Comparison of Types of Masks
Updated: Jul 20, 2021
Any discussion about different materials for masks can get pretty complicated looking at size of pores versus size of virus particles, versus electrostatic charges of different materials, versus thread counts, etc. However, last week we looked at two very convincing epidemiologic studies that suggest that any type of mask that fully covers the mouth and nose is highly effective in blocking transmission of respiratory viruses, regardless of the material, especially if both people are wearing a mask. So, in that context, a valid question is, “Should we really worry about what the mask is made of?” And the simple answer is, “Probably not.”
A few definitions will be helpful. First, a “medical grade mask” is made of layers of paper and is disposable. A “surgeon’s mask” is a medical grade mask that ties behind the head and neck while an “isolation mask” usually has ear loops to hold it in place. This is because it is easier to safely remove a contaminated mask by lifting off the ear loops instead of fiddling around with a knot to untie. Medical grade masks also usually come with a small wire or metal strip along the top so that the mask can be fitted better over the bridge of the nose so as to reduce fogging of one’s glasses. Some medical masks also have a thin adhesive over the bridge of the nose for the same purpose. However, the material that the mask is made of is generally the same. A “cloth mask” refers to any of a number of designs made at home by anyone who is handy with a sewing machine. An N-95, as we will discuss in more detail later, is a particularly thick mask, also known as a “respirator”, that is designed to filter out very fine dust particles or aerosols. N-95s are made for medical use as well as use in construction settings and workshops.
Personally, I think that the most important elements of a good mask are that it be comfortable to wear and even more importantly, comfortable to breathe through. For example, one of the problems with an N-95 mask is that it takes extra work to breathe through which can be fatiguing after a while. Also, if the mask doesn’t fit well and you are always having to reach up to readjust it, then that is not good, or even worse is when it constantly falls down. The same goes for a mask that leads to your glasses fogging up all the time. All of these annoyances will lead to people avoiding wearing the mask and that circumvents the point which is to have everyone wearing a mask when around others outside of your social bubble.
Most of the research comparing different mask materials has been in protecting against influenza. Since these two viruses, SARS-2 (COVID) and influenza, are very similar in size and means of transmission, then these studies are useful. I think one of the best such study was published in the British Medical Journal in 2015 in which a comparison was made of influenza transmission in 1,607 healthcare workers in 14 different hospitals between those who wore cloth masks and those that wore medical grade masks (BMJ Open / March 26, 2015 / 5: e65770). This study did show a reduction in the risk of catching influenza if a medical grade mask was used over a cloth mask. However, one potential shortcoming in the study is that the medical grade masks were disposed of daily, while the cloth mask were reused. Also, the cloth masks were made of layers of gauze which has a very low density of threads per inch. However, both types of masks did significantly reduce the risk of influenza, with the medical grade mask performing better. Based on this and similar studies, current guidelines are that healthcare workers taking care of patients should wear medical grade masks. Translating that to a recommendation for the general public is difficult given the more casual encounters that occur while passing each other in a grocery store aisle versus being a nurse caring for an ill, hospitalized patient in close contact. Incidentally, the study showed that masks combined with strict handwashing was significantly more protective against infection than masks alone.
Another interesting study, forwarded to me by another Dr. Miller, in this case our local psychologist Dr. Richard Miller, looked at different cloth fabrics and their ability to trap small droplets and particles in the laboratory. This was done by researchers at the University of Wisconsin and published in the journal of the American Chemical Society in May (ACS Nano / May 14, 2020 / 14(5);6339-6347). In this study, they compared different fabrics including cotton, flannel, silk and various synthetic fibers along with different combinations of each as well as varying fabric density (ie., thread count and thickness). They found that double layered masks are more effective than single layer, no surprise there, but that varying the fabrics of the two layers was better than two layers of the same fabric. The best combinations appear to be cotton−silk, cotton−chiffon, and cotton−flannel. Also, higher thread counts were associated with better performance, once again no surprise there, but that there wasn’t much benefit beyond 600 threads per inch (TPI). This is important because most of us would find it difficult to breathe through a thread count much above that.
N-95 face masks are not really advised for the general public. The reason is that the main transmission of viruses like SARS-2 is from respiratory droplets which are much larger than aerosols. N-95s are designed to trap much smaller particles called aerosols. Aerosols are really a concern in medical and dental settings where procedures such as high-speed dental drills or medication nebulizers create large amounts of aerosols. In those settings, an N-95 is required. However, once again for casual encounters in the general public, such a mask is really overkill in my opinion. Plus, they are much more difficult to breath through because of their construction needing to trap the smaller particle.
N-95s are commonly found in hardware stores with a one-way mechanical valve. These masks are designed to trap small dust particles such as from wood working or metal polishing. As I mentioned, N-95s are difficult to breath through, so for work shop applications many come with a one-way valve that makes it easier to exhale. However, since our public health strategy in mask wearing is mainly to prevent people who are infected with the virus from spreading it to others, then masks with one-way valves are not as effective. Similarly, simply wearing a plastic face shield without a mask does not cut it at all as your breath simply cascades all around the face shield. Healthcare workers wear these in addition to our masks to reduce the risk of splashes into the eyes and not to take the place of a mask.
As we saw in last week’s article, science supports that masks are a very effective part of our public health strategy to limit this epidemic. In the end, it probably doesn’t matter which type of mask you choose to wear. Again, if is comfortable, fits well on your face to cover both nose and mouth, and is easy to breathe through then that is enough. I think that for the most part, people now get the need for wearing a mask in public. However, we still have new cases and a lot of those are due to social gatherings, especially parties and family get-togethers, where there may be great social pressure not to wear a mask. If you venture outside of your social bubble to go to a party or other large gathering, you better be wearing a mask. Do it not only for yourself, but for the rest of us. The sooner we get this pandemic behind us, the sooner we can go back to social and economic normalcy. And, thank you to all of you who do wear your mask.
[Miller Report for the Week of September 28, 2020; by William Miller, MD. ]