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Explained: Why wearing a weak mask may carry more risk than no mask at all

Filtration efficiency of a mask decreases with multiple use. How does a mask influence the airflow around the face? Study looks at flow speeds, tracks deposition of particles on folds, the nose, and the lungs.

Written by Kabir Firaque , Edited by Explained Desk | New Delhi |
Updated: December 23, 2020 12:17:36 pm
why old mask is worse than no mask, how masks protect us, why should I wear a mask, mask benefits, are masks necessary, indian express, express explainedWomen wearing masks during the Covid-19 pandemic in India. (Reuters Photo/File)

When you wear a mask, it is primarily to protect other people around you. From the perspective of your own protection, how does your mask influence the airflow around your face, the particles you inhale, and the way these particles are deposited in your upper airway or lungs? A new study in Physics of Fluids, a journal of the American Institute of Physics, looks at the effect of wearing a three-layer surgical mask on such airflows and particle deposition.

Key findings

* If you wear a used mask with low efficiency in filtering particles, this can be riskier than wearing no mask at all;

* A mask protects the upper airway best from particles that are larger than 10 micrometres (a micrometre, also called a micron, is a millionth part of a metre)

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* A mask protects the face and lungs best from particles smaller than 10 micrometres (such particles are PM10, by definition)

These findings were derived from simulations using a computational model of a person wearing a surgical mask of three layers with pleats, and then using numerical methods to track the particles through the mask. 📣 Follow Express Explained on Telegram

Used mask vs no mask

The efficiency of a mask to filter particles varies greatly between new and old masks. The simulations showed that a new surgical mask has a filtration efficiency up to 65%, which can drop to 25% after multiple uses.

Study author Jinxiang Xi, a biomedical engineer at the University of Massachusetts at Lowell, explained how airflow around the face differs with and without a mask. “Without a mask, air enters the mouth and nose following a narrow region. With a mask, the airflow comes close to the mouth/nose throughout the entire mask face,” Xi told The Indian Express, by email.

Also, airflow around the face is slower when you are wearing a mask. “The mask can greatly slow down the inspiratory airflow and spread (droplets) over the entire mask surface. The slower airflow favours the inhalation of droplets into the nose,” Xi said.

The amount of droplets you inhale depends, of course, on how many of these manage to cross the mask — which makes filtration efficiency crucial.

“The major finding of this study is that an old mask with low filtration efficiency may not project you well… When the mask filtration efficiency drops below 30% (after 2-3 usages), more aerosols can be inhaled into the nose due to the favourable airflows than without a mask, and thus ‘an old mask’ offers relatively poor protection to the upper airway from smaller particles,” Xi said.

The nose & the lungs

An efficient mask can reduce deposition in the lungs by threefold for particles in the size range 1–10 micrometres. In the upper airway, a 65%-efficiency mask also reduces deposition of all particle sizes, except those of size 1–3 micrometres.

“Our noses also act as filters,” Xi explained. “Without a mask, the nose will filter out all large particles, only allowing smaller particles entering lung. When wearing a mask, the mask will block nearly all large particles, thus save the nose from large particles. The slower flow speed now makes more small particles deposit into the nose, thus saving the lung from small particles.”

Pleats & particles

Once particles have crossed the mask, they are deposited in high concentrations in the pleats (or folds). The study found high flow speed in the pleats.

“More droplets crossed the folds (or pleats) and have a slightly higher speed (i.e., 10 cm/s). In addition, airflows through the folds are more orientated toward the nose, leading to a higher nose inhalation for slightly faster-moving flows,” Xi said.

The broad takeaway is that the findings make think twice about the belief that wearing a mask is always better than without a mask. “It is true for a wearer to protect others; it is also true for self-protection from large aerosols,” Xi said. “But it is not always true for self-protection from small droplets (PM2.5). Wearing a used mask with low filtration efficiency may pose higher risks to the wearer.”

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