Abstract: A respirator exhalation unit (10) comprises a negative pressure valve (120) and a selectively actuable positive pressure valve assembly (130). The position of the positive pressure valve assembly (130) is selectively adjustable to convert the exhalation unit (10) for use in multiple operating modes, such as a negative pressure mode, a powered air mode, and a self-contained breathing apparatus mode (SCBA). Additionally, a closed circuit breathing apparatus (CCBA) adapter assembly (200) can be attached to the exhalation unit for conversion to a CCBA operating mode. Further, the negative pressure valve (120) divides the interior of the exhalation unit into two chambers, one of which functions as a dead space that protects the user from exposure to any harmful contaminants at the end of exhalation.