December 16, 2020 7:28:36 pm
The effectiveness of a mask for filtering particulate matter varies greatly between new and old masks. The simulations showed that a new surgical mask has a filtration efficiency of up to 65%, which can drop to 25% after multiple uses. (Representation photo: Reuters)
When you wear a mask, it is primarily to protect the people around you. From the perspective of your own protection, how does your mask influence the flow of air around your face, the particles you inhale, and the way these particles settle in the upper airways or lungs? A new study in fluid physics, a journal of the American Institute of Physics, discusses the effect of wearing a three-layer surgical mask on such airflows and particle deposition.
Key results
* If you use a used mask with low efficiency in particle filtering, this may be more risky than not wearing a mask;
* A mask better protects the upper respiratory tract from particles larger than 10 microns (a micron, also called a micron, is one millionth of a meter)
* A mask better protects the face and lungs from particles less than 10 micrometers (such particles are PM10, by definition)
These findings were derived from simulations using a computational model of a person wearing a pleated, three-layer surgical mask, and then using numerical methods to trace the particles through the mask.
Used mask vs no mask
The effectiveness of a mask in filtering particles varies greatly between new and old masks. The simulations showed that a new surgical mask has a filtration efficiency of 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 through the mouth and nose following a narrow region. With a mask, the airflow is close to the mouth / nose across the face of the mask, ”Xi said. The Indian Express, By email.
Also, the airflow around your face is slower when you wear a mask. “The mask can greatly slow down the inspiratory airflow and spread (droplets) over the entire surface of the mask. The slower airflow favors the inhalation of drops into the nose, ”Xi said.
The number of drops you inhale depends, of course, on how many of them make it through the mask, making filtration efficiency crucial.
“The main finding of this study is that an old mask with low filtration efficiency may not project well … When the filtration efficiency of the mask drops below 30% (after 2-3 uses), they can be inhaled more aerosols in the nose due to the favorable airflows than without a mask, and thus ‘an old mask’ offers relatively poor upper respiratory protection from smaller particles, ”Xi said.
Nose and lungs
An effective mask can reduce deposition in the lungs by three times for particles in the size range of 1 to 10 microns. In the upper airway, a 65% efficient mask also reduces deposition of all particle sizes except 1-3 microns.
“Our noses also act as filters,” Xi explained. “Without a mask, the nose will filter out all the large particles and only allow the smallest particles to enter the lungs. When wearing a mask, the mask will block almost all large particles, thus saving your nose from large particles. The slower flow rate now causes more small particles to be deposited in the nose, thus saving the lung from small particles. “
Folds and particles
Once the particles have passed through the mask, they settle in high concentrations in the folds (or folds). The study found a high flow velocity in the folds.
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“More droplets crossed the folds (or folds) and had a slightly higher velocity (ie 10 cm / s). Also, the airflows through the folds are more oriented towards the nose, leading to a higher nasal inhalation for flows that move slightly faster, ”Xi said.
The overall conclusion is that the findings make one think twice about the belief that wearing a mask is always better than no mask. “It is true that a user must protect others; it is also true for the self-protection of large aerosols, ”Xi said. “But it is not always true for the self-protection of small droplets (PM2.5). Wearing a used mask with low filtration efficiency can present greater risks to the user. “
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