SARS-CoV-2, the novel coronavirus that causes COVID-19, is believed to transmitted from infected individuals primarily by respiratory droplets. Such droplets released into the atmosphere by coughs or sneezes can travel up to six feet  before settling on nearby surfaces, posing an inhalation risk to people nearby. Wearing face masks puts a physical barrier between you and the droplets from someone’s sneeze.
While properly fitted NIOSH N95 respirators are the gold standard for reducing transmission, even simple masks can be helpful in reducing exposure to infectious droplets. Importantly, such masks do not filter particles as small as free viruses on their own, but they can help to minimize inhalation of germ-carrying droplets (typically >5 microns) and reduce the risk for those in close proximity to infected individuals. Masks are also particularly valuable when worn by community members, as they can help limit the dispersion of coughs and sneezes.
As a result of the COVID-19 Pandemic, there is a global shortage of personal protective equipment (PPE) including surgical face masks and respirators. Many clinicians and other health care providers are already short on masks, leading them to reuse existing masks and to make their own. The CDC provides the following information for Health Care Provider use of non-NIOSH approved masks or homemade masks:
“In settings where N95 respirators are so limited that routinely practiced standards of care for wearing N95 respirators and equivalent or higher level of protection respirators are no longer possible, and surgical masks are not available, as a last resort, it may be necessary for HCP to use masks that have never been evaluated or approved by NIOSH or homemade masks. It may be considered to use these masks for care of patients with COVID-19, tuberculosis, measles, and varicella. However, caution should be exercised when considering this option.” 
The MakerMask project team has reviewed extensive literature on the science and manufacturing of NIOSH-approved respirators and masks. We selected a multi-layered design that balanced increased filtration capabilities with permeability for breathing. The spunbond nonwoven polypropylene (NWPP) outer layers use the same type of material used in medical-grade commercial masks, providing water-resistance for blocking droplet transmission and mechanical filtration from the structure of the fibers. While a meltblown polypropylene inner filter is ideal, international supply chains are disrupted due to the current crisis, so we have substituted a third layer of NWPP. To improve usability for communities with latex allergies against common elastic, as well as to facilitate disinfection/sterilization procedures, we use finished strips of bias tape or NWPP for tying the mask around the head.
We believe our designs offers a substantial step beyond cotton alternatives and are pursuing rigorous testing to provide data on particle filtration efficiency, fluid penetration resistance, and breathing resistance. Because these are do-it-yourself designs, we cannot control the manufacturing of the masks, but we have worked to ensure that the designs are simple and backed up with good science to ensure the best solution in this circumstance. Even a simple NWPP layer fit over other masks may help reduce instantaneous penetration of aerosols.
To quote the National Academies, “in the absence of data to the contrary, masks are likely to provide less protection against aerosols than an N95 filtering facepiece, but may offer better protection than cotton masks, homemade alternatives such as handkerchiefs and scarves, or no protection at all. No device is fail-safe, and its effectiveness depends on fit, level of exposures, and appropriate use.”
We hope our designs will help communities to make safer choices in these challenging times.[Edit 5/7/2020] For more information on mask materials see: The Big 4: Criteria for Community Mask Materials
 Xie X1, Li Y, Chwang AT, Ho PL, Seto WH (2007) How far droplets can move in indoor environments–revisiting the Wells evaporation-falling curve. Indoor Air. 2007 Jun;17(3):211-25.
 CDC: Strategies for Optimizing the Supply of N95 Respirators: Crisis/Alternate Strategies, Health Care Provider use of non-NIOSH approved masks or homemade masks. Accessed at https://www.cdc.gov/coronavirus/2019-ncov/hcp/respirators-strategy/crisis-alternate-strategies.html on March 15, 2020. Rengasamy S, Eimer B, Shaffer RE. (2010) Simple respiratory protection–evaluation of the filtration performance of cloth masks and common fabric materials against 20-1000 nm size particles. Ann Occup Hyg. 2010;54(7):789–798. doi:10.1093/annhyg/meq044. Accessed at https://academic.oup.com/annweh/article/54/7/789/202744 on March 27, 2020.
 Institute of Medicine. 2006. Reusability of Facemasks During an Influenza Pandemic: Facing the Flu. Washington, DC: The National Academies Press. https://doi.org/10.17226/11637. Accessed at https://www.nap.edu/download/11637 on March 12, 2020.