Help me prove that trees are safe!

Hey r/trees, I have a friend (no, really, it isn't me) whose mother isn't so sure about him smoking trees. I need some articles backed up with facts to prove it to her that trees aren't that dangerous. I realize this should just be a simple Google search, but I really can't find anything that isn't covered in images of potleaves or filled with sensationalism. Please help me find some refutable sources, guys!