Which type of active humidifier would involve directing gas over the surface of water?

The correct choice is the passover humidifier. This type of humidifier operates by passing a flow of gas over a surface of water, allowing the gas to pick up moisture as it travels. The design typically includes a reservoir of water where the gas flows over, either directly or through a heated element, enabling efficient saturation of the gas with water vapor.

In a passover humidifier, the increased surface area of the water allows for optimal evaporation and moisture transfer, improving the efficacy of heat and moisture exchange for patients requiring respiratory support. Passive systems do not utilize heated water directly; instead, they may rely on other mechanisms to achieve humidification, making the passover humidifier distinct in its application of directing gas over the water surface.

Other types like bubble humidifiers involve bubbling gas through water, which does not utilize direct surface contact as prominently as a passover humidifier, while wick humidifiers draw water from a reservoir through a wick, increasing the surface area for evaporation but not actually "directing gas" over water. Heat and moisture exchangers (HMEs) use a different mechanism, focusing on capturing exhaled moisture rather than actively humidifying inhaled gas.