Should We Wear a Face Mask?

Like a hound from hell, COVID-19 has arrived. We have to make a number of choices. One of these choices is causing great consternation. Should we wear a face mask?

The topic of face masks and COVID-19 wasn’t something we planned to write about; however, the world forced us to learn about face mask wearing. We wanted to share what we’ve learned.

The image is from Pixabay. Image by melli666 from Pixabay

The Verdict Is In

If your feathers are in a fluff about your right to wear or not wear a face mask as you choose, you’re in the minority. In a poll conducted by Huffington Post on May 14 to 16, 2020, eighty-five percent, or the majority, of Americans believed that face masks protect you and/or someone else from germs. The majority also believed that masks are a matter of public health policy rather than personal choice. Most believe we should wear a face mask.

According to an ABC News/Ipsos poll conducted in early April 2020, 55% of Americans who’d left their homes in the previous week had worn a face covering. Many more of us have worn a mask in public at some time during this pandemic.

Evidence Is Confusing

The evidence on whether we should wear face masks during this viral pandemic is confusing for experts. Take this quote for example.

[…} little is known about the relative importance of [contact and respiratory] modes in transmission of other common respiratory viruses. Uncertainties similarly apply to the modes of transmission of COVID-19. (Leung et al., 2020)

Mendacity?

Making matters worse is misleading information about the value of wearing a face mask. We aren’t surprised to learn that something we read on the internet is pure fabrication. What if that information is from the Center for Disease Control (CDC)?

Although some information on the CDC website is valuable, some isn’t. The CDC provided a chart on the protection offered by face masks. The chart suggests that you have a 70% chance of being infected by a COVID-19 carrier who isn’t wearing a mask. This holds true even if you’re wearing a face mask. If the carrier wears a mask, your chance of being infected drops to 5%. If both parties wear masks, the contagion probability drops to 1.5 percent. This chart supports what many health experts are saying. Masks don’t protect the wearer, they protect others.

We should wear a face mask. Chart taken from CDC website illustrating how mask wearing reduces the spread of COVID-19. Unfortunately, there is no support for the numbers presented. Watch out for misinformation during the pandemic.
CDC Chart on Face Mask Protection

Do these claims hold up to scrutiny? Snopes gave this chart a “Mostly False” rating. What’s true is that the CDC is recommending that people wear face masks to decrease the spread of COVID-19. The CDC believe we should wear a face mask. What’s false are: 1) the effect of masks on the spread of COVID-19 hasn’t been established; and 2) the percentages on the chart can’t be accurate because there’s no scientific consensus. Where did these numbers come from? We don’t know.

Hey! What about we lay guys? We’re really adrift. Let’s try to sort this out.

Wearing Masks during a Flu Epidemic Isn’t a New Idea

A little more than a century ago, the Spanish Flu epidemic of 1918 or the Spanish Lady epidemic was a major health concern in the United States. During the viral outbreak, many wore cloth masks. Like now, some wore masks because the government ordered them to. Others wore them because they wanted to do everything they could to stay well. And still others, thumbed their noses at mask wearers.

Government thought we should wear a face during the 1918 pandemic. Vintage pictures of doughboys from WWI sitting in a football stadium with other fans watching a football game while wearing a face mask. Image illustrates that wearing a mask isn't a new idea.

WWI Doughboys and civilians wearing masks while watching a football game during the Spanish Flu epidemic.

In 1920, W. K. Kellogg, M.D., Director of Public Health in California, published an extensive study of the efficacy of gauze facial masks during the epidemic (Kellogg & MacMillan, 1920). They concluded that face masks hadn’t been proven efficient enough to “warrant compulsory application for the checking of epidemics.” (Yes, this is the same Kellogg who invented breakfast cereal.)

Isn’t it odd how everything changes, but everything stays the same?

What Makes a Good Mask?

What properties must a mask have to protect us against COVID-19? First, a mask requires a user who knows how to handle a mask. Most of us don’t have a clue. When researchers ask their subjects to wear masks, they must teach many of them how to wear and handle a mask. There’s a lot to know. Do yourself a favor and watch these YouTube videos. The first video tutorial shows how you can increase your risk of being exposed to COVID-19 if you use a mask or gloves incorrectly. The second shows you how to put on and remove several types of masks. If you only have time to watch one tutorial, watch the first.

The second property a mask must have is the ability to efficiently filter particles which are smaller than 300 nanometers.

How Are Masks Rated?

A face mask is basically an air filter. Like the filter on your car or your furnace, each filter has a particle size range that it  can collect. Above and below this range, particles will be collected with greater or lesser efficiency. When mask filtering efficiency testing is done, the most important data point is the point of inefficiency. Below this point, particles are no longer collected as air flow increases (Brosseau & Sietsema, 2020).

Worst Case Conditions

The best filter test uses worst-case conditions: high flow rates per minute with particle sizes in the least efficiency range. This manner of testing guarantees that filter efficiency will be high at typical, lower flow rates for all particle sizes. The N95 designation means the filter exhibits at least 95% efficiency in the least efficient particle range. Masks are considered efficient if their rating is 95% (Neupane et al., 2019). The surgical grade N95 mask provides the highest level of protection available. It’s filtration efficiency against particles the size of COVID-19 is 85% (El-Atab et al., 2020). For other medical masks, the filter efficiency ranges from 38 to 55 % (Brousseau & Sietsema, 2020).

Medical Masks

All filters or face masks have pores or openings where substances of varying sizes can pass. COVID-19 is tiny, approximately 65-125 nanometers. There are many kinds of facial protection most of us can purchase in 2019. These range from the cone style respirator mask with a humidity valve to the cloth mask. Within the medical fields, there are a range of masks designed for a variety of purposes. The least protective of these is the surgical mask. This is the familiar three-layer face mask your doctor or dentist pulls on for routine medical procedures. The most protective of these is a full-face respiratory mask. These masks are only used for highly contagious activities such as intubating a COVID-19 patient.

Make Up of Medical Masks

Medical masks are made up of a variety of materials. Some materials absorb or block water, some provide padding, some have properties that enhance their ability to block pathogens, and some provide structure to the mask. An N95 filtering facepiece respirator is constructed from electret fiber material with electrostatic attraction for the additional collection of all particle sizes.

Pore Sizes of Medical Masks

The pore size of standard medical masks are shown in the table below (Leonas & Jones, 2003). We’ve rounded the numbers and used mean pore size. For reference, one micrometer = 1,000 nanometers.

MaskThickness mmPore Size micrometer
Tie-on Surgical.324
Classical Surgical.219
Sofloop Extra Protection.416
Aseptex Fluid Resistant.635
Surgine II Cone Mask.451
Surgine Grande Cone .531
Purchasing Medical Masks

Whenever tragedy strikes, we can count on scammers to leap into the breach. In the case of masks, questionable companies are offering surgical masks. You can purchase several different kinds on Amazon.

So, why can we buy surgical procedure masks online when hospitals are fighting to obtain them? The answer is simple and sad. If you can find a mask for sale, chances are better than even that the mask is a fake.

Composition of a Surgical Mask

Let’s look at the innards of a surgical procedure mask. These masks have three layers. The inner layer is a soft absorbent non-woven layer. The outer layer is water resistant and non-woven. The middle working layer is a meltblown filter.

Meltblown technology is an advanced technique involving thermoplastic resins which have good absorption and cohesive properties and can be drawn down to tiny fibers and molded in a variety of ways. Complex equipment is required to manufacture the meltblown inner layer of surgical procedure masks.

Is Your Mask a Fake?
Surgical Procedure Mask

How can you tell if the surgical procedure mask you purchased is genuine? There are two ways. First, you can test the middle layer. There are several YouTube videos which show you how. Second, you can go to the CDC website and check out the manufacturer. Has the company passed the strict guidelines for producing medical masks?

Here is a link to a website which lists suppliers of quality masks. https://www.thomasnet.com/products/surgical-masks

-50111608-1.html

N95 Mask

Another popular mask style is the cone N95 mask. The N95 mask is the gold standard in the mask world because it has an 85% efficiency rating (El-Atab et al., 2020). This mask is beloved by allergy sufferers because it filters the smallest of pollens.

N95 masks, although more complicated, are being faked. A Huffington Post article tells you how to determine the authenticity of these masks. https://www.huffpost.com/entry/n95-coronavirus-mask-defective-fake_n_5eb2ee3cc5b613518d49525f

Who dat?

We looked at highly rated surgical procedure mask availability on Amazon. None of these high quality masks were available for purchase by the public. What we found instead were masks from companies such as Marakym. What do we know about Marakym? There was no Google listing for Marakym. We did find a company with that name. They make clavicle braces, not masks! The Better Business Bureau has no listing for Marakym. Got the picture?

Although the public is actively discouraged from purchasing surgical grade masks to save this scarce resource for medical providers, many of us do have these surgical and N95 masks. Allergy sufferers are ahead of the game. We already know about the dangers of the stuff we breathe, so most of us own a variety of masks. Since we’re also very vulnerable to COVID-19, we must know how to reuse and preserve these valuable health resources.

Cloth Masks

Of the mask types on the market, fabric masks have the largest pores. The pore size of a fabric mask varies with the type of fabric used (Neupane et al. 2019). Due to large pore size, fabric masks are the least effective at filtering a virus (Neupane et al., 2019). Fabric masks can intercept large pollutants before they enter your lungs.

Performance of Fabrics

The National Institute for Occupational Safety and Health (NIOSH) studied filter performance of commercial fabrics which are mass marketed for air pollution and allergens — sweatshirt, t-shirt, scarf, towel, scarf (Brosseau and Sietsema, 2020). They tested the fabrics with a wide range of particles, and T-shirts had 10% efficiency, scarves 10 to 20%, cloth masks, 10 to 30%, sweatshirts 20 to 40%, and towels 40%. All of these materials had near zero efficiency at the .3 µm particle size which easily penetrates into the lungs and is much larger than all COVID-19 aerosols.

How Many Layers?

In another study, cloth masks filter efficiency varied by the number of layers. One layer had 2% efficiency, but 3 layers of cloth had 13% efficiency. The best of the cloth masks had a filter efficiency of 38%, and the poorest had a filter efficiency of 2% (Bousseau & Sietsema, 2020).

Buying a Cloth Mask

It probably doesn’t matter which fabric mask you buy, despite supplier claims to the contrary. Cloth masks must be cleaned after each wearing, so you’ll need more than one.

Most cloth masks are made up of two layers of fabric. If you’re using a cloth mask (usually made of cotton but can be silk), you’ll want a mask with a metal nose piece you can mold to your nose. The mask should cover your mouth and nostrils and fit under your chin. The mask should be held in place with elastic strips which hold the mask tight against your face. The point is to breathe through the mask as much as possible, not around the edges of the mask. When the elastic strips stretch, replace them.

Adding Filtration to a Cloth Mask
We should wear a good face mask and a filter might help. Image of Puraka mask and PM2.5 filter as effective protection against COVID-19.
Carbon Filters and COVID-19

Some cloth masks have a pocket which will accommodate a filter. This filter may capture small particles such as viruses. Puraka Masks is one manufacturer of these masks. There is no direct evidence that these filters block COVID-19.

Puraka Masks use a removable composite Filter PM2.5 composed of three materials:

  • Two layers of spunbond fabric
  • One layer of activated carbon
  • Two layers of meltblown fabric

The spunbond fabric is a non-woven fabric that filters out larger particles, provides structure, and holds the finer non-woven layers in place. The carbon filter layer traps particulates by absorption and serves as an antibacterial. The meltblown fabric has electrostatic properties (like the N95 mask) that capture microscopic particulates.

Manufacturer Claims

The manufacturer claims that their filters block the “vast majority of airborne particles, even the very smallest nanoparticles.” They note that research has shown that the material will capture nanoparticles of a similar size to COVID-19. This research is an inhouse research report and didn’t use live viruses. The manufacturer cites one peer reviewed study (Adhikari et al., 2018).

Purchase PM2.5 Filters

The PM2.5 filters are flat and easily stored. A single filter can be used for 16 to 24 hours continuously and for 1 to 2 weeks occasionally. A face mask and 2 filters can be purchased from purakamasks.com for $12.50. This is a Chinese company. Replacement filters are available and sell for approximately one dollar each. There are knock offs of this product.

Industrial Face Masks

One mask type to avoid is the industrial face mask. These masks are designed to catch sawdust or metal filings, not microbes or allergens. Don’t use an industrial mask to protect yourself against COVID-19!

Make Your Own Fabric Face Mask

Many face mask patterns are posted on the internet. Two of the masks recommended by the CDC are the Olson Face Mask and the FU Face Mask. The Olson face mask is top rated and includes resources for linings which may improve the efficiency of the mask. Links to the masks are given in the order listed.

The patterns are posted on https://sewitonline.com/pages/download-free-pattern and https://freesewing.org/blog/facemask-frenzy/

There are many YouTube tutorials which provide step-by-step instructions for making face masks. These tutorials include a nose piece.

Do Medical Masks Protect Against COVID-19?

How do you decide whether to wear a face mask? Do you use laboratory data on pore and particle size? Do you measure how many copies of a virus penetrate a mask in a laboratory? Do you look at how well wearing face masks works at controlling the spread of disease outside the laboratory? Each of these methods has been used, and the results aren’t in perfect agreement. We know that the filtering ability of masks in a laboratory setting varies widely.

What happens when masks are worn in the real world? You’d think given the number of people worldwide who’ve died from influenza over the last century that we’d know more than Dr. Kellogg did in 1920. There is still little research on how effective masks are in the real world. Let’s take a look at what we have.

Do Surgical Masks Cut Down Viral Spread?

Surgical masks may reduce overall viral particle release from patients with tuberculosis, cystic fibrosis, and influenza (Brosseau & Sietsema, 2020). Surgical masks decreased emission of large particles twenty-five fold and small particles 3-fold.

Universal use of surgical masks may limit the spread of infection to vulnerable people. For stem-cell transplant patients who are immune suppressed, the use of surgical masks by everyone in contact with the patient reduced the rate of respiratory viral infection (Sung et al., 2016).

Health Care Workers

Healthcare workers who wore N95 masks had 53% fewer respiratory viral infections, but those who used medical masks had 12% fewer respiratory infections than those who hadn’t worn a mask (Bousseau & Sietsema, 2020).

New York and COVID-19

Essential workers such as law enforcement, health care, and transit workers have lower infection rates than the general population. In downstate New York, 11-14% of essential workers were infected with COVID-19 (based on presence of antibodies in their bodies), but 20% of the general population tested positive for antibodies.

How Masks are Tested

All the data we’ve discussed are indirect measures. Most of the existing evidence on filtering efficacy of facemasks and respirators comes from in vitro experiments with nonbiological particles, which may not be generalizable to infectious respiratory virus droplets (Leung et al., 2020). There is little information on the efficacy of facemasks and filtering respiratory viruses and the reduction of viral release for an individual with respiratory infections.

How Do Masks Work Against a Real Virus?

How do we mimic a real-life situation and determine how well a face mask filters a virus? Leung and colleagues did just that (Leung et al., 2020). They recruited several thousand volunteers, and they screened these volunteers for viral RNA of three types: influenza, rhinovirus, and coronavirus. COVID-19 samples weren’t used, but COVID-19 is a coronavirus.

One hundred and eleven of the original sample tested positive for one of the viruses. The volunteers breathed normally while a 30-minute sample of exhaled breath was collected. Half of the volunteers wore a surgical face mask, and the other half did not. The exhaled breath was analyzed for viral RNA. The mask used was the Kimberley-Clark Lite One Face Mask 62356. Researchers also collected nasal and throat swabs.

Experimental Study Results

The results of the analysis of aerosols are summarized in the chart (numbers are means). Two viral particle sizes were considered (large >5 micrometers and small <5 micrometers). Although a surgical mask did not completely control the spread of a respiratory virus, it was particularly helpful in reducing the spread of larger aerosols.

We should wear a face mask. Chart of how well a medical procedure masks protects against small and large particles. Shows masks can protect us from spread of virus. 

Description automatically generated

These researchers conclude that surgical masks can reduce the emission of influenza virus particles into the environment.

[…}Surgical facemask[s] could be used by ill people to reduce onward transmission. Leung et al. 2020

When Face Mask Wearing Becomes the Norm

The people of Hong Kong were traumatized by the SARS epidemic of 2003. They know from sad experience how deadly a novel coronavirus can be.

During the COVID-19 epidemic, 96.6% of the people in Hong Kong wore a face mask to control the spread of the virus. As a result, the incidence rate and crude fatality rate were amongst the lowest in the world (Chan et al., 2020).

Animal Simulation Study of Viral Spread

Harry the hamster says, “Wearing a face mask reduces viral transmission by 75%”

Dr. Chan and colleagues (2020) developed an animal model to study the transmission of COVID-19. They used Golden Syrian Hamsters as a model for human to human transmission because hamster cells have a similar binding affinity for ACE2, the enzyme COVID-19 must have to enter a human cell.

Could COVID-19 Spread without Direct Contact?

Would COVID-19 aerosolized droplets transmit the disease from one hamster to another? Would a filter between the animals reduce the spread of COVID-19? Since hamsters won’t wear masks, the researchers developed a special cage and airflow system. They could filter each hamster’s air supply using a special isolation system and surgical mask material.

Each hamster has its own cage, and all the cages are in a closed system where airflow to and from each cage is controlled. Filtering material can be inserted between the cages to simulate the effect of wearing a mask.

Experimental Conditions

Using virus samples isolated from confirmed COVID-19 patients, some hamsters were exposed to the virus. Other hamsters were healthy. Three exposure simulations were done.

  • No filter material between healthy and exposed hamsters
  • Surgical mask filter material on the exposed hamster’s side (simulating a sick person wearing a mask)
  • Surgical mask filter material on the healthy hamster’s side (simulating a healthy person trying to avoid exposure).
Masks Help Reduce the Airborne Spread of COVID-19

The results were clear cut. Non-contact transmission occurred between the hamsters. Two-thirds of previously healthy hamsters contracted COVID-19 due to airborne transmission of virus-laden droplets. When the infected hamster wore a mask, the transmission rate dropped to 16.7%. When the naïve or healthy hamster was protected by the filter material, the transmission rate was reduced by 33%. When both the exposed and healthy hamsters were protected, the infection rate dropped by 75%.

Masks Aren’t Perfect

Non-contact transmission still occurred in our hamster model despite a reduction of transmission when the naïve hamsters are protected by mask partitioning.

Chan et al. (2020)

Chan and colleagues study isn’t definitive because the sample size is small, but when the results of all studies are combined, the conclusion is inescapable. Everyone should be wearing a mask.

Disinfecting a Face Mask

For cloth face masks, nothing beats soap. You can wash your face masks in a washing machine, but we recommend hand washing and air drying for two reasons. First, you don’t damage the nose piece. If you use a washing machine, the nose piece will twist or fall out. Second, machine washing and drying cotton fabric stretches and damages the fabric (Neupane et al., 2019).

What we do is: 1) fill a basin with very hot water; 2) add castile soap or other non-allergenic soap; 3) soak the mask for 10 minutes; 4) rub the fabric gently to make sure all areas of the mask are clean; 5) rinse the mask in hot water; and 6) air dry on a clean surface.

Another Way to Use Your Rice Cooker

Washing in soap and water will destroy a medical mask. To reuse these, there are several methods. You can use medical grade sanitizers, but these are in short supply. You can use UV, but you must have a UV box (costs more than $100.00) or you can use your rice cooker or InstaPot. In Taiwan, it’s a common practice to decontaminate medical face masks in a rice cooker.

Picture of a rice cooker being used to heat an N95 face mask. Shows a method for decontaminating and reusing masks.

Li et al. 2020 tested the rice cooker method for decontaminating face masks which had been inoculated with pathogens. Disinfection requires moist heat at about 150 degrees Fahrenheit (65 C) for approximately 20 minutes (Li et al., in press).

Li and colleagues infected medical respirators and face masks with several bacterial pathogens. They then used an Aroma model rice cooker. The masks were heated in the cooker for 8 to 10 minutes with lid open, and the lid was closed for 5 minutes. The second cycle is the steam cycle. Li et al. also tested a standard oven at a temperature of 150 degrees. They then looked at the infectious pathogens which remained on the masks.

Moist Heat Is as Effective as Methods Hospitals Use

Both the oven and the rice cooker methods reduced the infectious potential of the masks, but moist heat was more effective. The moist heat was more effective than UV treatment. Moist heat was nearly as effective as aerosolized peracetic acid and hydrogen peroxide (Li et al., in press).

Does Moist Heat Damage a Face Mask?

The moist heat method doesn’t significantly change respirator or mask performance. We found a fascinating YouTube tutorial produced by a mask designer. He shows how several methods of treating a mask affect the filtering fabric layer of the mask. https://www.youtube.com/watch?v=Tal_LiRRAr8

A medical face mask may be cleaned and reused several times. The exact number of times a mask can be cleaned relative to efficacy hasn’t been established. Hospitals reuse masks five times according to television news reports.

Don’t Melt Your Mask
Image shows how direct heat melts many medical grade masks. Don't use moist heat when sterilizing a mask.

Don’t be this guy who’s wearing a melted mask. Pay attention when you heat your mask!

This guy put his mask on the bottom of the inner pot of his rice cooker. Look what happened. The mask melted. You must use indirect heat.

Watch a tutorial before trying this method. This link takes you to a demonstration of the rice cooker method from Taiwan. https://www.youtube.com/watch?v=ogCTP3xCWkM&t=21s

This tutorial shows you how to clean N95 cone style masks in a rice cooker using a steamer basket. https://www.youtube.com/watch?v=ekl_B7lNt4k

Final Thoughts

We’ve looked at the data from front to back, from top to bottom, from side to side. We reached three conclusions:

  • Wearing a mask is an important tool in the control of the spread of infections viruses.
  • Use the best quality mask you can. Use a surgical grade mask if you can purchase one.
  • Combine good hand hygiene with mask wearing.

COVID-19 is not going to go away. It’s going to hang around. We wear a face mask to protect others from us, to show our concern for the health of others, and to reduce our chances of getting COVID-19.

Other Posts Related to COVID-19

You might be interested in our post on the relationship between being overweight and influenza. https://carbohydrateconfessions.com/obesity-increases-risk-of-dying-covid-19/

You might also be interested in our post on the war with COVID-19. It was the researching of this post which sent us down the rabbit hole which is COVID-19. https://carbohydrateconfessions.com/at-war-with-covid-19/

References

Adhikari, A., Mitra, A., Rashidi, A., Ekpo, I., Schwartz, J., & Doehling, J. (2018). Field Evaluation of N95 Filtering Facepiece Respirators on Construction Jobsites for Protection against Airborne Ultrafine Particles. Int J Environ Res Public Health, 15(9), 1958. doi:10.3390/ijerph15091958

Brosseau, L. M., Sietsema, M. (2020). Commentary: Masks-for-all for COVID-19 is not based on sound data. Retrieved from https://www.cidrap.umn.edu/news-perspective/2020/04/commentary-masks-all-covid-19-not-based-sound-data

Chan, J. F.-W., Yuan, S., Zhang, A. J., Poon, V. K.-M., Chan, C. C.-S., Lee, A. C.-Y., Yuen, K.-Y. (2020). Surgical mask partition reduces the risk of non-contact transmission in a golden Syrian hamster model for Coronavirus Disease 2019 (COVID-19). Clinical Infectious Diseases. doi:10.1093/cid/ciaa644

El-Atab, N., Qaiser, N., Badghaish, H., Shaikh, S. F., & Hussain, M. M. (2020). Flexible Nanoporous Template for the Design and Development of Reusable Anti-COVID-19 Hydrophobic Face Masks. ACS Nano. doi:10.1021/acsnano.0c03976

Kellogg, W. H., & Macmillan, G. (1920). AN EXPERIMENTAL STUDY OF THE EFFICACY OF GAUZE FACE MASKS. Am J Public Health (N Y), 10(1), 34-42. doi:10.2105/ajph.10.1.34

Leonas, K., & Jones, C. (2003). The Relationship of Fabric Properties and Bacterial Filtration Efficiency for Selected Surgical Face Masks. J Textile Apparel Tech Manage, 3.

Leung, N. H. L., Chu, D. K. W., Shiu, E. Y. C., Chan, K. H., McDevitt, J. J., Hau, B. J. P., Cowling, B. J. (2020). Respiratory virus shedding in exhaled breath and efficacy of face masks. Nat Med, 26(5), 676-680. doi:10.1038/s41591-020-0843-2

Li, D. F., Redmond, S. L. N., Jones, L. D., Donskey, C. J. (In press). It’s not the heat. It’s the humidity. Am. J. Infect. Control.

Neupane, B. B., Mainali, S., Sharma, A., & Giri, B. (2019). Optical microscopic study of surface morphology and filtering efficiency of face masks. PeerJ, 7, e7142-e7142. doi:10.7717/peerj.7142

Sung, A. D., Sung, J. A. M., Thomas, S., Hyslop, T., Gasparetto, C., Long, G., . Horwitz, M. E. (2016). Universal Mask Usage for Reduction of Respiratory Viral Infections After Stem Cell Transplant: A Prospective Trial. Clinical Infectious Diseases, 63(8), 999-1006. doi:10.1093/cid/ciw451

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