Pressure-treated wood: Old poisons, new cautions

I decided to build a raised border to contain my puny vegetable garden. I was excited to try some of the new pressure-treated wood that's become available since the government decided that the old green-hued stuff, impregnated with chromated copper arsenate (CCA), should be voluntarily phased out at the beginning of 2004.

Why the Fuss?

When a multi-billion dollar industry voluntarily phases out a hugely popular product, there must be a reason. But both the Canadian and U.S. governments say there's no known human health risk with arsenic-treated lumber, as long as it is used 'properly'. To find out what 'properly' entails, read the Consumer Safety Information Sheet on CCA Preserved Wood.

Scary.

In a nutshell, you shouldn't touch CCA-treated lumber with your bare skin when you're building with it, and if you do, you should wash your skin thoroughly. And you should never breathe CCA-treated sawdust, or get it in your eyes, or work with it in an unventilated area. You should never burn it either(a tablespoon of its ash contains a lethal dose of arsenic).

You shouldn't let CCA-treated lumber touch anything that might come into contact with the water table or the food chain. And you should wash your clothes (separately from other laundry) after working with it.

One of the most worrisome things about having an existing CCA-treated deck is that it can continue to release arsenic for at least 10 to 15 years. Technically, arsenic is supposed to have been permanently 'fixed' to the wood fibres during the pressure-treating process, but a University of Florida study released last year found that CCA-treated decks caused surface soil arsenic concentrations underneath the decks to be 2000% higher, on average, than normal.

Children playing on or around pressure-treated decks and play sets put their hands in their mouths an average of 16 times an hour. Ingesting arsenic regularly increases a human being's lifetime risk of developing lung or bladder cancer from a range of 2 in a million to 100 in a million.

Now What?

Since the 1970s the majority of pressure-treated wood used in residential settings was treated with CCA. If you're not sure what your deck or play set is made of, you should assume it is CCA-treated lumber and care for it accordingly.

Based on limited data, some environmental groups suggest that applying coatings to older CCA-treated structures will limit the amount of arsenic that comes out of the wood. UV-resistant acrylic paint has been shown to be most effective at preventing arsenic from leaching out of CCA-treated wood. Pigmented stain is the next most effective sealant. Least effective is clear water-repellent sealant. Because most commercial wood sealants lose their effectiveness at trapping arsenic after about 6 months, the sealing process must be repeated regularly.

Huge Safety Alert

You should NEVER use bleaches, deck cleaners or brighteners on old CCA-treated wood because they oxidize the chromium-3 that's already in CCA-treated lumber, turning it into the highly carcinogenic chromium-6.

Besides sealing your existing CCA-treated structures, you can replace the boards that get the most skin contact with alternatives to CCA-treated lumber. My favourite is Western red cedar, but it's expensive (i.e. approximately 20 dollars for an eight-foot long 4" x 4" post).

Or you can try replacing your CCA-treated decking with some of the new plastic composite lumber.

Or you can try the new pressure-treated lumber. The two most widely available preservatives that replace CCA are alkaline copper quaternary (ACQ) and copper azole (CA). They're very low in toxicity. But here's the deal; lumber treated with ACQ or CA has a much higher concentration of copper (from 18% to 96%) so it's more expensive than the old stuff. That's not all.

The high copper content means that any other metal (like nails, screws, bolts, or flashing) that comes in contact with the pressure-treated lumber sets up a mild electrical charge with the copper, making traditional steel fasteners and aluminium flashing corrode like crazy (up to five times faster than they'd corrode in the old CCA-treated lumber).

This risk of corrosion presents the possibility of structures becoming unstable or even collapsing because of incorrect fastener usage.

More on that later. And hopefully something to joke about too, since nothing about the old pressure-treated lumber is funny.