The U.S. Center for Disease Control and Prevention recommends that people wear masks in public to prevent the spread of SARS-CoV-2, the new coronavirus that causes COVID-19, but many have questioned the effectiveness of cloth masks. The virus is thought to spread through respiratory droplets when an infected person coughs, sneezes, speaks or breathes. These droplets form in a wide range of sizes, but the tiniest ones, called aerosols, can easily slip through the openings between certain cloth fibers, leading some people to question whether cloth masks can actually help prevent the disease. In order to test the effectiveness of cloth masks, VBFF Fellow Supratik Guha (class of 2018) has discovered and demonstrated that the best material for homemade face masks for effectively filtering out aerosol particles is a combination of cotton with natural silk or chiffon. Guha and his team studied the ability of common fabrics, alone or in combination, to filter out aerosols similar in size to respiratory droplets. The team of researchers used an aerosol mixing chamber to produce particles ranging from 10 nm to 6 μm in diameter. A fan blew the aerosol across various cloth samples at an airflow rate corresponding to a person's respiration at rest, and the team measured the number and size of particles in air before and after passing through the fabric. One layer of a tightly woven cotton sheet combined with two layers of polyester-spandex chiffon—a sheer fabric often used in evening gowns—filtered out the most aerosol particles (80-99 percent, depending on particle size), with performance close to that of an N95 mask material. Professor Guha published these results in a report to the American Chemical Society’s ACS Nano, acknowledging the use of the U.S. Department of Defense’s Vannevar Bush Fellowship along with other sources in funding the study.