HOMEMADE MASKS MORE EFFECTIVE THAN THOUGHT: ‘Exhaustive’ study shows cloth masks help protect both wearers and those nearby. But not just any cloth mask: “The research, led by airborne disease transmission expert Linsey Marr, found, based on testing different materials, that a three-layer mask consisting of outer layers of a flexible, tightly woven fabric and an inner layer made from one of several materials designed to filter small particles, should provide at least 74 percent material filtration efficiency for the most penetrating particle size.”