Is pressure treated wood safe?

Modern pressure-treated wood (ACQ, CA-C, MCA) is considered safe for residential use by the EPA and AWPA. The older CCA (chromated copper arsenate) treatment, which contained arsenic, was voluntarily phased out of residential use in 2004. Modern treatments use copper-based preservatives without arsenic. ACQ, CA-C, and MCA-treated wood is approved for playground equipment, garden beds, picnic tables, and all residential applications. The EPA has determined that modern treated lumber poses no unreasonable risk to human health when used as intended. However, basic precautions apply: wear gloves when handling, wash hands before eating, and avoid burning treated wood (the smoke and ash contain concentrated copper compounds). What is the difference between above ground and ground contact treated lumber?

What is the difference between above ground and ground contact treated lumber?

The difference is the preservative retention level, which determines how much rot-fighting chemical is in the wood. Above-ground treatment (UC3B) has a lower retention level, sufficient for wood exposed to weather but not in direct soil contact (deck boards, railings, fence pickets). Ground-contact treatment (UC4A) has a significantly higher retention level, required for wood touching or embedded in soil (fence posts, landscape timbers, deck posts, foundation elements). Using above-ground rated wood in ground contact is a common mistake that leads to premature rot. Always check the end tag or stamp on treated lumber for the Use Category rating. When in doubt, use ground-contact rated lumber. How long does pressure treated wood last?

How long does pressure treated wood last?

Properly treated wood can last 40+ years in above-ground applications and 20-40 years in ground contact, depending on the treatment level and environmental conditions. Ground-contact rated (UC4A) lumber in moderate climates commonly lasts 25-40 years. Above-ground rated (UC3B) lumber in weather-exposed applications lasts 20-30+ years. Factors that affect longevity: treatment retention level (higher is better), species (Southern Yellow Pine accepts treatment better than most species), climate (hot, humid environments accelerate decay), and maintenance (sealing cut ends and maintaining finish). The most common failure point is cut ends that weren't treated with end-cut preservative, allowing rot to enter from exposed, untreated wood.

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Woodworking & Lumber

Treated Lumber Types Explained: ACQ, CA-C, MCA & Ground Contact Ratings

Lumberama Editorial Team, Pressure-Treated Lumber & Wood Preservation Specialists Feb 27, 2025 11 min read

How Pressure Treatment Works

Pressure treatment forces chemical preservatives deep into the wood's cellular structure using a combination of vacuum and pressure in a sealed cylinder. The wood is loaded into the treatment cylinder, a vacuum removes air from the wood cells, preservative solution fills the cylinder, and high pressure (150-200 PSI) forces the solution into the wood. The process takes 2-4 hours. The result: preservatives penetrate the outer 1-2 inches of the wood (the sapwood absorbs treatment readily; heartwood resists penetration).

The preservatives make the wood toxic to the fungi and insects that cause decay. Properly treated wood can last 40+ years in ground-contact applications — far longer than untreated wood, which typically rots within 1-5 years in soil contact.

Modern Preservatives

ACQ (Alkaline Copper Quaternary): The most common residential treatment. Contains copper (which kills fungi and some insects) and a quaternary ammonium compound (which kills copper-tolerant organisms). ACQ-treated wood has a greenish-brown color when fresh that weathers to gray. ACQ is the standard treatment at most home centers (Home Depot, Lowe's). It's safe for residential use, playground equipment, and garden beds. The high copper content means ACQ is corrosive to standard galvanized fasteners — use hot-dipped galvanized (HDG), stainless steel, or ACQ-rated fasteners.

CA-C (Copper Azole Type C): Contains copper and tebuconazole (a fungicide also used in agriculture). CA-C is functionally equivalent to ACQ for residential applications. Slightly less corrosive to fasteners than ACQ due to lower copper content. Some manufacturers prefer CA-C because it's easier on processing equipment.

MCA (Micronized Copper Azole): A newer formulation that uses microscopic copper particles instead of dissolved copper. MCA-treated wood has a cleaner appearance (less green discoloration), is less corrosive to fasteners, and dries faster than ACQ. MCA is becoming increasingly popular and is available at many lumber dealers. The micronized copper particles are less mobile in the environment than dissolved copper.

Use Categories (UC)

The American Wood Protection Association (AWPA) classifies treated lumber into Use Categories based on the exposure conditions. Each category specifies the minimum preservative retention level required.

UC3A (Above Ground, Coated): Wood that's above ground, protected from rain by a roof or overhang, and coated with a finish. Lowest retention level. Example: covered deck framing, porch ceiling boards.

UC3B (Above Ground, Uncoated): Wood that's above ground but exposed to weather. Higher retention than UC3A. Example: deck boards, railings, pergola rafters, fence pickets. This is the most common residential treatment level.

UC4A (Ground Contact, General Use): Wood in contact with the ground or fresh water. Significantly higher retention than UC3B. Example: fence posts, landscaping timbers, deck posts, playground structures. Use UC4A for any wood that touches or is embedded in soil.

UC4B (Ground Contact, Heavy Duty): Wood in critical structural applications with ground contact. Higher retention than UC4A. Example: permanent wood foundation posts, utility poles (lower portion), agricultural structures.

UC4C (Ground Contact, Extreme Duty): Wood in extreme conditions: salt water splash, heavy insect exposure, or tropical environments. Highest retention level. Example: marine piling in brackish water, wood in extremely challenging decay environments.

Important Rules

Cut ends must be treated: When you cut pressure-treated wood, the cut end exposes untreated wood from the interior. Brush or dip the cut end with end-cut preservative (copper naphthenate, sold as "cut-end treatment" at home centers) to maintain protection.

Fastener compatibility: ACQ and CA-C are highly corrosive to standard galvanized fasteners. Use hot-dipped galvanized (HDG), stainless steel (Type 304 or 316), or fasteners specifically rated for ACQ/CA-C treated wood. Using the wrong fasteners causes premature corrosion, black staining, and structural failure.

Don't burn treated wood: Burning pressure-treated lumber releases toxic copper compounds into the air and concentrates heavy metals in the ash. Treated wood scraps must be disposed of in a landfill, not burned in fire pits, stoves, or fireplaces.