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Editor's Note: The Annals of Internal Medicine cited below on June 2 because the authors said they had "not fully recognized the concept of limit of detection" of the in-house assay, rendering the values below the limit of detection "unreliable" and the authors' findings "uninterpretable."

A small study from South Korea cast doubt on the ability of surgical or cotton face masks to effectively prevent dissemination of COVID-19 coronavirus from the coughs of infected patients.

Median viral loads did not differ significantly when comparing coughing samples of COVID-19 patients without a mask, with a surgical mask, and with a cloth mask, suggesting these masks were ineffective at filtering SARS-CoV-2, the virus that causes COVID-19, reported Sung-Han Kim, MD, of University of Ulsan College of Medicine in Seoul, South Korea, and colleagues.

In a , they cited the size of viral particles as a possible reason for masks' poor ability to filter the virus, despite their effectiveness against other respiratory infections. In particular, prior studies found surgical masks, as well as N95 respirators (which were not tested in the current analysis), help prevent dissemination of influenza virus.

Citing concern about asymptomatic and pre-symptomatic spread of COVID-19, the CDC recommended late Friday night that all Americans should wear cloth masks in public.

Kim and colleagues examined data from four COVID-19 patients admitted to negative pressure isolation rooms. They were instructed to cough five times each onto separate petri dishes while wearing no mask, a surgical mask, a reusable 100% cotton mask, and no mask again.

Nasopharyngeal and saliva samples had a median viral load of 5.66 log copies/mL and 4.00 log copies/mL, respectively. However, median viral loads for patients 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, swabs from the outside of the mask were positive for SARS-CoV-2, while most swabs from the inside of the mask were negative, the authors said.

The authors pointed to earlier research showing particles 0.04 to 0.2 μm "can penetrate surgical masks." For the coronavirus responsible for SARS, particles were estimated to be within that range at 0.08 to 0.14 μm, they said.

"Assuming that SARS-CoV-2 has a similar size, surgical masks are unlikely to effectively filter this virus," they wrote. "The size and concentrations of SARS-CoV-2 in aerosols generated during coughing are unknown."

When noting greater contamination on the outside versus the inside of the mask, they speculated it may be due to "the mask's aerodynamic features" or "the small aerosols of SARS-CoV-2 generated during a high-velocity cough might penetrate the masks." The finding reinforces the importance of hand hygiene after touching the outside of masks, Kim and colleagues suggested.

The researchers acknowledged that the study didn't examine actual transmission of COVID-19 illness nor whether the masks "shorten the travel distance of droplets during coughing."

Whether face masks decrease transmission from asymptomatic individuals with COVID-19 or those who are not coughing needs further study, they added.

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