Masks come in many styles and materials, but they generally work in the same way. Layers of fibers capture large respiratory droplets and smaller airborne particles known as aerosols that can carry the coronavirus. This process is known as filtration, and a material’s ability to trap particles is called filtration efficiency. Tightly woven cotton outperforms most common fabrics. A nonwoven material like that of an N95 respirator is most effective.
Larger particles are easier to trap. They slam straight into the fibers and get stuck when the air stream brings them within touching distance, or when their momentum causes them to veer off course.The smallest particles are bounced around by air molecules in a random zig-zag pattern, increasing the time they spend in the fiber forest and their chances of getting captured. Medium-size particles are the hardest to filter. They evade capture because they follow the air flow, twisting and turning around the fibers.
Here are the fibers of an N95 respirator. They are made of synthetic material, vary in size and are arranged randomly.And they have an extra feature: an electrostatic charge that attracts and captures particles of all sizes. Where most masks fall short, N95s excel. Their material can filter at least 95 percent of the elusive medium particles and even more of the large and small ones. Fibers are not the only factor in filtration. Masks of all kinds vary in filtration efficiency based on their shape and fit. Loose-fitting masks or those pressed against facial hair allow aerosols to leak.
A good mask will have a large surface area, a tight fit around the edges, and a shape that leaves space around your nostrils and mouth. This creates a larger breathing zone to catch the particles and increases the chance they will encounter a fiber.
Some masks have valves that make it easier to exhale, but without filters, these valves do not trap the aerosols you breathe out, so they do little to protect others. A well-fitting N95 is the gold standard, but don’t worry if you can’t get your hands on one. When everyone wears a mask, the combined filtration efficiency increases.
Let’s say the mask you’re wearing filters half of the particles you exhale. The particles that escape disperse through the air and are further diluted. The particles that eventually reach someone else’s mask get filtered again, reducing the number that get through. “It’s become clear that cloth masks, even though they’re not as effective as the N95s, are still effective at reducing transmission,” said Linsey Marr, an aerosol expert at Virginia Tech. “Even if you’re not achieving that 95 percent reduction, something is better than nothing.”