Airflow is improved for multi-agent chemical biological air-purifying apparatus by providing a filter module that has multiple filter elements contained within a substantially sealed housing. The housing secures the filter elements in a spaced relation such that the top and bottom faces of each filter element are exposed to the flow of air and provides an air path within the housing adapted to channel air through the multiple filter elements in a parallel flow. The filter module may be made from a flexible and resilient plastic material and may be contoured to conform to the shape of a helmet liner or similar headgear.