Being in the building business, we generate some scrap AdvanTech flooring pieces. This stuff is great. water resistant, flat, and inexpensive compared to 3/4" plywood. On the down side, it is heavy. Unpainted, this stuff holds up well to the elements, painted, it would last forever. I made a couple of bottom boards out of it once, they are holding up well. After reading the MSDS sheet, I don't see any environmental issues. I am attaching a link to the MSDS, a second opinion would be welcome.
http://huberwood.com/media/pdf/MSDS/AdvanTech%20MSDS.pdfSteve