How Do Masks Work?

While face coverings primarily prevent the wearer from spreading the disease, they also can protect the wearer from others.

Two lines of evidence support the effectiveness of face coverings in reducing the spread of COVID-19:

Epidemiological data demonstrate a decrease in COVID-19 growth rates following mandates to wear face coverings in multiple U.S. states, with similar results found internationally.

Laboratory experiments visualizing respiratory droplets during various activities provide a qualitative illustration that face coverings reduce the emission of particles by the person wearing the covering. These experiments do not provide information on the usefulness of these coverings as personal protection, since the particles that would be inhaled by the wearer from inside the covering cannot be observed.

What type of face coverings are the most effective?

Filtering masks, such as N95 respirators, are the most effective at filtering out particles in the air while also restricting the number of droplets spread by an individual.

KN95 respirators are similar to N95 respirators, except they are developed to meet Chinese standards, which differ slightly from U.S. standards. Surgical masks follow next in terms of effectiveness.

Face coverings made from other materials vary in efficacy, depending on characteristics such as the type of material used, the number of layers present, and fit. Face coverings should completely cover the wearer's nose and mouth. Those with one-way valves do not provide community protection, since the valve opens to allow easy escape of exhaled breath and any particles that it contains.

Caltech's Richard Flagan, the Irma and Ross McCollum-William H. Corcoran Professor of Chemical Engineering and Environmental Science and Engineering, is applying his expertise in aerosols to inform studies of the release of virus particles into the air. He also is analyzing the effectiveness of different mask materials.

Based on preliminary studies, Dr. Flagan notes the following:

• The fit of the mask is important. Masks should be tight all the way around the face. Leakage around the edges reduces the mask's effectiveness at protecting both the wearer and people nearby. Tip: If your glasses fog up when you are wearing a mask, the mask is leaking.
• All of the fabric face coverings tested were markedly less efficient than the non-woven N95, KN95, and surgical masks tested. Why? In fabric coverings, the fibers are twisted into threads to enable weaving, making only a small fraction of the fibers available for filtration.
• Fabric face coverings should have at least two layers, and high-quality coverings tend to have four layers.

Read more about Flagan's findings in a paper published by the journal Aerosol Science and Technology in December 2021.

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Professor Flagan answers questions from the public about mask effectiveness.