with copper serving as the primary fungicide. Chromium acted as a fixing agent, binding the other chemicals to the wood fibers so they wouldn’t wash out easily. Arsenic provided insect-killing power.
CCA gave wood a distinctive greenish tint and was extremely effective. It was used in virtually everything: decks, playgrounds, picnic tables, fences, landscaping timbers, and boardwalks. The problem was the arsenic. Over time, small amounts leach out. Forest Service research on CCA-treated decking found that arsenic release was highest when the wood was new, with anywhere from 0.16% to 0.72% of the arsenic escaping per rainfall event depending on rain intensity. Slower, lighter rain actually drew out more chemicals than heavy downpours.
In 2003, the EPA phased out CCA for all residential uses, effective December 31 of that year. The ban covered decks, playground equipment, picnic tables, fences, patios, walkways, and boardwalks. CCA is still legally used in industrial applications like utility poles, highway construction, marine pilings, and permanent wood foundations, but you won’t find it at a home improvement store. If your deck or fence was built before 2004, there’s a good chance it contains CCA.
What’s in Today’s Treated Wood
The two preservatives that replaced CCA for residential lumber are both copper-based but arsenic-free.
- Alkaline Copper Quaternary (ACQ) combines copper oxide with a quaternary ammonium compound, the same class of antimicrobial chemical found in many household disinfectants. The copper kills fungi and provides long-term decay resistance, while the ammonium compound adds insect protection.
- Copper Azole (CA) pairs copper with a fungicide called tebuconazole, which is also used in agriculture. This combination offers strong protection against both rot and insect damage.
Both of these preservatives still give treated wood a green or brown color (depending on whether a tint is added). They’re significantly less toxic to humans than CCA, but the higher copper content can be corrosive to certain metals. If you’re building with modern treated lumber, you need stainless steel or specially coated fasteners and hardware to prevent corrosion.
Micronized Copper: The Newer Formulation
A more recent variation uses micronized copper azole (MCA), sometimes sold under brand names like MicroPro. Instead of dissolving copper in a liquid solution, this process grinds copper carbonate into particles smaller than 31 micrometers (roughly a third the width of a human hair) and pushes them into the wood cells.
The key difference is how the copper works once it’s inside the wood. Some of the tiny copper particles dissolve immediately and bind to the wood fibers, providing instant protection. The rest stay as solid particles that slowly release copper ions over time, creating a reservoir effect. This gradual release is designed to extend the wood’s useful life. Micronized copper treatments also tend to be less corrosive to metal fasteners than the older dissolved-copper formulations, which is one reason they’ve become popular at retail lumber yards.
Borate-Treated Wood for Indoor Use
Not all treated wood is meant for outdoor projects. Borate-treated lumber uses sodium salts (sodium octaborate, sodium tetraborate, or sodium pentaborate) dissolved in water and is designed exclusively for interior applications. It’s effective against decay fungi and insects, including termites, and it’s commonly used for framing lumber in areas with high termite risk.
Borates have very low toxicity to humans and other mammals. The treated wood is odorless, colorless, and can be painted or stained without any issues. The trade-off is that borate remains water-soluble after treatment, meaning it will wash right out if exposed to rain or ground moisture. That’s why borate-treated wood is only standardized for interior use where it stays dry or, at most, occasionally damp.
How the Chemicals Get into the Wood
The treatment process uses industrial pressure to force preservatives deep into the wood’s cellular structure. Lumber is loaded into a large steel cylinder, and in the most common method (the full-cell process, used since 1838), a vacuum first pulls air out of the wood’s cells. Then the cylinder floods with preservative solution and pressure ramps up to 140 to 150 psi, held for several hours. This forces the chemicals into the empty cell cavities. Afterward, excess chemical is drained and a final vacuum cleans the surface.
An alternative called the empty-cell method starts with pressurized air (35 to 40 psi) compressed into the wood cells before the preservative is introduced. Pressure then increases to the same 140 to 150 psi range. When the pressure is released, the compressed air inside the cells expands and pushes excess preservative back out, leaving a thinner coating on cell walls. This method uses less chemical while still providing good penetration.
Use Categories and What the Stamps Mean
Every piece of treated lumber carries an end tag or ink stamp with a use category code from the American Wood Protection Association (AWPA). These codes tell you how much chemical is in the wood and what conditions it can handle:
- UC1: Interior, dry conditions (primarily for insect protection)
- UC2: Interior, occasionally damp
- UC3A: Exterior, partially sheltered from weather (like siding under eaves)
- UC3B: Fully exposed exterior, above ground (like deck boards)
- UC4A: Ground contact or above-ground use in high-decay conditions
- UC4B: Heavy-duty ground contact
- UC4C: Severe ground contact, structurally critical
Higher category numbers mean more preservative was forced into the wood. A UC4A post buried in the ground contains substantially more copper than a UC3B deck board. If you’re buying treated lumber, matching the use category to your project is one of the most important decisions. Using UC3B lumber for a ground-contact application, for instance, will lead to premature rot because it simply doesn’t contain enough preservative.
Safety When Cutting and Handling
Sawing, drilling, or sanding treated wood creates dust that contains whatever preservative is in the lumber. For modern copper-based treatments, the risk is lower than it was with CCA, but copper dust still irritates the lungs, eyes, and skin. Wear a dust mask, eye protection, and gloves when cutting treated lumber, and work outdoors or in a well-ventilated space whenever possible.
OSHA identifies local exhaust ventilation as the primary method for controlling wood dust in shop settings, recommending exhaust hoods on saws and full enclosures on sanders. For most homeowners doing occasional deck work, a fitted N95 respirator and safety glasses are practical measures. Wash your hands thoroughly after handling treated wood, and wash work clothes separately.
Never burn treated wood of any kind. The smoke and ash concentrate the preservative chemicals, and burning CCA-treated wood releases arsenic compounds into the air. Even modern copper-treated lumber produces toxic fumes when burned. Treated wood is not classified as hazardous waste under federal law, so in most areas you can dispose of it in regular construction debris landfills, but state and local rules vary. Chipping or composting treated wood is also not recommended, as the chemicals can leach into the surrounding environment.