Abstract: A breathing assistance system includes a gas delivery system including a gas flow generation device configured to generate a gas flow; a humidifier system including a liquid water chamber and an outlet, and a microporous filter. The gas delivery system and the humidifier system at least partially define a gas flow path from the gas flow generation device to the outlet of the humidifier system. The microporous filter is located along the gas flow path such that the gas flow generated by the gas flow generation device flows through the microporous filter, subsequently becomes humidified in the liquid water chamber, and subsequently flows through the outlet of the humidifier system. The microporous filter is gas-permeable and liquid water-impermeable, such that the microporous filter prevents liquid water in the liquid water chamber from flowing into the gas flow generation device in any orientation of the gas delivery system and humidifier system.