New study: Wearing a dirty or overused mask might be worse than wearing no mask at all

A new study released by researchers in the field of fluid physics purports to show that, overall, mask-wearing does reduce the chances of COVID-19 transmission. However, the study also contains a warning that using a mask with a poor filtration rate — e.g., a mask that has been worn numerous times and has gotten dirty — might actually increase the risk of infection with COVID-19.

The researchers conducted their study by creating a computerized model of airflow through and around various kinds of protective face masks, and evaluating the extent to which those masks were effective at preventing particles of various size from reaching the areas of the body that are known to be receptors for the COVID-19 virus, like the nose and lungs.

The researchers tested a wide variety of particle sizes because, although the COVID-19 virus itself is small enough to pass through holes in all commercially available masks, the aerosol droplets to which the virus attaches can vary in "shape and size distribution" and are susceptible to being caught by properly fitted and worn masks.

Indeed, the study found that wearing masks — regardless of their filtration efficiency — was effective at preventing particles of all sizes from reaching the lungs, which is one of the sites to which the novel coronavirus attaches.

With regard to the nasal cavity, however, the findings were more complex, and revealed what the study authors characterized as an "unexpected finding" that should raise an "alarm" with policymakers: masks that are unduly restrictive of airflow, particularly those that may have become clogged and dirty, might actually increase the risk that COVID-19 will settle in the nasal cavity. According to the study:

The results of this study show that wearing a zero-filtration mask can lead to a higher deposition rate of particles smaller than 10 µm (i.e., PM10) in the upper airway for all flow rates (15 l/min–60 l/min) and mask resistance matrices considered. This seemingly counterintuitive observation may be attributed to the altered pressure and airflow fields caused by the mask, which further changes the inhalability of the particles and subsequent deposition in the upper airways. 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.

However, the study's authors were adamant that wearing a clean, properly functioning face mask would reduce the overall viral load that would reach the nose and upper airways, as well:

Luckily, the adjusted dosimetry of ambient aerosols is lower with a mask than without one for all particle sizes considered (1 µm–20 µm) in the face, upper airway, and lungs. Considering that the nasal epithelium is one of three sites in the human body binding with the SARS-CoV-2 virus,46,47 wearing a 65%-filtration mask can reduce the nasal deposition (viral load) by half for 3 µm–10 µm aerosols and by four to five times for 15-µm aerosols (Fig. 13).

The study has practical implications for policymakers, given that at this point in the pandemic, a sizeable portion of the mask-wearing population has traded in disposable, single-use masks for more "permanent" face coverings that they reuse over and over, often with little or no washing. The results of this study suggest that people who are using masks in this fashion may be worse off than those who are using no masks at all.