Mask/circuit connectors must follow an ISO standard (ISO 5356-1) which is also captured in the ISO standard for BVM connectors, ISO 10651-4. The masks must either have a 22mm female connection or a 15mm male connection. The is why most masks have both, with a 22mmOD/15mm ID connector. Infant masks usually have the smaller connector (15mm O.D.). Some CPAP masks have a swivel/elbow which can get in the way, but the connections should conform to the same standards. Connector bushings may be needed if attaching certain nebulizer tees or other ancillary equipment.