The American Chemical Society published a study by researchers at the University of Chicago for testing the filtration efficiencies of various commonly-used fabric materials, like silk, cotton and polyester-spandex chiffon. The studies involved the use of fan blowing aerosols particles at the breathing rate of the resting state. The number of particles in the air before and after these passed the mask were noted. It was discovered that masks made with a combination of fabrics, like a tightly woven cotton sheet with two layers of chiffon (a blend of 90% polyester and 10% spandex), filtered out to 90% of aerosol particles. Flannel, when used to replace chiffon, also rendered similar results.