The obvious advantage of using untreated lumber is that there are no health risks involved. It's as close to wood in its natural form as you're going to get without grabbing an axe and chopping down your own tree. If you're building anything that people will regularly be coming in contact with -- such as playground equipment, lawn furniture or benches -- you should always use untreated wood [source: Houlihan and Wiles].
Untreated lumber should also be used when you're dealing with gardening, whether you're building a raised garden bed, a flowerpot, or just making mulch. Some of the chemicals used to treat wood are meant to kill insects, which means they probably won't do wonders for your soil either. They can damage your flowers and make their way into your vegetable gardens -- ruining your prized tomatoes.
An obvious advantage of untreated lumber is its price; it's much cheaper than treated lumber. Since CCA-treated lumber was taken off the market, new treatment techniques use high levels of copper, which is more expensive. As a result, the cost of treated wood has risen considerably [source: Morrison].
When working with untreated wood, you don't have to worry about protecting your skin. You may want to wear a mask to keep from breathing in sawdust, but you can work in short sleeves and/or shorts without any fear of endangering yourself. The same can't be said for treated wood. In fact, when working with treated lumber, you should be adequately covered -- long sleeves, long pants and eye goggles are all a good idea. Afterward, make sure to clean any sawdust from you or your clothing thoroughly. And, of course, you want to avoid breathing in any sawdust particles from treated lumber [source: McClintock]. At a minimum, you should wear a dust mask/facemask while working with treated lumber. And, if you have one, a respirator would be even better.
At the end of the day, the ease of mind that comes with using untreated lumber may be worth it. You don't need to worry about potential health concerns, even if the wood comes in contact with your soil or water supply.