Effects of mask-wearing on the inhalability and deposition of airborne SARS-CoV-2 aerosols in human upper airway
…In this study, we found that the protective efficacy of a mask for the nasal airway decrease at lower inhalation flow rates. Particularly at 15 l/min, the nasal retention of 1 µm–3 µm ambient aerosols is even higher by wearing a 65% filtration mask than without a mask at all.
…Even though face masks are well accepted as tools useful in reducing COVID-19 transmissions, their effectiveness in reducing viral loads in the respiratory tract is unclear. Wearing a mask will significantly alter the airflow and particle dynamics near the face, which can change the inhalability of ambient particles. The objective of this study is to investigate the effects of wearing a surgical mask on inspiratory airflow and dosimetry of airborne, virus-laden aerosols on the face and in the respiratory tract. A computational model was developed that comprised a pleated surgical mask, a face model, and an image-based upper airway geometry. The viral load in the nose was particularly examined with and without a mask. Results show that when breathing without a mask, air enters the mouth and nose through specific paths. When wearing a mask, however, air enters the mouth and nose through the entire surface of the mask at lower speeds, which favors the inhalation of ambient aerosols into the nose.
…Wearing a mask can notably distort the inhalation aerodynamics in comparison to that without a mask.
…The nose has a unique role in this COVID-19 pandemic for several reasons. It is the first physical barrier of our body to keep ambient aerosols from getting into the respiratory tract; unlike the mouth, the downward nostrils can effectively prevent large particles from being inhaled due to their large inertia. The nasal mucus and immune cells constitute the second line of defense against invading viruses. However, the nasal goblet secretory cells are also one of the three confirmed binding sites for COVID-19 viruses, where two necessary enzymes for cell invasion, ACE2 (angiotensin-converting enzyme 2) and TMPRSS2 (type II transmembrane serine protease), coexist.
…The other two sites with these two enzymes coexisting are the surface epithelial cells of the alveoli and the ileal absorptive cells in the small intestine.
…The overall lower speeds of the respirable particles after wearing a mask, as well as an increased area of respiration, can increase the chance of respirable particles to land on the face or being inhaled into the mouth and nose. This unexpected finding raises an alarm that wearing masks with very low filtration efficiencies may lead to a higher chance of deposition of ambient aerosols and thus can do more harm than protection.
https://www.bbc.co.uk/bitesize/guides/z3xq6fr/revision/2
In summary, the effects of wearing a three-layer surgical mask on airflow and aerosol dynamics were examined in a mask–face-airway model in comparison to without a mask. A better understanding of the factors involved in determining the dosimetry of ambient aerosols on the face and in the respiratory tract was obtained. Specific findings are as follows:
1. Wearing a mask significantly slows down inspiratory flows and extends respiration zones, which favors the inhalability of ambient aerosols into noses.
2. High flow speed and elevated particle concentrations are observed in the mask pleats.
3. Wearing a mask significantly reduces particle penetration into the lungs, regardless of the filtration efficiency of the mask. Wearing a 65%-filtration mask can reduce lung deposition by three folds for particles of size 1 µm–10 µm.
4. With a 65% mask filtration efficiency that is typical for a three-layer surgical mask, deposition is reduced by wearing a mask for all particle sizes considered, except 1 µm–3 µm, for which equivalent dosimetry in the upper airway was predicted.
5. Wearing a mask protects the upper airway (particularly the nose and larynx) best from particles larger than 10 µm, while it protects the face and lungs best from particles less than 10 µm (PM10).
6. The mask protection of the nasal airway, whose goblet secretory cells are binding sites for SARS-CoV-2, decreases at lower inhalation flow rates (15 l/min or less).
Makes this twitter thread a more interesting read.
