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The research project RP-1360 investigated the nature of dust collected on general ventilation air filters and how the filter performance changes over time due at least in part to the collection of that dust. A "Four-City Study" investigated the performance of in-situ filters over their lifetimes for pressure drop, dust weight gain, and changes in filtration efficiency. To investigate the nature of dust collected by filters for a large number of locations, a "Multi-City Study" was undertaken. This study focused solely on properties of the dust collected on filters and included the dust's elemental composition, organic and inorganic carbon content, particle morphology, the presence of fibers, and dust color. Filter efficiency as it changes over time is not well represented by the dust loading steps in the ASHRAE Standard 52.2 test. The Appendix J conditioning step was needed to bracket the minimum in-situ efficiency of the electret filter. Reporting values calculated from the minimum efficiency curve are much more relevant to in-situ performance than the use of average or final dust-loaded values.