What is pressure-treated lumber, and how does it forestall decay?

Wood is an amazing building material. Its strength-to-weight ratio is greater than that of most types of steel, it is easy to carve and cut, and it is abundant and affordable—though the home improvement boom that came with the pandemic made it less so.

It’s also a tasty snack, at least for fungi and insects such as termites. On a molecular level, wood is about three-quarters polymerized sugars that are indigestible to humans and most other animals. But not bugs and mushrooms. “Hah! More for us,” they say as they slowly eat your neighbor’s deck.

Unless, that is, your neighbor followed most building codes and used pressure-treated lumber. The basics of pressure-treated lumber are close to common knowledge: it’s what you use outdoors, it often has a greenish tint at the store, and you’re not supposed to burn it, because that vaporizes the chemicals that preserve it against decay. But what are those chemicals, and how do they protect wood from being munched?

The molecular recipe has changed over the years, says Colin McCown, CEO of the American Wood Protection Association (AWPA), but the principle is the same: impregnate the wood with a biocide that kills the decay bugs and other organisms. The AWPA writes the technical standards for wood preservation in the US and internationally.

For lumber used before 2003, McCown says, the most likely preservative was chromated copper arsenate, or CCA, which chemist Sonti Kamesam invented in the 1930s while at India’s Forest Research Institute. Lumber processors like Georgia-Pacific applied it to wood as an aqueous solution of chromic acid (H₂CrO₄), arsenic acid (AsO(OH)₃), and copper oxide (CuO). The copper fights off microorganisms and the arsenic kills insects; the chromium mostly helps the other ingredients bind to cellulose and lignin in the wood.

Although chromium and arsenic can be toxic, inside the wood, the elements are in their less-hazardous and less-mobile oxidation states, Cr(III) and As(IV) (Environ. Sci. Technol. 2004, DOI: 10.1021/es0351342). Nonetheless, at the beginning of the 21st century, the chemicals in pressure-treated lumber changed. “The wood preservative manufacturers—the chemical companies—worked with EPA to voluntarily restrict the arsenical chemicals,” McCown says, referring to the US Environmental Protection Agency. “So the industry started using the nonarsenicals for consumer products beginning in 2004.”

These days, most pressure-treated lumber that you might get at a local home improvement store is one of two kinds: ground contact and non–ground contact.

Ground-contact lumber, which can be placed on the dirt or even buried, uses copper as the primary fungicide and insecticide, alongside the azole compounds tebuconazole and propiconazole to fight off copper-tolerant fungi. Older formulations also contained boric acid, but that has been mostly phased out in consumer lumber.

Meanwhile, non-ground-contact pressure-treated lumber doesn’t contain copper. Instead it uses imidacloprid as an insecticide. The formulations can be boosted with either the same azoles as the copper-containing formulations (tebuconazole and propiconazole) or dichlorooctylisothiazolinone, often abbreviated as DCOI or DCOIT, which can also kill bacteria.

Omitting the copper makes the lumber cheaper and less prone to discoloring other things it might touch. But it doesn’t hold up as well when in contact with dirt, leaves, and water and the bugs and fungi they harbor.

Industrial-grade lumber, used for things like telephone poles and railroad ties, uses a different set of chemicals, though there is some overlap.

The AWPA works with chemical manufacturers and testing labs to characterize new options and new applications for existing formulas. McCown has a background in wood science, and though he knows his chemistry, “I’m a bugs-and-fungus guy,” he says.

Testing wood preservatives is complicated because it involves measuring the activity of biological organisms, an activity that generally has to occur outside in settings that mimic where the lumber will be used, he says. “You can imagine that that’s extraordinarily variable.” Results from a field test in May 2019, for example, can’t be compared meaningfully with those from a test in April 2022.

In one common category of test, wood scientists go to a site known to have a high population of a target pest, and they bury three samples of wood, each about a meter apart. One sample will be treated with a new formulation, one will be treated with a known preservative, and one will be untreated.

“One of the favorite places for fungal testing is on the windward side of the Big Island of Hawaii because they get over 130 inches of rain per year,” McCown says. Former sugarcane plantations in that area still have insecticides in the soil, so there are few termites. “Whereas you can go up the hill to a macadamia nut plantation, and there are relatively few insecticides used up there, so the termite load is huge.”

Parts of Australia, in contrast, offer extremely voracious termites in dry climates with little fungi.

The length of time needed for the tests also complicates things, McCown says. A typical test will yield initial data after 3 years, but the longest-running experiments can last decades. “Sometimes if you’re testing for fungal decay and a colony of termites moves in, you’re kind of screwed,” he says.

The main companies providing the preservative formulations in the US are Arxada, Koppers Performance Chemicals, and Viance, which is a joint venture between Venator Materials and DuPont, McCown says. About 200 pressure-treating plants serve the North American consumer lumber market by applying the chemicals made by these firms. The plants are concentrated in the Southeast and Pacific Northwest of the US. “Because transportation is a major cost in forest products, most plants tend to be near the resources,” McCown says.

Though the chemicals have changed, the pressure part of the process is more or less the same. Operators tow a train of palletized lumber—30 m³, which is roughly 1,800 16 ft two-by-fours—into an above-ground, thick-walled cylindrical steel tank roughly 2 m in diameter and 16 m long. The operators then fill the tank with the preservative and crank it up to pressures around 1 megapascal to drive the chemicals deep into the wood.

Prepandemic, the consumer pressure-treated lumber industry sold about 5.5 billion board feet—13 million m³—per year, he says. A rush of do-it-yourself projects by homebound homeowners caused a huge spike in demand in 2020, raising lumber futures on the commodities market higher than $1.60 per board foot that April. The price usually hovers around $0.35.

While the lumber market continues to be strained by supply chain issues and demand that’s hard to predict, the chemical innovation around wood preservation is following familiar patterns. A handful of academic and industrial labs are publishing and presenting on new chemistries, and chemical companies are cautiously interested in bringing the best of them to market.

McCown says the industry is watching new, nonbiocidal wood protection methods. The two most advanced are chemical and thermal modification. In the former, wood is cured with a chemical such as acetic anhydride, which makes the wood hydrophobic and unrecognizable to digestive enzymes in the pests that normally munch on it.

Thermal modification also changes the wood polymers into an inedible form, but it’s a balancing act. “The hotter you cook it, the more durable it becomes, but the greater the strength loss,” McCown says. “Charcoal is extremely durable, but it’s not structurally sound.” He’s also seen academic papers and presentations on biocidal essential oils, though none have been commercialized for wood preservation.

In the meantime, copper-based ground-contact lumber is the best-selling consumer pressure-treated lumber, and some stores are phasing out non-ground-contact wood to reduce customer confusion about when to use what. New preservation methods may eventually compete, but for now, copper is king at the lumberyard.