ASHRAE recently accepted our final report for project RP1649, a large investigation of residential filtration. We found that high-efficiency filters may not actually reduce particle concentrations in homes because of issues like run time (how often the heating/cooling system operates), in-situ filtration efficiency degradation because of face velocity, air bypass, and particle loading, and other loss mechanisms like ventilation. ASHRAE is working on a residential filtration guideline that will help homeowners and HVAC technicians get the most out of their filters. Congrats to all the students who worked on this project. The final report is available to ASHRAE members for free on the ASHRAE Website (and for a nominal fee for non-members). We have published four papers on this work thus far (see publications below) and have another two in review.