You can isolate pressure-treated wood by lining the inside of a bed with heavy plastic to prevent leaching of chemicals from the wood into the soil. Top your bed with boards of untreated lumber to make a flat top to sit on and to avoid contact with the wood, particularly if you have CCA-treated wood in your garden.
I put in an 8' x 4' raised bed last year using 2×12 boards.. By the way, untreated wood from Lowe's/Home Depot/etc. will break down in about .
If I use untreated 2X pine for raised beds, how long do you think the wood would last? There doesn't seem to be any definitive proof that treated .