For more than half a century, pressure-treated wood was a simple material to understand and purchase. Most of the PT lumber used in residential work was treated with chromated copper arsenate, or CCA. Most of the rest was treated with ammoniacal copper zinc arsenate, or ACZA. Different suppliers used different brand names, but all the wood — CCA and ACZA — looked, handled, and performed about the same.

And the performance was good: While treated lumber might check, splinter, or warp from the effects of weathering, it generally wouldn’t rot or get eaten by termites. This has been confirmed by decades of field experience as well as testing by manufacturers and the U.S. Department of Agriculture Forest Products Laboratory.

In both ACZA and CCA, the main preservative is copper — which, in high enough concentrations, will suppress or kill most kinds of fungi and insects but doesn’t hurt people or pets. The arsenic in both formulas serves as a “co-biocide”; it controls a few copper-tolerant fungi that would otherwise attack the wood and puts the final nail in the coffin of termites and other wood-eating bugs. The chromium in CCA and the zinc in ACZA are there primarily as binders — they help lock the copper and arsenic into the wood by “fixation,” which means that they form chemical bonds between the biocides and the wood fibers.

The big difference between CCA and ACZA is the carrier: CCA uses mostly water, whereas ACZA includes an ammonia solution to help it better penetrate some hard-to-treat Western softwood, like Douglas fir.

Up until the 1990s, treated wood was treated wood. But a few years ago, all of that changed. The chemical companies that supplied the lumber treatments were facing increasingly negative publicity about their products, particularly in regard to the potential exposure of children to arsenic in backyard structures and playground equipment. While no public health threat from pressure-treated wood has ever been clearly established, arsenic is listed as a known human carcinogen by the EPA and the International Agency for Research on Cancer (IARC), and the resulting bad press and threat of lawsuits proved to be impossible for wood treaters to overcome. In 2003, the leading chemical suppliers for the wood-treating industry decided to compromise and entered into a voluntary agreement with the EPA: In 2004 they would stop selling preservatives containing arsenic and chromium to residential lumber treaters and switch to new formulas for residential applications.

This new policy set the stage for an ongoing parade of new treated-wood products and formulas (see table, facing page). First up was ACQ — ammoniacal copper quaternary or (depending on the carrier used) alkaline copper quaternary. This formula keeps the relatively benign copper but replaces the arsenic and chromium co-biocides with a quaternary compound, or “quat.”

Compared with arsenic, quat is pretty mild stuff; it’s basically a sanitizing soap based on ammonia. Quats are a universal ingredient in shampoo, for example. And the typical quat used in ACQ, known as “DDAC,” for didecyl dimethyl ammonium chloride, is also an active ingredient in household products like Mr. Clean sanitary wipes and Febreze antimicrobial fabric freshener.

The other early entry was copper azole, or CA — which, like ACQ, has copper as the primary fungicide and insecticide, but uses carbon-based “azole” compounds called tebuconazole and propiconazole to do the job of arsenic. The azoles are pesticides, widely used in agriculture to control insects and fungi. Unlike arsenic, azoles are approved by the EPA for use on food crops. Seed treating is a common use for azoles; in California, the biggest users of tebuconazole are grape growers who spray it on their plants.

So there’s no question about it: ACQ and CA have fewer scary ingredients than CCA. But in use, they turned out to have a few significant drawbacks. The new treated boards and timbers had significantly higher copper content than the older CCA-treated wood, and they tended to leach a lot of copper. That heavy leaching, along with the surfactant action of the quat components and the “amine” carrier (ammonia or alkaline), has been blamed for leaving greenish stains on painted surfaces — for example, where water runoff from a deck flows onto trim or siding.

In addition, says wood scientist and consultant Mike Freeman, the amine carrier used to dissolve the copper in ACQ and copper azole turned out to be an excellent mold food. “The plants that converted over in 2002 and 2003 had a huge mold crisis,” says Freeman. “Millions of dollars’ worth of wood almost had to be destroyed — or at least rewashed or retreated with moldicide. The levels of moldicide that were used with CCA just did not work well on the amine-based formulations.”

The most significant problem, however, was the effect of ACQ and CA on metal. The high levels of free copper that remain in the wood had a strong tendency to corrode fasteners, hardware, and flashing.

Corrosion Concerns

No sooner did ACQ hit the market in 2004 than complaints started to pour in. According to Freeman, building code officials got 6,000 complaints about fastener or hardware corrosion in the first year after CCA was withdrawn. Some of those complaints may just reflect closer scrutiny, says Oregon Department of Transportation engineer Quentin Smith, who’s chairing an ASTM committee on fastener corrosion standard

s: “It’s possible that people are just paying more attention then they used to, and that the corrosion’s not actually any worse than it was.” But objectively, there’s no disputing that ACQ and CA are more destructive of metal than CCA is. In comprehensive testing using methods standardized by the American Wood Protection Association (AWPA), New Zealand researcher Gareth Kear and his colleagues found that “ACQ-treated timbers are more corrosive towards mild steel and hot-dipped galvanized steel than any other type of treated timber.” ACQ wood corroded mild steel about five times as fast as CCA did, and corroded hot-dipped galvanized steel anywhere from five to 19 times as fast. Galvanized steel lasted longer than mild steel, but only stainless steel held up without significant damage: “The 316 stainless steel performed very well in terms of corrosion resistance within all of the preservative treatments examined,” reported the researchers.

In response to the increased corrosion and other problems associated with ACQ and CA, within just four years of their introduction these second-generation formulations were largely replaced by a newer generation of “micronized” — or “dispersed” — formulas with names like micronized copper azole (MCA), micronized copper quat (MCQ), and dispersed copper azole (µCA). As of 2008, says Freeman, MCA, MCQ, and µCA accounted for 80 percent of the residential market, with ACQ and CA squeezed down to 20 percent or less.

These new third-generation formulas contain copper in the form of very finely ground particles, rather than in dissolved form. They don’t need the nitrogen-rich amine carrier, and because they are less prone to leaching, they can be treated with less total copper and still maintain adequate long-term concentrations. Manufacturers also claim that these formulas are less corrosive to fasteners. Osmose, for instance, says that its SmartSense treated wood with MicroPro, a micronized copper quaternary formula, “exhibits corrosion rates on metal products similar to CCA pressure-treated wood and untreated wood.”

Peter Laks, a professor at the School of Forest Resources and Environmental Science at Michigan Tech, confirms that there is a lot of test data to support this claim. “It also just makes sense from the chemistry,” he says. Dissolved-copper systems leave lots of free copper ions in the wood that act to corrode other metals like steel, zinc, or aluminum, he explains. Moreover, the amine carrier — ethanolamine — that is used to help dissolve the copper “is inherently quite a corrosive material,” he says.

The micronized approach, says Laks, uses a “very different kind of protection mechanism” that avoids the corrosive effects of ethanolamine. The active ingredient, finely ground particles of copper carbonate, has a low level of water solubility. “Those particles penetrate through the micro-pore structure of the wood and then just reside within the wood structure ... so that when the wood gets wet, a tiny little bit of that copper carbonate particle dissolves and diffuses into the cell wall of the wood and protects it.”