English follows Japanese
新型コロナウイルスや花粉症でのマスク装着に関する日本エアロゾル学会の見解
○「繊維の隙間より小さい粒子はマスクのフィルターを通過する」は間違い
○ 大事なことは「マスクのフィルター性能より、マスクの縁と顔表面との隙間からの漏れ（侵入）を少しでもなくすこと！」
→ ［見解（全文）はこちらをご覧下さい］（2020.2.21掲載，2020.3.26一部修正）
On Mask Wearing for COVID-19 and Hay Fever
- The statement “Particles smaller than the gaps in the fibers can pass through the mask filter” is incorrect.
- What is important is to minimize leaks (infiltration) from the gaps between the mask edges and the surface of the face, rather than focusing solely on the filter performance of the mask!
- [View the full statement here] (Published on February 21, 2020, with partial revisions on March 26, 2020)
An English translation of the Japanese statement (PDF) from the society (JAAST) regarding mask use for COVID-19 and pollen allergy.
JAAST’s View on Mask Use for COVID-19 and Pollen Allergy
(Revised on March 26, 2020)
Masks are essential not only for people with colds but also for those with pollen allergies. However, there is often misinformation about mask use in media reports and online articles. A typical example is:
“Because viruses or pollen allergens are small, they pass through the gaps in mask fibers, so only special masks for viruses (such as N95 masks) are effective.”
This type of statement was also commonly seen in reports about high concentrations of PM2.5 in Beijing (Jan–May 2013), with “virus” or “allergen” replaced by “PM2.5.” As explained below, this is scientifically incorrect in two ways. Here, “masks” refer to medical or surgical masks commonly available at drugstores.
1. Particle Capture by Masks
If viruses or pollen allergens remained as spherical particles of about 0.1 μm in diameter in the air, they would indeed be about 1/300th the size of typical cedar pollen (30 μm diameter). However, the mechanism by which filters (including those in masks) capture airborne particles is fundamentally different from simply sieving out particles larger than the mesh size.
For example, for a 0.1 μm spherical particle, even an N95 mask filter has gaps much larger than the particle’s diameter. So, how are particles captured? The mechanisms are:
- Inertial Impaction
- Interception
- Brownian Diffusion
Particles adhere to the surfaces of the fine fibers in the filter (see diagram). Brownian diffusion becomes dominant as particle size decreases. Contrary to common belief, particles around 0.1 μm or smaller are actually more easily captured by the filter due to this effect. Some mask filters also use electrostatic forces to enhance the capture of small particles. In many cases, regular mask filters capture small particles with high efficiency (for example, Mask-1 captured 97% or more of particles 0.025–0.21 μm in diameter). Therefore, the explanation that “viruses or allergens are small and pass through mask fibers” is fundamentally incorrect.
2. What Are “Virus-Specific Masks”?
The behavior of airborne aerosol particles is governed by physical principles, regardless of whether they are biological or non-biological. Cedar pollen (30 μm), droplets (10 μm), or viruses (0.1 μm) all exist as stable liquid or solid particles in the air and are all considered aerosols. N95 masks and Japanese disposable dust masks (e.g., DS2) are tested using sodium chloride particles. For those concerned about biological particles, organizations in Japan and the US also test masks for their Bacterial Filtration Efficiency (BFE) and Viral Filtration Efficiency (VFE), using aerosols of Staphylococcus aureus or bacteriophages. These tests follow standards such as ASTM F2101. However, the mist particles used in these tests are 1 μm or larger, not targeting particles as small as 0.1 μm. This should be kept in mind.
3. Points to Note When Wearing Masks
Even when wearing an N95 or DS2 mask to block particles around 0.1 μm, the proportion of particles inhaled depends on the presence and size of gaps between the mask’s edges and the face—i.e., leakage. The filter’s capture performance is secondary. Air containing particles will flow more easily through gaps than through the mask’s fibers. While N95 and DS2 masks fit more tightly, even regular medical or surgical masks can be effective against various particle sizes if worn to minimize leakage around the nose and chin.
4. In Conclusion
This statement refers to N95 masks and medical/surgical masks with performance indications, not all masks. While this is not direct evidence, from the perspective of cough etiquette, even masks without performance indications, such as cloth masks, are necessary to prevent the direct spread of droplets. People with symptoms such as coughing or sneezing are recommended to wear masks.
References
- National Institute of Infectious Diseases, “What is Coronavirus?”
https://www.niid.go.jp/niid/ja/kansennohanashi/9303-coronavirus.html - Aerosol Research Journal: Toshihiko Myojo, “PM2.5 and Masks,” 28, 287-291 (2013). https://jaast.jp/?p=125
- ASTM F2101-19, Standard Test Method for Evaluating the Bacterial Filtration Efficiency (BFE) of Medical Face Mask Materials, Using a Biological Aerosol of Staphylococcus aureus, ASTM International.
- Ministry of Health, Labour and Welfare, “Cough Etiquette”
https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/0000187997.html