Background:
During respiratory viral infection, face masks are believed to prevent transmission (1). It is uncertain whether face masks worn by coronavirus disease 2019 (COVID-19) patients prevent environmental contamination (2, 3).
A previous study reported that surgical masks and N95 masks were equally effective in preventing the spread of the influenza virus (4), so surgical masks could help prevent the transmission of severe acute respiratory syndrome coronavirus 2 (SARS). – CoV-2). However, the SARS-CoV-2 pandemic has contributed to shortages of N95 and surgical masks, and cotton masks have gained interest as a substitute.
Aim:
To evaluate the effectiveness of surgical and cotton masks in filtering SARS-CoV-2.
Methods and results:
The institutional review boards of 2 hospitals in Seoul, South Korea, approved the protocol, and we invited patients with COVID-19 to participate. After giving informed consent, patients were admitted to negative pressure isolation rooms.
We compared disposable surgical masks (180 mm × 90 mm, 3-ply [inner surface mixed with polypropylene and polyethylene, polypropylene filter and polypropylene outer surface], pleated, bulk packaged in carton; KM Dental Mask, KM Healthcare Corp) with 100 Reusable % cotton masks (160 mm × 135 mm, 2 layers, individually packed in plastic; Seoulsa).
One Petri dish (90 mm × 15 mm) containing 1 ml of viral transport medium (sterile phosphate-buffered saline with bovine serum albumin, 0.1%; penicillin, 10,000 U/ml; streptomycin, 10 mg ; and amphotericin B, 25 µg) was placed approximately 20 cm from the patients’ mouths. Patients were instructed to cough 5 times each onto a Petri dish while wearing the following sequence of masks: no mask, surgical mask, cotton mask, and again no mask.
A separate Petri dish was used for each of the 5 cough episodes. The mask surfaces were swabbed with aseptic Dacron swabs in the following sequence: outer surface of the surgical mask, inner surface of the surgical mask, outer surface of the cotton mask, and inner surface of the cotton mask.
The median viral loads of nasopharyngeal and saliva samples from the 4 participants were 5.66 log copies/ml and 4.00 log copies/ml, respectively.
Median viral loads after coughing 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. ml, respectively.
All swabs from the external surfaces of the masks were positive for SARS-CoV-2, while the majority of swabs from the internal surfaces of the masks were negative .
Discussion:
Neither surgical nor cotton masks effectively filtered SARS-CoV-2 during coughing in infected patients.
Previous evidence that surgical masks effectively filtered influenza virus (1) informed recommendations that patients with confirmed or suspected COVID-19 should wear face masks to prevent transmission (2).
However, the size and concentrations of SARS-CoV-2 in aerosols generated during cough are unknown. Oberg and Brousseau (3) demonstrated that surgical masks did not exhibit adequate filter performance against aerosols of 0.9, 2.0, and 3.1 μm in diameter.
Lee and colleagues (4) showed that particles of 0.04 to 0.2 μm can penetrate surgical masks. The SARS-CoV particle size from the 2002-2004 outbreak was estimated to be 0.08 to 0.14 μm (5); Assuming SARS-CoV-2 is of similar size, surgical masks are unlikely to effectively filter this virus.
Of note, we found greater contamination on the external mask surfaces than the internal one.
Although it is possible for virus particles to cross from the inner surface to the outer surface due to the physical pressure of the smear, we clean the outer surface before the inner surface. The consistent finding of viruses on the external surface of the mask is unlikely to have been caused by experimental error or artifact.
The aerodynamic characteristics of the mask may explain this finding. A turbulent jet due to air leakage around the edge of the mask could contaminate the outer surface. Alternatively, small SARS-CoV-2 aerosols generated during high-velocity coughing could penetrate masks.
However, this hypothesis can only be valid if coughing patients do not exhale large particles, which would be expected to deposit on the inner surface despite the high velocity. These observations support the importance of hand hygiene after touching the external surface of masks.
This experiment did not include N95 masks and does not reflect the actual transmission of infection from COVID-19 patients wearing different types of masks. We don’t know if masks shorten the travel distance of droplets during coughing. More studies are needed to recommend whether face masks decrease virus transmission from asymptomatic individuals or those with suspected COVID-19 who are not coughing.
| In conclusion , both surgical and cotton masks appear to be ineffective in preventing the spread of SARS-CoV-2 from the cough of COVID-19 patients to the environment and the external surface of the mask. |
