Homemade masks are everywhere, but how effective are they? To answer that question, a team at Virginia Tech led by Linsey Marr analyzed the protective properties of a variety of masks. The results confirm earlier studies and suggest that while an N95 is the gold standard, a well-fitting cloth mask will help protect the wearer and people nearby. Marr and coworkers began the study in March after they recognized that wearing cloth masks would be important during the COVID-19 pandemic. The results are available as a preprint, meaning the study has not yet undergone peer review (medRxiv 2020, DOI: 10.1101/2020.11. 18.20233353). The researchers tested the filtration efficiency of 10 materials, including thin cotton, materials used in surgical masks, and a microfiber cloth of the type used to clean glasses. Then they fitted masks made from those materials onto manikins to determine how well they protect the wearer. The researchers found that in addition to trapping particles exhaled by the wearer, masks also protect the wearer from inhaling airborne particles, such as those exhaled by other people. They note, however, that the tests were performed in ideal lab conditions. The team recommends people use three-layer masks that are flexible enough to provide a close fit. The team reports that the best results come from masks made from a tightly woven material for both outer layers and a filter material sandwiched in the middle.