Over the last two months, many health experts around the world have begun to tell us that wearing a non-medical or surgical mask can provide work to decrease the number of respiratory droplets we may expel1,2. Facemasks, when used along with proper handwashing and social distancing, can protect from COVID-191.
An important question to ask is how do we know wearing a mask can decrease the expulsion of our respiratory droplets? Well, to find out, let’s dive into the mechanisms behind the three most common types of masks that became a part of our norm: the N95s (otherwise known as respirators), surgical masks, and the homemade non-medical mask.
Before we begin going over how those types of masks work, we need to first understand the basics of a mask and how masks became a part of the arsenal of protective equipment used by healthcare workers. Before fancy N95s and disposable surgical masks were the norms, a face mask made of “a piece of gauze tied by two strings to the cap and sweeping across the face so as to cover the nose and mouth and beard” was worn by surgeon Jogann Mikulicz. He used this makeshift mask in 1897 to prevent his respiratory droplets from falling into a patient under his knife3. This was a revolutionary act during his time, as germ theory was only just starting to take shape3.
The idea of covering your nose and mouth with a reusable, cotton-based mask was made even more prominent within hospitals after the Manchurian plague of 1910-11 and the well-known 1918 influenza pandemic. By the early 20th century, medical practitioners came to the agreement that the function of the mask was to prevent the wearer from expelling respiratory droplets and to protect others from receiving those respiratory droplets. As scientists and doctors experimented with different designs of the medical mask, they identified that when certain masks used properly, they could offer protection from infection. Around the 1960s, a more snug and disposable mask was introduced: surgical masks were mostly made of synthetic woven fibres, thus replacing the early cotton masks.3
Now that we know the history behind why our healthcare workers wear masks, let’s dive into how each model works to prevent respiratory droplets and aerosols.
The N95
N95 respirators are used in two common settings: in the construction industry and the healthcare setting4. In the healthcare setting, people use N95 respirators that have been designed to form a seal around the nose and mouth when worn4,5. These masks should fit the wearer snugly (they normally have to be fitted for you before you wear them) and the filter has to be able to capture more than 95% of the particles in the air that passes through it5,6. Because an N95 has a tight fit and creates a seal, air (that may contain contagious aerosols) can’t find a way to a wearer’s mouth or nose unless it passes through the respirator first.
The filters in the N95 respirator are constructed from nonwoven mats of fine fibres. They work to capture more than 95% of the particles. These filters use three mechanical mechanisms: inertial impaction, interception, and diffusion5.
What the heck do those words even mean? Well, the inertial impaction and interception are involved in trapping large particles within the fibre and preventing them from reaching the wearer’s mouth or nose. Diffusion works by forcing smaller particles to move away from the airstream and to connect with the filter fiber5.
With these three mechanisms in place, the infectious aerosols have a higher chance of becoming trapped in the filter and removed from the air before they reach the wearer’s mouth and nose. The particles also won’t leave the fibres once they stick there because of a strong molecular attractive force, known as an electrostatic attraction. Electrostatic attraction works by forcing particles with the opposite charge from the fibre to bind the fibre and “stick”.5
To measure the filter performance, the most penetrating particle size (MPPS) is looked into. A higher MPPS performance means that small and large particles will be collected more efficiently5.
The surgical or face mask
Unlike the snug fit of an N95 respirator, surgical masks are loose fitting and don’t create a seal around the nose and mouth. A surgical mask can create a physical barrier between the mouth and nose of the wearer and their environment. When used and worn properly, a surgical mask can help block large particles from reaching the wearer’s mouth and nose. A surgical mask also reduces the chances of the wearer’s respiratory aerosols reaching others4,5.
Due to their design, a surgical mask can’t block or filter small particles that the wearer may breathe, but like a properly made N95 respirator, a surgical mask is made of synthetic woven fibres. Surgical masks should also only be worn once as reusing them can significantly decrease the ability to trap the wearer’s expelled respiratory aerosols5.
The homemade cotton masks
Cotton masks were once widely manufactured and were the norm throughout history before N95s and disposable surgical masks became commonplace amongst healthcare workers3. Homemade masks made of tightly woven cotton can provide a physical barrier between the wearer and their environment like a surgical mask, but cannot filter aerosols in any way2,7.
The good thing about homemade masks is that they can be reused after proper washing and sanitation, and wearing one can significantly reduce the number of respiratory aerosols that the wearer expels2,8. Many of our health experts and scientists recommend that now we wear a surgical or homemade mask when we go out in public to help decrease the transmission of pathogens between people.
However, it is crucial to remember that a homemade mask is not regulated like surgical masks and N95 respirator. This means that there are no testing standards to make sure they work effectively, and homemade masks may not entirely protect the wearer from small particle sizes that the wearer may inhale.
Well, now you know how these three types of masks work! As you can see, these masks work in varying degrees to filter the air the wearer breathes and prevent respiratory aerosol expulsion from the wearer to others. Most of our top health officials recommend that surgical and N95 respirator be kept for healthcare workers as they are the ones most exposed to pathogens. That being said, they also recommend that we should continue to wear our homemade masks to at least prevent most of our respiratory aerosols from getting to and potentially infecting the other people around us2.
References:
1. Advice on the use of masks in the community, during home care and in healthcare settings in the context of the novel coronavirus (COVID-19) outbreak. https://www.who.int/publications-detail/advice-on-the-use-of-masks-in-the-community-during-home-care-and-in-healthcare-settings-in-the-context-of-the-novel-coronavirus-(2019-ncov)-outbreak.
2. Canada, P. H. A. of. COVID-19: About non-medical masks and face coverings. aemhttps://www.canada.ca/en/public-health/services/diseases/2019-novel-coronavirus-infection/prevention-risks/about-non-medical-masks-face-coverings.html (2020).
3. Strasser, B. J. & Schlich, T. A history of the medical mask and the rise of throwaway culture. The Lancet 0, (2020).
4. Health, C. for D. and R. N95 Respirators and Surgical Masks (Face Masks). FDA (2020).
5. N95 Respirators and Surgical Masks | | Blogs | CDC. https://blogs.cdc.gov/niosh-science-blog/2009/10/14/n95/.
6. Proper N95 Respirator Use for Respiratory Protection Preparedness | | Blogs | CDC. https://blogs.cdc.gov/niosh-science-blog/2020/03/16/n95-preparedness/.
7. Can face masks protect against the coronavirus? Mayo Clinic https://www.mayoclinic.org/diseases-conditions/coronavirus/in-depth/coronavirus-mask/art-20485449.
8. Davies, A. et al. Testing the efficacy of homemade masks: would they protect in an influenza pandemic? Disaster Med. Public Health Prep. 7, 413–418 (2013).
