Abstract: A respiratory filtration mask that offers protection from hazardous particulate aerosols, vapors, and the like and which is particularly useful for providing respiratory protection from biological warfare agents. The mask provides a military level of protection against biological agents in a half mask configuration, thereby avoiding the disadvantages of a full face mask such as restricted vision, heat buildup, weight, and feeling of enclosure. The mask has a housing enclosing a filter compartment and a gas transfer compartment and an air inlet which allows unfiltered air into the filter compartment. There is a filter medium in the filter compartment which removes contaminants from the unfiltered air and provides filtered air substantially free of contaminants to the gas transfer compartment. A one way air valve regulates air flow between the filter compartment and the gas transfer compartment.