Our masks are made with a woven anti-microbial polyester fabric. The synthetic fibers leave little room for bacteria build-up and is also entirely machine washable. This is a similar fabric to higher end sportswear, medical gowns, and certain pillowcases.
Google "anti-microbial fabric" for more specific information on the mechanisms at work. Please note this is not anti-viral and is intended for long-term everyday use.