Effectiveness of Surgical and Cotton Masks in Blocking SARS–CoV-2: A Controlled Comparison in 4 Patients

BY Herschel Smith
3 years, 11 months ago

Annals of Internal Medicine.

Background: During respiratory viral infection, face masks are thought to prevent transmission (1). Whether face masks worn by patients with coronavirus disease 2019 (COVID-19) prevent contamination of the environment is uncertain. A previous study reported that surgical masks and N95 masks were equally effective in preventing the dissemination of influenza virus, so surgical masks might help prevent transmission of severe acute respiratory syndrome–coronavirus 2 (SARS–CoV-2). However, the SARS–CoV-2 pandemic has contributed to shortages of both N95 and surgical masks, and cotton masks have gained interest as a substitute.

Objective: To evaluate the effectiveness of surgical and cotton masks in filtering SARS–CoV-2.

Methods and Findings: The institutional review boards of 2 hospitals in Seoul, South Korea, approved the protocol, and we invited patients with COVID-19 to participate. After providing informed consent, patients were admitted to negative pressure isolation rooms. We compared disposable surgical masks (180 mm × 90 mm, 3 layers [inner surface mixed with polypropylene and polyethylene, polypropylene filter, and polypropylene outer surface], pleated, bulk packaged in cardboard; KM Dental Mask, KM Healthcare Corp) with reusable 100% cotton masks (160 mm × 135 mm, 2 layers, individually packaged in plastic; Seoulsa).

A petri dish (90 mm × 15 mm) containing 1 mL of viral transport media (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 with no mask. A separate petri dish was used for each of the 5 coughing episodes. 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.

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. 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. All swabs from the outer mask surfaces of the masks were positive for SARS–CoV-2, whereas most swabs from the inner mask surfaces were negative.

Discussion: Neither surgical nor cotton masks effectively filtered SARS–CoV-2 during coughs by infected patients. Prior 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 coughing are unknown. Oberg and Brousseau (3) demonstrated that surgical masks did not exhibit adequate filter performance against aerosols measuring 0.9, 2.0, and 3.1 μm in diameter. Lee and colleagues (4) showed that particles 0.04 to 0.2 μm can penetrate surgical masks. The size of the SARS–CoV particle from the 2002–2004 outbreak was estimated as 0.08 to 0.14 μm (5); assuming that SARS-CoV-2 has a similar size, surgical masks are unlikely to effectively filter this virus.

Of note, we found greater contamination on the outer than the inner mask surfaces. Although it is possible that virus particles may cross from the inner to the outer surface because of the physical pressure of swabbing, we swabbed the outer surface before the inner surface. 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. 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. However, this hypothesis may only be valid if the coughing patients did not exhale any large-sized particles, which would be expected to be deposited on the inner surface despite high velocity. These observations support the importance of hand hygiene after touching the outer surface of masks.

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 do not know whether masks shorten the travel distance of droplets during coughing. Further study is needed to recommend whether face masks decrease transmission of virus from asymptomatic individuals or those with suspected COVID-19 who are not coughing.

In conclusion, both surgical and cotton masks seem to be ineffective in preventing the dissemination of SARS–CoV-2 from the coughs of patients with COVID-19 to the environment and external mask surface.

This paper was retracted and one of the comments was: “This is likely to aggravate ongoing controversy regarding personal protective equipment (PPE).”  There is also this comment.

According to included table, when coughing onto a Petri dish without a barrier, the 4 patients release detectable viral load. When coughing through a cotton mask, in 2 cases the viral load is not detectable (ND), and in the other 2 it is reduced more than 10 times. Yet, according to the average (the authors use the word “median”, while they actually compute averages) viral loads presented by the authors as main results, the viral load is reduced only 5 times. This is apparently because in the computations, the averages are taken over whole rows of the table with the ND instances ignored. This is a serious methodological error. If the virus was not detected in 3 patients instead of 2, the average could have been even higher.

They need more data.  They need to properly assess that data.

If you’d like some background on what I’ve previously said about nuclear grade HEPA filters, this reference will be sufficient for now.  There are many more.

I’ll provide a link at the bottom of this page with prior posts, but let’s review what I’ve said so far.

  1. First, the SARS-CoV-2 virus is 80 nm in diameter.
  2. HEPA filters remove particles down to 0.3 µm in size.
  3. This means that a SARS-CoV-2 virus is 80E-9 / 0.3E-6 = 0.27 the minimum size necessary for even the most expensive nuclear grade HEPA filters to remove it from an air stream.
  4. Even with N95 masks, the bulk of air flow to the breather goes over the top of and under the bottom of the mask.
  5. To get efficient filtration of air, a fitted full face respirator must be worn, leak tested and verified.
  6. That FFR must have a charcoal filter in order to remove viruses of this diameter.
  7. The necessity of charcoal is because charcoal will remove organics, including particles with a charge (especially charcoal impregnated with TEDA).  Viruses are weakly charged, water is polar.  Therefore, charcoal filters will remove water as well as other contaminates, paint fumes among them.  Water and paint fumes can actually decrease the efficiency of charcoal filters, which is why nuclear air filtration systems have air pre-heaters to reduce relative humidity.
  8. Cotton rags are completely ineffective at removing particles of this size.
  9. N95 masks are not leak tested and fitted.
  10. Surgical masks are little better than cotton rags at removing particulate matter.

I said other things, but this is a good primer for where we start.

This should be sufficient to do away with the fairy tale notions of the use of rags and N95 masks for removing free floating viruses in the air.  As one commenter previously said, use of masks is like trying to stop a mosquito with a chain link fence.  But what about viruses attached to water molecules?

A water molecule is 2.75 Angstroms in diameter.  This is not sufficient to create a large enough particle for interception by a HEPA filter.  But what about a virus being attached to spittle?  In this case, a mask of some sort might be effective at catching the spittle on a temporary basis, but there is re-evolution of the virus into the air, evaporation of the water, and buildup of the virus on the mask to consider.

We have pointed out that asymptomatic carriers are not contagious.  Their spittle will not be a concern.  If someone is symptomatic and coughing, he should stay home.

Now, Dr. Paul W Leu of the University of Pittsburgh doesn’t like the study.

The conclusions of this study by Bae et. al are not only erroneous but misleading. 1. The main result of this study is that higher concentrations of SARS-CoV-2 were found on the outside of masks that were coughed into as opposed to the inside. The fact that the virus was determined to be present on the outside of the mask is unsurprising. Surgical and cotton masks are fabrics which will simply absorb any droplets they come into contact with. The higher concentrations found on the outside of the masks may be due to their swabbing the outside of the masks first (which may remove some of the virus) as opposed to the inside. Results should be compared with swabbing the inside first and then the outside. 2. The presence of SARS-CoV-2 on the outside of masks of infected people is of very limited concern for transmission. Most people put on and remove their own masks and do not touch each other’s masks. 3. The results of this study do NOT show that masks are “ineffective in preventing the dissemination of SARS–CoV-2 from the coughs of patients with COVID-19 to the environment.” As the authors acknowledge, their study does NOT evaluate the ability of the masks to shorten the trajectory of droplets emitted during coughing. The function of the mask is to reduce how far aerosol droplets travel during breathing, speaking, singing, sneezing, or coughing. This is the same reason one should cover one’s mouth or nose with your forearm, inside of your elbow, or tissue when sneezing. CDC guidelines advise the wearing of face coverings to “slow the spread of the virus and help people who may have the virus and do not know it from transmitting it to others.”

I consider this comment to be a misdirect on a number of levels.  First of all, no one has told medical workers not to wear masks.  Medical workers wear masks for all sorts of reasons, most particularly, to prevent blood borne pathogens from entering their mouths (as my daughter, an NP, does in the ER and OR).  Further, a face shield should be worn for the very same reasons, i.e., to intercept airborne particles in their trajectory.  I wouldn’t trust a surgical mask to do that.

The researchers have focused a great deal of attention on demonstrating whether masks are effective at removal of viruses.  If one claims that masks are effective, the burden is on him to prove the point, not detractors from proving theirs.  That’s how science is done.

I do find it odd and tiring that people think that they are the first to consider these things and that no research has been done to date on air filtration engineering, industrial hygiene, and reduction of contaminates and toxicants in the air.  This science has been going on for decades, and focuses on real data and mathematical modeling, not well wishes or suppositions.  Considering what the air filtration engineers have accomplished in the nuclear industry would be a good place for people to start.

I do find it interesting that the researchers found that the previously published SARS-CoV virus diameter is 0.08 to 0.14 μm, whereas my source gives 80 nm.  This is fairly close correspondence in data, but also note that there is a range of diameters.  This leads me to my challenge problem for Paul W Leu and other researchers.

I do not believe any of your challenges to the findings of this report.  I barely believe this report.

I won’t believe any of your models or data until [at least] the following has been done.  Assemble an interdisciplinary team of experts, in fields such as industrial hygiene, air filtration engineering, physics, chemistry, and medicine.  Formulate hypotheses on the distribution of particle sizes (there isn’t one particle size, there is a distribution, and it may be a normal distribution, or it may not, it may be a right skewed distribution, or it may be a left skewed distribution); back up your hypothesis with experimental data; assemble a panel of experts to test filters of various types, from cotton, to N95, to HEPA filters, on those particle sizes; report the results; next, do the same with the [possibly] polar composition of viruses and their travelling companion water molecules or other particles, and report results; results shall at least include and consider (a) trajectory, (b) evaporation, (c) re-evolution of particles and viruses into the air stream, (d) and where the collection of particles occurs.

Determine, based on this team’s judgment, whether there is an unhealthy buildup of viruses on the masks you have tested, both for the patient and the worker (or any passerby).  Include in this analysis not only SARS-CoV-2 viruses, but other pathogens as well.  Specifically include in your analysis the buildup and concentration of Legionella bacteria, what we found to be so problematic at the Bellevue-Stratford Hotel when the HVAC engineers directed intake air flow over the top of the condensate discharge from the evaporator units.  Masks collect moisture.

Considering the whole of the findings of this investigation, perform a probabilistic risk analysis for various populations wearing masks under various conditions (including people who have a low oxygen saturation level anyway).  After coming to agreement between the entirety of the committee of experts, prepare a formal report under the authority of a professional engineer’s seal and signature.  Publish all mathematical models, data and test results for peer review.  I want this seal because the researchers have nothing to lose if the contents are wrong.  A professional engineer has his reputation and livelihood to lose.

Only then are you doing science.  Only then will I believe anything you have to say.


New England Journal of Medicine on What Masks Can’t Do Regarding SARS-CoV-2

Concerning the Effectiveness of Masks to Filter SARS-CoV-2

Asymptomatic Carriers of SARS-CoV-2 Are Not Very Contagious



  1. On June 2, 2020 at 6:48 am, Jacob said:


    Just FYI, they retracted the paper. https://www.acpjournals.org/doi/10.7326/L20-0745

    We had not fully recognized the concept of limit of detection (LOD) of the in-house reverse transcriptase polymerase chain reaction used in the study (2.63 log copies/mL), and we regret our failure to express the values below LOD as “<LOD (value).” The LOD is a statistical measure of the lowest quantity of the analyte that can be distinguished from the absence of that analyte. Therefore, values below the LOD are unreliable and our findings are uninterpretable. Reader comments raised this issue after publication. We proposed correcting the reported data with new experimental data from additional patients, but the editors requested retraction.

  2. On June 2, 2020 at 8:54 am, Ned2 said:

    Dr Pam Popper has another video on the mask issue etc..

  3. On June 2, 2020 at 9:03 am, Herschel Smith said:


    Yes. I specifically stated so.

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