In a brief report in Annals of Internal Medicine, Seongman Bae (Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea) and colleagues report that neither surgical nor cotton masks appear to be an effective approach for preventing the dissemination of SARS-CoV-2 from the coughs of patients with COVID-19. They add that further research is needed to determine the effectiveness of cotton and surgical masks in stopping the spread of the virus from asymptomatic or non-coughing patients.
Bae et al note that previous research has suggested that surgical masks may be equally as effective as N95 masks at preventing the dissemination of the influenza virus, so surgical masks could potentially be used as an alternative to N95 masks during the current pandemic. However, they add: “The SARS-CoV-2 pandemic has contributed to shortages of both N95 and surgical masks, and cotton masks have gained interest as a substitute.” Therefore, the aim of their study was to evaluate the effectiveness of both surgical and cotton masks in preventing the dissemination of virus particles.
The authors asked four COVID-19 patients, while in a negative pressure isolation room, to cough five times into a petri dish while not wearing a mask. They were then asked to repeat this process while wearing a surgical mask, and then again while wearing a cotton mask (each time coughing into a different petri dish). “Mask surfaces were swabbed with aseptic Dacron swabs in the following sequence: outer surface of surgical mask, inner surface of surgical mask, outer surface of cotton mask, and inner surface of cotton mask,” Bae et al explain.
They report that 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. Furthermore, regardless of the type of mask (surgical or cotton), all swabs on the outside of the mask tested positive for SARS-CoV-2, but most swabs on the inside did not. According to Bae et al, these findings suggest that surgical or cotton masks do not adequately filter SARS-CoV-2 during coughs by infected patients.
The authors comment that while the size and concentrations of SARS-CoV-2 in aerosols generated through coughing are unknown, SARS-CoV particles from the 2002–2004 epidemic were estimated to be 0.08 to 0.014μm in diameter. They explain that a previous study found that surgical masks did not provide adequate filter performance against aerosols measuring 0.9, 2, and 3.1μm. “Assuming that SARS-CoV-2 has a similar size [to the 2002–2004 outbreak], surgical masks are unlikely to effectively filter this virus,” Bae et al note.
As to why there was SARS-CoV-2 on the outer surface of the masks but not the inner surface, the authors speculate that this could relate to the “aerodynamic features” of the masks. “Alternatively, the small aerosols of SARS-CoV-2 generated through a high-velocity cough might penetrate the masks,” they say. Whatever the reason for the finding, Bae et al state: “These observations support the importance of hand hygiene after touching the outer surface of a mask.”
Looking to the future, the authors say that further studies are needed to review whether face masks “decrease transmission of virus from asymptomatic individuals or those with suspected COVID-19 who are not coughing”. Another unknown is whether masks shorten the distance of droplets during coughing.
Study author Sung-Han Kim (Department of Infectious Diseases, Asan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea) told Cardiovascular News that their findings did not mean that surgical or cotton had “no role” to play in stopping the spread of the virus. He explained: “Based on empirical evidence, masks might shorten the distance of aerosol containing virus and redirect the turbulent jets in less harmful directions (outward protection)—although the targeted studies using SARS-CoV-2 are unavailable. Furthermore, the inhaled air might have different aerodynamics in terms of low velocity particles with adherence of masks to face by depressurising. So, the ineffectiveness of outward protection of surgical or cotton masks in coughing COVID-19 patients do not mean ineffective inward protection of these masks. We recommend wearing any kind of masks in public settings with hand hygiene.”