Cloth can be dangerous. An effective filter for COVID-19 is able to block out COVID-19 sized particles. How big are those particles? Research suggest that the hardest particle size to capture is 0.3 microns. If you took 250 of those particles and stacked them in a row, that would be equal to the diameter of one human hair. These particles are not visible to the naked eye.



How do you check if your material is a good choice? Try #TheWindowTest. If you hold most cloth up to a light, you’ll notice that you can see pin holes of light coming through. If you can see the pin holes of light with your naked eye, then COVID-19 particles can get through.



In addition to the fabric’s ability to filter, you must also consider stretchiness, breathability, and water resistance. If a fabric is too stretchy, the pin-hole size can increase while you're wearing it, making it an ineffective filter. If it’s not breathable enough, our health care workers won’t be comfortable while wearing them and won't be able to do their jobs effectively. If not resistant to splashes of water (like a paper filter, for example), the filter could degrade, or, via capillary action, virus-laden particles could travel from the outside of the mask to the inside of the mask.