You don't know what chemicals they were treated with, anti-fungal, silver birch has a lot of oils in it, pine has tar/sap that could come out.. There are lots of reasons not to, if you really want to, then find a piece of driftwood that's been exposed to sun and water (typical of driftwood) just make sure it's big enough, then cut it into boards, using premade boards or green wood can have very un-predictable contents.