Typical construction of a shower door is to lay the bottom rail in a bed of caulk. The side jambs are then installed and screwed into the wall and may be caulked. The top rail is laid over the jambs and fits by compression. To remove the bottom, take out the doors, (lift up and out), take off the top rail, remove the jambs, cut the caulk line by slipping a thin metal strip or putty knife between the tub and rail. No problems with asbestos in bathtub caulk. Some building and roofing caulks were fiber reinforced, but still don't pose a hazard because the caulk binds the fibers.