Treated lumber is a popular building material that plays a key role in the construction of many structures in the Pacific Northwest. Our good friend, Dennis McWhirter of Exterior Wood, is an expert on pressure-treated wood—with nearly three decades of experience in the industry. In this series, Dennis provides answers to some common questions and reminds us why treating lumber is one of the best things we can do to help preserve our greatest renewable resource.

In today’s episode, Dennis shares a bit of background on arsenic—and why it’s naturally present in treated lumber. 

arsenic in treated wood

Why is there arsenic in treated lumber? 

Arsenic is found everywhere. There are 90 elements that make up the Earth’s crust, and one of those is arsenic. It’s in food, water, our bodies—it’s natural. 

And so, yes, there is arsenic in our treated wood—it was in the wood before we even treated it, because it was pulled out of the ground by the tree’s root system. Just like when you build a raised garden bed and fill it with soil, there’s likely to be trace levels of arsenic in your dirt. That said, there is no extra arsenic chemical added to our pressure-treated lumber designed for residential use.

pressure treated lumber

Can you use treated lumber for a raised garden bed?

The short answer is yes. The chemicals used in today’s treated lumber are chemicals we use in everyday life. I recently did an interview where I asked a participant to hold out her hand, and I put six pennies in her palm. I asked, “Do you feel uncomfortable holding this amount of money, or this amount of copper?” She laughed and said no, she didn’t. I said, “What if I told you that in the average raised garden bed, you’d have about the equivalent of six pennies of copper?”

The other chemical present in our treated lumber is azole, an organic fungicide used on produce. And then we also use water—that’s it. We don't use more chemicals than that.

Dennis McWhirter of Exterior Wood

We hope this video helps you understand more about pressure-treated lumber so you can decide on the best materials for your next project. For more on treated lumber, check out the history of pressure-treated wood’s chemical composition or how pressure-treated wood is manufactured. And be sure to stay tuned for the rest of our series with Dennis.