What Chemicals Are Actually in Pressure Treated Wood — And What You Need to Know Before You Build

The Chemicals in Pressure Treated Wood, Identified Clearly

Pressure treated lumber sold in the United States today is predominantly treated with one of three wood preservation chemicals: Alkaline Copper Quaternary (ACQ), Copper Azole (CA), or Micronized Copper Azole (MCA). These have been the standard residential preservatives since 2004, when the EPA phased out Chromated Copper Arsenate (CCA) for most consumer lumber applications due to arsenic content concerns.

Each of these chemical treatments for wood works by driving biocidal compounds deep into wood fibers under high pressure, creating a barrier against fungi, bacteria, and wood-boring insects. The result is lumber that can last 20 to 40 years or more in ground contact or wet conditions — compared to just 1 to 5 years for untreated pine in the same environment.

Industrial and specialty applications use a broader range of wood preservation chemicals, including creosote, pentachlorophenol (penta), and various oil-borne preservatives. Understanding which chemical is in your lumber — and why — helps you make better decisions about safe handling, hardware selection, and end-use suitability.

How the Pressure Treatment Process Actually Works

The term "pressure treated" refers to the delivery method, not a specific chemical. A wood treater loads lumber into a sealed cylindrical retort vessel, then uses a vacuum-and-pressure cycle to force preservative solution deep into the wood's cellular structure — far beyond what surface soaking or brush application can achieve.

The standard process, known as the Full-Cell (Bethell) Process, works in this sequence:

1. An initial vacuum removes air from the wood's cells.
2. Preservative solution floods the retort under pressures typically between 150 and 200 psi.
3. Pressure is maintained for a set period — minutes to hours depending on species and retention level.
4. The solution is drained and a final vacuum pulls excess chemical back out, leaving preservative fixed within the wood fibers.

The amount of preservative retained in the wood — measured in pounds of chemical per cubic foot (pcf) — is called the retention level. Higher retention levels are required for more demanding exposure conditions, such as ground contact, freshwater immersion, or saltwater splash zones.

The Main Wood Preservation Chemicals Used Today

Alkaline Copper Quaternary (ACQ)

ACQ is the most widely used residential preservative in North America. It contains copper oxide (67–69%) and a quaternary ammonium compound (31–33%). Copper is toxic to fungi and most wood-destroying organisms; the quaternary ammonium compound provides additional protection against copper-tolerant fungi and insects.

ACQ comes in several formulations — ACQ-B, ACQ-C, ACQ-D — that vary in the specific carrier and quat compound used. All are water-borne and leave the wood with a greenish tint that weathers to gray over time. ACQ is significantly more corrosive to metal fasteners than CCA was, which is why building codes now require stainless steel, hot-dipped galvanized, or other corrosion-resistant hardware with ACQ-treated lumber.

Copper Azole (CA)

Copper Azole combines copper with an azole fungicide — typically tebuconazole or propiconazole — to combat fungi that have developed copper tolerance. CA-B formulation contains 96% copper and 4% tebuconazole. The azole component targets the ergosterol biosynthesis pathway in fungi, making it effective even against species that copper alone cannot control.

CA-treated lumber is slightly less corrosive to fasteners than ACQ but still requires corrosion-resistant hardware. It is widely used for decking, fencing, and structural applications and carries the same above-ground, ground-contact, and freshwater immersion use categories as ACQ.

Micronized Copper Azole (MCA)

MCA represents the next evolution in copper-based chemical treatment for wood. Instead of dissolved copper salts, it uses micronized copper particles — typically less than 1 micron in diameter — suspended in the treating solution. These particles lodge in the wood's cell walls rather than dissolving into the cell lumen.

The practical benefit is meaningfully reduced metal corrosion compared to ACQ and CA. Independent testing has shown that MCA-treated lumber is approximately 50–70% less corrosive to galvanized steel than ACQ. This makes MCA increasingly preferred for projects where fastener longevity is a concern, such as elevated decks and structural framing in wet environments.

Chromated Copper Arsenate (CCA) — Legacy Use

CCA was the dominant residential pressure treated chemical from the 1940s through 2003. It contains copper, chromium, and arsenic — all three are highly effective biocides, and the chromium acts as a fixing agent that bonds the other metals into the wood. CCA-treated lumber is identifiable by its distinctive green color and the presence of a "CCA" stamp on older pieces.

The EPA phased CCA out of residential applications in 2004 following concerns about arsenic leaching. It remains legal and widely used for industrial applications — utility poles, marine pilings, and agricultural timbers — where its superior performance in harsh environments justifies controlled use. If you are working with lumber from structures built before 2004, assume CCA treatment and take appropriate precautions: do not burn it, wear a dust mask when cutting, and wash hands thoroughly after handling.

Preservatives for Wood Used in Industrial and Specialty Applications

Creosote

Creosote is a coal-tar derivative that has been used as a wood preservative for over 150 years. It remains the treatment of choice for railroad crossties, wooden bridge timbers, and marine structures because of its exceptional resistance to water, fungi, and marine borers. Creosote-treated wood is not available for residential consumer purchase; it is restricted to industrial use due to skin irritant and carcinogen concerns associated with prolonged dermal exposure.

Pentachlorophenol (Penta)

Pentachlorophenol is an oil-borne preservative used primarily for utility poles, cross arms, and marine pilings. It is a Restricted Use Pesticide in the United States, meaning only certified applicators can purchase and apply it. Penta-treated wood has a distinctive oily appearance and petroleum carrier odor. Like creosote, it is not available at retail for consumer projects.

Sodium Borate (Borate Treatments)

Borate-based preservatives — including disodium octaborate tetrahydrate (DOT) sold under brand names like Tim-bor and Bora-Care — are water-borne treatments used primarily for above-ground, interior, dry-use applications. Borates are highly effective against termites, wood-boring beetles, and decay fungi, and they have extremely low mammalian toxicity.

The critical limitation of borate treatments is leachability: borates are water-soluble and will leach out of wood exposed to sustained moisture or ground contact. They are therefore not suitable for exterior applications without a protective coating system, but are an excellent choice for treating interior framing, joists in crawl spaces, and log home structures.

Zinc Naphthenate and Copper Naphthenate

These oil-borne preservatives are available in retail formulations for brush or dip application — common as field-treatment solutions when cutting pressure treated lumber. When you cut a piece of ACQ- or CA-treated lumber, the cut end exposes untreated wood fiber. Applying a copper naphthenate end-cut solution restores some preservative protection to the exposed surface. They are not a substitute for factory pressure treatment but are an important part of correct installation practice.

Retention Levels and Use Categories: Reading the Lumber Tag

Every piece of pressure treated lumber sold in North America should carry an end-tag or stamp from an accredited third-party inspection agency such as the American Wood Protection Association (AWPA). This tag tells you exactly what chemical was used and at what retention level — critical information for matching lumber to its intended application.

Using UC3B lumber in a ground-contact application is a common and costly mistake. The lower retention level is not sufficient for sustained soil contact, and the wood can begin decaying within 3 to 7 years — far short of the expected service life. Always match the use category to the actual exposure condition.

Safe Handling Practices for Pressure Treated Lumber

Modern ACQ, CA, and MCA treatments are significantly safer than CCA-era lumber, but they still contain biocidal compounds that warrant basic precautions during cutting, sanding, and installation.

• Wear a dust mask (N95 or better) when cutting or sanding pressure treated lumber. Preservative-laden sawdust should not be inhaled.
• Wear safety glasses to protect against sawdust and chemical splash when crosscutting.
• Wash hands and exposed skin thoroughly before eating, drinking, or touching your face after handling treated lumber.
• Do not burn pressure treated wood — combustion releases concentrated preservative compounds in smoke and ash. Burning treated lumber is illegal in many jurisdictions and poses a genuine inhalation hazard.
• Dispose of treated lumber scraps through normal solid waste channels, not by burning or composting.
• Apply an end-cut preservative solution (copper naphthenate) to any field cuts to restore protection at exposed surfaces.
• Use only stainless steel, hot-dipped galvanized (HDG), or manufacturer-specified hardware with ACQ and CA treated lumber. Electroplated zinc fasteners corrode rapidly in contact with copper-based preservatives and can fail structurally within a few years.

Choosing the Right Preservative for Your Project

The right chemical treatment for wood depends on three factors: the exposure condition, the species being treated, and any specific environmental or contact sensitivities relevant to the application.

• Residential decks, fences, and railings above grade: ACQ or CA at UC3B retention is the standard. MCA is a good upgrade if fastener corrosion is a priority concern.
• Fence posts and deck footings in ground contact: UC4A minimum; UC4B if the site has high moisture, poor drainage, or a history of termite pressure.
• Interior crawl space framing and sill plates: Borate treatment (DOT) is preferred for its low toxicity and effectiveness against termites and fungi in dry or semi-protected conditions.
• Vegetable garden beds: Use naturally decay-resistant species such as cedar, redwood, or black locust rather than chemically treated lumber. While modern ACQ/CA are considered low-risk, many gardeners prefer to avoid any preservative-treated wood in direct contact with food crops.
• Marine, freshwater dock, or saltwater applications: Specify UC4C or UC5 retention; for severe marine exposure, CCA-treated pilings or creosote-treated timbers (installed by licensed contractors) may be specified by a structural engineer.