Abstract: A passive humidifier membrane includes polyolefin with a plurality of pores, wherein the average pore size of the plurality of pores is 0.05 ?m to 0.4 ?m as established by a PMI Capillary Flow Porometer. The humidifier membrane is virtually airtight while providing a high transfer rate of water. Transfer of heat is also high. The humidifier membrane is particularly suitable for heat exchange and water transfer between fluids and, it is particularly useful for applications inside or outside fuel cells such as PEMFC. The membrane is also particularly useful in the application as humidifier for air or oxygen.