A respirator that fits perfectly on one face can leak badly on another, which is why fit testing is not optional in any workplace where it is required. The test seals the mask and exposes the wearer to a harmless test substance with a strong taste or smell; if it can be detected, the seal has failed. Facial hair along the seal line is the most common cause of failure, followed by scars and unusual jaw structure. A respirator that passes inspection and looks correctly worn can still be leaking without the wearer knowing, since the sensation of wearing a mask feels the same whether or not it seals.