What fabrics make the most effective homemade masks?

Tightly woven cotton, such as that used to make high-thread-count sheets; four layers of silk; and mixtures of fabric, such as cotton and flannel or cotton and chiffon, are good materials for making masks to prevent the spread of the novel coronavirus, SARS-CoV-2, scientists report. According to a new study, masks made from these fabrics are good at filtering saltwater aerosols 10 nm to 6 µm—the same size as droplets that are known to spread respiratory viruses (ACS Nano 2020, DOI: 10.1021/acsnano.0c03252). Droplets this size tend to stay suspended in air. Last month, upon learning the Centers for Disease Control and Prevention would recommend that people wear face coverings, scientists at the Pritzker School of Molecular Engineering, the University of Chicago, and Argonne National Laboratory decided to test the filtration properties of various fabrics. “There was very little scientific data taken with laboratory-grade instruments on this,” says Supratik Guha, who led the research. Guha built an apparatus to flow aerosols through fabric and paired it with sophisticated equipment from Argonne that measures the size and distribution of aerosols. After observing which aerosols penetrate 15 types of fabrics, Guha recommends using at least two layers of fabric but not so many layers that airflow becomes blocked. Also, he says, gaps between the contours of the mask and the face can significantly degrade a mask’s performance.