They are non permeable, nothing gets in and nothing gets out. Re x-ray from what I understand they do not cause a reflection as they are not think enough, the organic material in the MBB will still be visible on the scanners though so if it's getting scanned it probably gone anyway, the entire purpose of using MBB's is so the dogs do not react to the package and therefore it doesn't get scanned at all, can you imagine if customs decided to scan all letter mail? never going to happen.