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How Do Face Masks Work at the Microscopic Level?

Updated: Feb 28, 2023

It has been scientifically proven that a swath of fabric covering your mouth and nose is effective at fighting the Covid-19 pandemic. But how do face masks work exactly?

Trapping particles

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 virus. 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.

Let’s take a closer look at how filtration works at the microscopic level.

Cotton masks

The fibers of cotton masks present a dense forest that the particles must navigate as they move with the air stream. The laws of physics influence how the particles interact with the fibers and how well the mask can capture them.

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.

N95 respirators/FFP2 masks

The fibers of an N95 respirator (or an FFP2 mask, as it is called in Europe) are made of synthetic material, vary in size and are arranged randomly. 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% of the elusive medium particles and even more of the large and small ones.

Shape and fit

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.

The collective benefit of mask-wearing

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.

Good ventilation and social distancing further reduce the risk of transmission. And if everyone wears a mask and keeps their distance, the collective benefit goes up.

The public health debate on masks is settled, Joseph G. Allen, director of the Healthy Buildings program at Harvard, told the New York Times. When you wear a mask, “you protect yourself, you protect others, you prevent yourself from touching your face”.

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