The current ASTM Standard Test Method for Categorization and Quantification of Airborne Fungal Structures in an Inertial Impaction Sample by Optical Microscopy (D 7391-09) has outlined the procedures for calculating airborne fungal structures (FS).
For slit impactors such as Air-O-Cell and Allergencos the calculation for fungal structures (or particulates) is based on traverses analyzed. For details click An Alternative Calculation Procedure for Airborne Particulates.