A pressurised gases source is provided, the source comprising a centrifugal impeller driven by a motor within a housing, the housing having a gases inlet, a gases outlet and a partition to define first and second interior regions. The first region is defined by the housing and the partition and comprises the gases inlet and the impeller, and the second region is defined by the housing and the partition and comprises the gases outlet. The first and second regions are fluidly connected by a crescent shaped opening formed in or by the partition.