Saturday, April 26, 2014

MERS: Are Two Surgical Masks Better Than One?


Photo Credit PHIL (Public Health Image Library)





A question was posed to Dr. Ian Mackay on twitter overnight as to whether wearing a double layer of surgical masks would be `safer’ against MERS than just a standard single layer.  Ian expressed doubt that it would help very much but said he didn’t know, then hand-passed it to me (legal under Aussie rules) to see if I had any references on the subject.


Well, references I’ve got.   Definitive answers, on other hand, are in somewhat shorter supply. The quick answer (if you can call it one), is:  Maybe . . . a little.


The problem is that while surgical (or in some regions, cloth) masks are ubiquitous in healthcare settings, tend to show up in public places during epidemic outbreaks, and have been recommended for wear in crowds in Saudi Arabia during the Hajj  – they are primarily designed to prevent the wearer from spreading germs.

Not, as most people assume, to protect the wearer.

That is not to say there isn’t some protective benefit to wearing surgical masks.  There is, although the degree of protection is subject to considerable debate (see The Great Mask Debate Revisited). 


For HCWs dealing with MERS, the CDC strongly recommends wearing a fit-tested N95 mask, eye protection, gown & gloves (see Survival Of The Fit-tested).  And I heartily agree.


But as the question posed dealt with surgical masks (double or single layer), we’ll limit ourselves to the wearing of masks in a community settings or low resource medical settings


Since there is a great deal we do not yet know about how the MERS virus is being transmitted (particularly now, with large nosocomial outbreaks in Saudi Arabia & the UAE), we have to look back to the SARS coronavirus outbreak of 2003 for guidance. 


One study that is particularly on point  (albeit conducted in a healthcare setting),  was published in in the Journal of Hospital Infection in 2005.


Protecting healthcare staff from severe acute respiratory syndrome: filtration capacity of multiple surgical masks

J.L. Derrick*, C.D. Gomersall

Department of Anaesthesia and Intensive Care, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong, China

Summary Guidelines issued by the Centers for Disease Control and Prevention and the World Health Organisation state that healthcare workers should wear N95 masks or higher-level protection during all contact with suspected severe acute respiratory syndrome (SARS). In areas where N95 masks are not available, multiple layers of surgical masks have been tried to prevent transmission of SARS. The in vivo filtration capacity of a single surgical mask is known to be poor. However, the filtration capacity of a combination of masks is unknown.

This was a crossover trial of one, two, three and five surgical masks in six volunteers to determine the in vivo filtration efficiency of wearing more than one surgical mask. We used a Portacount to measure the difference in ambient particle counts inside and outside the masks. The best combination of five surgical masks scored a fit factor of 13.7, which is well below the minimum level of 100 required for a half face respirator.

Multiple surgical masks filter ambient particles poorly. They should not be used as a substitute for N95 masks unless there is no alternative.


The results of this study found that two layered surgical masks provided nearly 40% more filtration than a single mask. Three layers boosted that to 70%, and five layers roughly doubled the filtration.

The author’s conclude by writing:


In conclusion, our data show that no combination of multiple surgical masks was able to meet the requirements for a respirator. If protection against airborne organisms is required, an N95 respirator or better should be used, as currently recommended by the CDC and WHO guidelines for SARS prevention.

Multiple surgical masks will reduce the number of viruses inhaled, but whether the degree of reduction is sufficient to produce significant protection is unknown and cannot be predicted at present. Multiple surgical masks should, therefore, only be used if N95 masks are not available.


So, the short answer is a double layer of surgical masks does appear to provide more filtration.  How much protection that affords in the real world, is another matter.


Cloth or other improvised masks are often used as an alternative to single-use surgical or N95 masks, particularly in resource poor nations, and again, the evidence of their effectiveness is lacking.   Last year,  the International Journal of Infection Control published:


Use of cloth masks in the practice of infection control – evidence and policy gaps

Abrar Ahmad Chughtai, Holly Seale, Chandini Raina MacIntyre


Cloth masks are commonly used in low and middle income countries. It is generally believed that the primary purpose of cloth masks is to prevent spread of infections from the wearer. However, historical evidence shows that they had been used in the past for protection of health care workers (HCWs) from respiratory infections. Currently there is a lack of evidence on the efficacy of cloth masks. In this paper, we examined the evidence on the efficacy of cloth masks and discuss the use of cloth masks as a mode of protection from infections in HCWs. We also discuss various methods to improve the effectiveness of cloth masks; for example; type of fabric, masks design and face fit. Further research is required to validate the use of cloth masks in HCWs for prevention of respiratory infections.


In 2006, the IOM published a report entitled Reusability of Facemasks During an Influenza Pandemic: Facing the Flu, which cited the expected demand for respiratory protective gear during a pandemic for HCWs, and for the American public.


CDC estimates that in the event of a severe influenza pandemic, at least 1.5 billion medical masks would be needed by the healthcare sector and an additional 1.1 billion would be needed by the public. Demand for N95 respirators by the healthcare sector could exceed 90 million for a 42-day outbreak (CDC, 2006)


Given current stockpiles, and that most of the manufacturing of these items has now moved offshore, shortages during a severe pandemic are considered likely. A topic we looked just over a month ago in NIOSH: Options To Maximize The Supply of Respirators During A Pandemic.


The 2006 IOM report mentioned above addresses the issue of `improvised’ masks during a pandemic, and while not exactly endorsing them, accepts the will likely be necessary.


Regulatory standards require that a medical mask should not permit blood or other potentially infectious fluids to pass through to or reach the wearer’s skin, mouth, or other mucous membranes under normal conditions of use and for the duration of use. It is not clear that cloth masks or improvised masks (e.g., towels, sheets) can meet these standards.

Without better testing and more research, cloth masks or improvised masks can not be recommended as effective respiratory protective devices or as devices that would prevent exposure to splashes.

However, these masks and improvised devices may be the only option available for some individuals during a pandemic. Given the lack of data about the effectiveness of these devices in blocking influenza transmission, the committee hesitates to discourage their use but cautions that they are not likely to be as protective as medical masks or respirators. The committee is concerned that their use may give users a false sense of   protection that will encourage risk-taking and/or decrease attention to other hygiene measures.


Personally, this is why I have a small stockpile of N95 masks in my prep closet, and encourage others to acquire a small quantity while supplies are adequate and supplies chains remain uncompromised.  I also have a couple of small boxes of surgical masks for street or `casual wear’. 


If I were out of N95s, and found myself caring for someone who was infected with the MERS virus (or any other respiratory virus), I’d probably `double up’ with the surgical masks.


I just wouldn’t have a whole lot of faith that I was getting much in the way of protection.  


Not exactly ideal.  Still, I have to believe that wearing any mask has got to beat having no mask at all.