“The ideal cloth masks that you really want to have is one that has three layers. So you have an inner layer, you’ve got the filter layer, and you’ve got your outer layer,” Aileen M. Marty, MD, a professor of infectious diseases at Florida International University, explained in an interview with Good Morning America. “If you add that filter, then you’re also adding a really good protection for yourself.” Marty explained that the material used in a mask’s composition also makes a difference—the external layer should be water-resistant, the inner layer should be absorbent, and the layer in-between should be a non-fiberglass filter.ae0fcc31ae342fd3a1346ebb1f342fcb So, what kind of filter material is most affective? In an interview with NPR, May Chu, an epidemiologist at the Colorado School of Public Health and advisor to the World Health Organization (WHO), noted that polypropylene, a fabric frequently also referred to as spunbond, is a great filter material, thanks to its electrostatic charge, which can help catch droplets instead of allowing them to escape the confines of the mask. If you don’t have access to polypropylene, Chu explained that two folded tissues—making a total of four layers of paper—will also work, albeit without the benefit of an electrostatic charge. RELATED: For more up-to-date information, sign up for our daily newsletter. However, even if you’re wearing a mask with a filter in it, that doesn’t mean you’re done protecting yourself. Marty noted that filters should be changed after every second or third use. Similarly, the CDC recommends that cloth masks should be washed after every use to limit the chance of infection or cross-contamination. Luckily, even if you can’t get your hands on a filter, wearing a mask at all can significantly reduce your risk of getting coronavirus: According to researchers at the University of California, Davis, simply wearing a mask can reduce a person’s risk of becoming ill with COVID-19 by 65 percent. And if you want to protect yourself, brush up on these 50 Essential COVID Safety Tips the CDC Wants You to Know.