It depends on the positive pressure device and circuit being used. If a single limb circuit without an active exhalation valve is being used (e.g. a home NIPPV machine), then a vented mask or vented circuit must be used (i.e. fixed orifice resistor outlet). If a dual or single limb circuit with an active exhalation valve is being used, then an NIV mask without a vented port should be used. Read more on circuit and mask setup for NIPPV/CPAP