Hair, goes back 90 days to 6 months. As I recall my agonizing search, it is the number one lab in the country used for forensics: Gas chromotography. If you smoke more than one joint every three months, you WILL NOT PASS. It looks at levels. So, there are acceptable thresholds. MJ has a very limited "Pass" score. If you shave all your hair, they will draw blood, which has even a tighter threshold.