Does One-Way Masking Work?
(COVID-19 Series, Part 4 of 7)
Table of Contents
Masking As A Form of Air Filtration
If you are the only person wearing a mask, are you protected? As we will explore today, this is not really a yes or no question. It will depend on the mask, the setting, and the amount of time spent there. Masks are excellent filters, and have been used in a variety of settings to protect people from contaminants from dust, to droplets, to wildfire smoke and other aerosols. However, they have their limitations. Just like a raincoat has its limitations in scenarios like hurricanes, one-way masking alone will not offer sufficient protection in situations of high viral densities for long periods of time. As we continue our journey through the complexities of COVID-19 protection, it’s essential to acknowledge that no single measure is a silver bullet.
In Part 3, we considered the critical roles of ventilation and filtration in mitigating the spread of the virus. In Part 4 of our series, we will take a closer look at optimizing masking as a form of air filtration. Whether it is to filter allergens, wildfire smoke, or airborne pathogens like COVID-19, we will look into how masking makes a difference in the air we inhale. We will specifically take a close look at one-way masking—a scenario where only the non-infected person wears a mask, and gauge effectiveness in different types of situations and environments.
Masking, while seemingly simple, holds many nuances that make a difference for personal safety and public health. For instance, by examining the filtration capabilities of various mask types, the importance of proper fit, and the overall effectiveness of one-way masking as a strategy, we aim to provide a comprehensive understanding of its role in reducing viral density in different settings. Join us as we explore how different masks perform and what this means for your protection against COVID-19.
What Type of Mask Is It?
In order to understand how effective a mask is for COVID-19 protection, we first need to look at what type of mask it is. This blog focuses on the most accessible options such as surgical, KN95s, N95 masks and the differences in protection offered between them, especially in one-way masked situations. Let’s examine the strengths and weaknesses of the different mask types.
Masks that Provide Droplet Protection
Surgical masks are loose-fitting, disposable masks that create a physical barrier between the mouth and nose of the wearer and potential contaminants in the immediate environment. They are meant to help block large-particle droplets, splashes, sprays, or splatter.
Although they have been popularized for use throughout the ongoing COVID-19 pandemic, they are not designed to capture aerosols– the very small particles in the air that can carry the SARS-CoV-2 virus (the virus that causes COVID-19).
Their filters are inferior to those of respirator masks, which are made to catch both droplets and aerosols. One study shows that the filters in surgical masks have at least 15% lower filtration efficiency than the filters of N95 respirators. However, surgical mask filters did perform better than the cotton and polyester cloth masks in the study, which had 70% lower filtration efficiency than N95 respirators [1].
Surgical masks also fit loosely on the face, so unfiltered air can get in through gaps by the nose and at the edges of the mask. They can be modified to reduce some of this leakage using a mask fitter, mask brace, or by using a knot-and-tuck method [2].
Masks That Protect From Both Droplets and Aerosols
Respirator masks, like N95s, KF94s, KN95s, FFP2s, FFP3s, and P100s, filter out both droplets and aerosols that carry viruses very efficiently (94-99.97%), and have a close facial fit to minimize leakages. For this reason, they are recommended for protection from diseases that travel through the air, like COVID-19, RSV, TB, influenza, and measles, as well as for wildfire smoke. The weave of the filter layers, as well as the addition of electrostatic charge, makes them proficient in capturing a wide variety of particle sizes, including those mostly likely to carry viruses. They also conform well to the face offering a better seal than other types of masks.
Although there are some differences between the different types of respirators due to variations in the filter materials, amount of electrostatic charge, and in the fit factor (how well it performs on fit tests to minimize leakage around the edges), they are all very effective.
N95 (the American Standard regulated by NIOSH) and FFP2 (the European Standard, Filtering Face-Piece 2) are approximately equivalent and are the minimum typically advised for COVID-19 protection. They block out 95% of particles that are .3 microns, as well as blocking out smaller and larger particle sizes at higher efficiencies.
KN95s and KF94s are not regulated by NIOSH, but instead by regulating agencies in other countries. They have ear loops as opposed to headbands, which may not provide as good of a seal for some people, resulting in more leakage than N95s with headbands. However, they are still much more protective than surgical masks.
One study shows that the filter layers of N95 respirators were 8-fold thicker and had 2-fold higher dipole (electrostatic) charge density than that of the Emergency Use Authorization (EUA)-approved KN95 respirators they tested. However, the KN95 respirators still showed filtration efficiencies as high as N95 respirators [1].
Another study that examined fit factors of different masks determined that KN95 and KF94 masks with adjustable ear loops had fit-test factors comparable to N95 masks, whereas those without adjustable ear loops fared worse [3].
Next are the N99 masks, and their European equivalents FFP3. These respirator masks offer even more protection than N95 masks, with over 99% efficiency.
A step above the common disposable respirator masks are the reusable respirators, which include elastomerics with P100 filtering cartridges and PAPRs, (Powered Air Purifying Respirators). These options have a higher up-front cost than disposable respirators, but ultimately reduce the amount of money spent and waste produced over a long period of time. Additionally, elastomeric masks and PAPRs provide an even higher level of protection against viral aerosols than disposable options, as well as protecting against a wider variety of environmental contaminants, such as gases, vapors, and other hazardous chemicals [4].
Finding a respirator mask type that fits your particular face well is important. Try different sizes and styles of masks to determine which has the best fit and comfort for you. Make sure you wear the mask correctly to cover your nose and mouth, and minimize any gaps around the edges and nose. To learn more about all the types of masks, along with more tips on achieving a good fit, check out the World Health Network’s Mask Resource [5].
Spoiler Alert: Research shows that the type and fit of the mask you wear for protection from COVID-19 makes a big difference in the amount of time and protection afforded, with respirator masks offering much more protection than surgical or cloth masks.
Let’s take a look at some of the research involving different one-way masked situations.
When the Infected Person Is Wearing the Mask (Source Control)
In a 2024 study, researchers looked at how much virus was exhaled by infected volunteers without masks and with wearing different types and fit of masks [6]. This type of study examines source control protection, that is, how well other people are protected if an infected person (the source) is wearing a mask. This study provides insights into how infectious aerosols travel through the masks’ filter – or around the filter, depending on mask fit and type – which provides important insights when looking at mask efficacy.
The researchers in this study aimed to mimic real-world mask use by not providing volunteers with prior training in proper use of, or professional fit tests for, the masks and respirators used in the study. They found:
“All masks and respirators significantly reduced exhaled viral load, without fit tests or training. A duckbill N95 reduced exhaled viral load by 98%, and significantly outperformed a KN95 as well as cloth and surgical masks. Cloth masks outperformed a surgical mask and (one of the) tested KN95.”
When looking at type, N95 respirators reduced viral load in total exhaled breath aerosol by 98%, followed by cloth masks with a reduction of 87%, KN95 respirators reduced exhaled viral load by 76%, and surgical masks reduced exhaled viral load by 74%.
When looking at fit: Surgical mask efficacy was improved by using the knot-and-tuck method, which reduces the gaps and leakages. One KN95 mask that did not conform well to wearers’ face was found to actually provide less protection than cloth and surgical masks, due to more unfiltered air escaping through the gaps than the filter. Well-fitting duckbill-style N95 masks performed best blocking 99% of large particles and 98% of small particles containing the virus in the exhaled breath.
Researchers concluded that N95 masks provide the best protection, and should be standard for use in healthcare settings, stating:
“Our data suggests a mildly symptomatic person with COVID-19, not wearing a mask or respirator, would exhale on average 2800 RNA copies per hour in their total exhaled aerosol or a little more than two infectious doses, quanta, per hour (may be an underestimate). However, wearing a N95 respirator would reduce the aerosol shedding rate to less than one tenth of a quantum per hour. This suggests that wearing a N95 respirator can lower the risk of transmission by a factor of 20.”
One-way masking by the person who is infected with COVID-19 (source control) offers more protection than one-way masking by the non-infected (also called “susceptible”) person, because so much of the virus is caught in the respirator mask, preventing it from accumulating in higher density in the room. However, COVID-19 is often spread pre-symptomatically (a period of high contagiousness before any symptoms develop and/or before a test can pick up infection) or asymptomatically (by a person who never develops symptoms themselves but still can transmit to others). Since the dropping of mask requirements, there is no guarantee that even a symptomatic or recently-exposed person will wear a mask, even though they are recommended to. As a result, more and more people are finding themselves in one-way masked situations as the non-infected person.
When Only the Non-Infected Person is Masking
A 2021 study by Bagheri, Gholamhossein, et al. [7] examines the infection probability of different masking situations by looking at inward leakage in respirators and unmodified surgical masks, and calculating the probability of infection over different amounts of time.
When looking at the results of the one-way masked situation – in which the non-infected person wore the mask while the infected person did not – there was a significant difference in time until infection depending on whether the non-infected person wore a respirator mask, specifically the FFP2 (Filtering Facepiece 2), or a surgical mask.
They found:
Respirator masks (FFP2s) provided significantly more protection and for longer amounts of time than the surgical masks, due to the respirator’s better filtration and fit, which reduced leakage.
Wearing an FFP2 mask without any adjustment whatsoever leads to total inward leakage of 53% for the smallest particle bin (0.3 μm to 0.37 μm), which decreases to 16% for 3 μm.
By simply adjusting the mask nosepiece to the nose the mask’s total inward leakage is improved by a factor of 4.3 for the smallest particles and by a factor of 7.5 for 3 μm particles.
The surgical mask was associated with the high total inward leakage, with the maximum value being in excess of 70% occurring for the smallest particle size. This was caused both through relatively high filter penetration (5% for particles around 0.3 μm) and the evidently high leakage.
Image 2: The FFP2 and Surgical Mask used in the Study
2. The infection risk over time for each one-way masked situation (the non-infected person wearing a surgical or FFP2) was:
- With a surgical mask their infection risk reaches 90% after 30 minutes.
- With an FFP2 mask their infection risk remains at about 20% even after 1 hour.
- When neither person is masked, even when 6 feet apart, the infection risk reaches 90% after only a few minutes.
Even when looking at a 20 minute period, this study shows one-way masking is not sufficient on its own, with the probability of COVID-19 infection risk to be 10% for a person wearing a fit-tested FFP2, and 70% for a person wearing a surgical mask, when standing 6 feet from an infectious non-masked person.
Image 3: Infection Risks of One-way Masking by the Susceptible with Surgical and FFP2 (N95-level) Masks
Notably, the study also showed when both people are wearing a fit-tested FFP2 mask, protection is very high (with only 0.4% infection risk after an hour). Universal masking situations can provide protection over long amounts of time. However, universal masked situations with respirators have become increasingly difficult to arrange and enforce.
What Does It All Mean?: One-Way Masking Is Not Enough on Its Own
One-way masking, while beneficial, is not a standalone solution in our fight against COVID-19. The studies and insights discussed highlight that the type and fit of the mask you wear can significantly impact your level of protection. Respirator masks like N95s and FFP2s offer superior filtration and fit, markedly reducing the risk of infection even in one-way masked scenarios. However, relying solely on one-way masking places a considerable burden on the non-infected individual to maintain high levels of protection in environments where others may not be masked. As we navigate the ongoing challenges of the pandemic, it becomes clear that a multi-layered approach by combining high-quality masks with proper fit, enhanced ventilation, and air filtration is essential for creating safer indoor environments.
If you mask in indoor public spaces we thank you. Whether you’ve been wearing cloth, surgical or N95, we salute you. Respirators, like N95s, are best since they offer aerosol protection from things like wildfire smoke, allergens such as dust and animal dander, and infectious diseases that spread through the air. We hope this post highlighted the importance of filtration, fit, and environment so you can find the best mask for the situation. Our next post in this series (Part 5: The Interplay of Time, Viral Density, and Filtration) will explore how different mitigation measures can work together synergistically to reduce the spread of airborne illness and create healthier indoor spaces for all.
Know better. Do better. Breathe better.
“A miracle is when the whole is greater than the sum of its parts. A miracle is when one plus one equals a thousand.”
[1] Yim, W., Cheng, D., Patel, S. H., Kou, R., Meng, Y. S., & Jokerst, J. V. (2020). KN95 and N95 Respirators Retain Filtration Efficiency despite a Loss of Dipole Charge during Decontamination. ACS Applied Materials & Interfaces, 12(49), 54473–54480. https://doi.org/10.1021/acsami.0c17333
[2] Brooks, J. T., Beezhold, D. H., Noti, J. D., Coyle, J. P., Derk, R. C., Blachere, F. M., & Lindsley, W. G. (2021). Maximizing fit for cloth and medical procedure masks to improve performance and reduce SARS-COV-2 transmission and exposure, 2021. Morbidity and Mortality Weekly Report, 70(7), 254–257. https://doi.org/10.15585/mmwr.mm7007e1
[3] Thapthim-On, M., Chaiear, N., & Mitsungnern, T. (2024). Which alternative well-fit masks can be used in medical procedures? Journal of Infection and Public Health, 17(2), 204–211. https://doi.org/10.1016/j.jiph.2023.11.023
[4] Health Care Workers Australia. (2021, October 5). Re-usable respirators – Health Care Workers Australia. https://healthcareworkersaustralia.com/elastomeric-mask/
[5] World Health Network Covid Action Group Mask Fit Matters – Don’t Share Your Air. WHN Science Communications 2021; 2 (2): 1-1. https://doi.org/10.59454/whn-2102-945
[6] Lai, J., Coleman, K., S.-H. Sheldon Tai. German, J., Hong F., Albert B., Esparza, Y., Rastogi, D., Srikakulapu, A., Kalliomäki, P., Schanz, M., Smith, A., Maldonado, I.S., Oertel, M., Fadul, N., Gold, T.L., McPhaul, K., Ma, T., Cowling, B., Milton, D. (2024). Relative efficacy of masks and respirators as source control for viral aerosol shedding from people infected with SARS-CoV-2: a controlled human exhaled breath aerosol experimental study. EBioMedicine, 105157. https://doi.org/10.1016/j.ebiom.2024.105157
[7] Bagheri, G., Thiede, B., Hejazi, B., Schlenczek, O., & Bodenschatz, E. (2021). An upper bound on one-to-one exposure to infectious human respiratory particles. Proceedings of the National Academy of Sciences of the United States of America, 118(49). https://doi.org/10.1073/pnas.2110117118
Sarah Masih, MD, MBA
Sarah Masih is the Executive Director of Air Support Project and has extensive experience in project management, fundraising, and communications. Her deep commitment to public health and her collaborative approach with community organizations and health experts make her a vital force in the fight for cleaner air and healthier communities.
SHEA O'NEIL
Volunteer
Creator of COVID-conscious advocacy works: writings, art, and social networking. BA degree in psychology, with over 11 years of continued education studies in allergy and autoimmune research, and continued research the past 3 years into airborne irritants and COVID-19. Parent, disabled rights advocate.
S. Riley
S. Riley is a mutlidisciplinary content creator and technology enthusiast with a background in crowdfunding and project management. He holds a BA in English, and is grateful to be able to utilize his unique skill set in service of Air Support Project's humanitarian mission.
Melissa Smallwood
Melissa Smallwood is Science and Technology Policy researcher with a background in neuroscience, psychology, and social science, and the author of The Future of Long COVID: A Threatcasting Approach. They are interested in how disability and healthcare justice intersect with emerging technologies, and are an advocate for COVID mitigation strategies to help end the pandemic.
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“Mask Type Matters” graphic describing the major differences between cloth, surgical, KN95/KF94/FFP2, N95/FFP3, and N99/P100/PAPR masks. Published under Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/ ).
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