Masks Fail to Filter Virus in Coughing COVID-19 Patients

All four patients were instructed to cough five times each onto separate petri dishes containing viral transport media while wearing no mask, a pleated three-layer disposable surgical mask, a reusable two-layer 100% cotton mask, and no mask again. The mask’s inner and outer surfaces were then swabbed.

The results showed that the median viral loads of nasopharyngeal and saliva samples from the four participants were 5.66 log copies/mL and 4.00 log copies/mL, respectively. The median viral loads after coughs without a mask, with a surgical mask, and with a cotton mask were 2.56 log copies/mL, 2.42 log copies/mL, and 1.85 log copies/mL, respectively. Interestingly, while all swabs from the outer mask surfaces were positive for SARS–CoV-2, most swabs from the inner mask surfaces were negative. The study was published on April 6, 2020, in Annals of Internal Medicine.

“The size of the SARS–CoV particle from the 2002–2004 outbreak was estimated as 0.08 to 0.14 μm; assuming that SARS-CoV-2 has a similar size, surgical masks are unlikely to effectively filter this virus,” concluded senior author Sung-Han Kim, MD, of University of Ulsan College of Medicine, and colleagues. “The SARS–CoV-2 pandemic has contributed to shortages of both N95 and surgical masks, and cotton masks have gained interest as a substitute. This experiment did not include N95 masks and does not reflect the actual transmission of infection from patients with COVID-19 wearing different types of masks.”

“We found greater contamination on the outer than the inner mask surfaces. The consistent finding of virus on the outer mask surface is unlikely to have been caused by experimental error or artifact. The mask's aerodynamic features may explain this finding,” added the authors. “A turbulent jet due to air leakage around the mask edge could contaminate the outer surface. Alternatively, the small aerosols of SARS–CoV-2 generated during a high-velocity cough might penetrate the masks. These observations support the importance of hand hygiene after touching the outer surface of masks.”

Standard N95 masks are efficient and are well tested. But such masks are not readily available in most developing countries, with a prohibitive cost of USD 3-4 each, and are too expensive for most consumers for use on a daily basis. By contrast, reusable cloth masks cost just 10-15 cents, and can be washed and worn for months.

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University of Ulsan College of Medicine
Sejong University