Abstract: A filter cassette for sampling of airborne substances comprising a body section having an air flow passageway that extends through the body section from an air inlet to an air outlet. A filter medium inside the body section is disposed across the air flow passageway for capturing airborne substances flowing through the passageway. The portion of the passageway directly downstream from the filter medium has a uniform frusto-conical contour converging in the direction of flow. The air inlet comprises openings in the body section upstream from the filter medium, the openings being arranged to direct air into the air flow passageway normal to the direction of flow onto the filter medium.