Statement on Mask-wearing (JAAST) マスク装着に関して(日本エアロゾル学会)


2020.Feb.21. 新型コロナウイルスや花粉症でのマスク装着に関して On wearing masks for new coronaviruses and pollen allergy

Point-1.「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 “Particles smaller than the fiber gap pass through the filter of the mask” is not accurate.

Point-2. 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 It is important to eliminate the leakage (intrusion) from the gap between the mask edge and the face surface, rather than the filter performance of the mask.

見解(全文)For full article:

The following is a machine-translation of the 1st statement (not an official version). Please refer to the original Japanese statement to preserve the correct expression.. | JAAST website |

The statement of JAAST (Japan Association of Aerosol Science and Technology) on wearing masks for new coronaviruses and pollen allergy (21 Feb. 2020)

Masks are indispensable not only for people who have a cold but also for people who have pollen allergy (hay fever). There is often incorrect information about wearing masks in mass media reports and web articles. Typical (incorrect) examples are as follows.

“Viral(virus) and pollen allergens (allergens in pollen) are so small that they pass through gaps in the mask fibers that it is meaningless if you do not use a special mask for viruses (or N 95 mask).” *Pass through* means “slip through” or “permeate”.

This type of discourse (statement), which was also common in the media reports on high concentration of PM2.5 (January to May 2013) in Beijing (In the above, “Viruses” and “allergen” were replaced by “PM 2.5”), is doubly scientifically incorrect as explained below. Masks here are medical and surgical masks that can be purchased in general at drugstores, and we will explain about these masks.

(1) Collection of particles in the mask
If virus and pollen allergens maintain a spherical shape with a diameter of approximately 0.1 μm (micrometer) in the air [Ref.1], it is certain that the size is approximately 1/300 of the typical diameter of cedar pollen of 30 micrometer. (1 micrometer = 1/1000 mm). However, the way air purifiers and other filters (The filter on the mask is one of them.) filter out particles in the air is fundamentally different from the way the “sieve” (screen) of the fiber traps particles larger than the mesh. For example, for a spherical particle with a diameter of 0.1 micrometer, the filter on the mask (even if it’s an N 95 mask) is full of empty spaces, and the spacing between the fibers that make up the filter is much larger than the diameter of the particle.

The way in which particles in the air are collected by the filter is based on the 3 mechanisms of “(a) Inertial collision, (b) Interruption, (c) Brownian diffusion”. The particles are collected by adhering to the surface of the elongated fiber (Refer to the figure.). Brown diffusion becomes dominant as the particle size decreases. As a result, for particles as small as 0.1 micrometer or less, the smaller the particles, the more likely they are to be trapped by the filter, contrary to most people’s expectations. The filters used in some masks are designed to attract and adhere particles to the fibers by electrostatic force, enhancing the ability to collect small particles. In many cases, filters with regular masks are highly efficient at capturing small particles. (Example 2 of Mask -1 (Myojo (2013) Ref.2) has a collection efficiency of 97% or more in the range of 0.025 ~ 0.21 μm in diameter.) Therefore, Because viruses and allergens are small, they pass through gaps in the mask fibers.…” is fundamentally wrong.

(2) What is a virus “specific” mask?
The behavior of suspended aerosol particles as described above is physically defined regardless of whether they are living or non-living. Aerosols can be defined as any system (state) in which cedar pollen with a diameter of 30 micrometer, droplets with a diameter of 10 micrometer or viruses with a diameter of 0.1 micrometer are stably present in the air as small liquid or solid particles.

The N 95 mask and the Japanese standard disposable dust respirator (DS2, etc.) are tested with sodium chloride (NaCl) particles. However, many people may be concerned that they can only collect and remove them because they are biological particles. For this reason, there are institutions in Japan and USA that measure the collection performance of Staphylococcus aureus aerosol particles (BFE) and bacteriophage (viruses that infect E. coli) aerosol particles (VFE). In the test, the liquid of biological particles is sprayed into mist (aerosol), and the resulting mist is passed through the test specimen to measure the number of biological particles (number of colonies, etc.) before and after the test specimen and calculate the collection efficiency. They are summarized as a standard in ASTM F 21013 [Ref.3]. The term “virus-specific-mask” probably refers to a mask that has undergone these tests. However, it should be recognized that the particle size of mist used in the tests is as large as 1 micrometer or more, and is not intended for small particles of about 0.1 micrometer or less.

(3) Precautions when wearing a mask
Even if a mask such as an N 95 mask or a DS2 section is worn to prevent particles smaller than about 0.1 micrometer, the percentage of inhaling particles is determined by the presence or absence of a gap between the mask edge and the face surface, and the degree of the gap, that is, air leakage (intrusion) from the gap, and the collection performance of the filter that constitutes the mask is secondary. The reason is that it is easier for air containing particles to flow through the gap between the face and the mask rather than through the fibers of the mask, which has resistance to ventilation. Masks such as N 95 and DS2 are more robust in shape, so it is certainly easier to stop the leakage of gaps between faces, but even medical and surgical masks that are generally available at drug stores can be expected to have a certain effect on particles of various sizes by wearing them to prevent air leakage from around the nose and jaw as much as possible [Ref. 2].

It is possible to some extent to prevent droplets from coughing and sneezing and the scattering of viruses and other pathogens contained in them by wearing any type of mask properly. People with symptoms such as coughing and sneezing are recommended to wear a mask.

[Ref.1] National Institute of Infectious Diseases, Website on Corona Virus.
[Ref.2] T. Myojo “PM2.5 and Mask”, Earozoru Kenkyu, 28, 287-291 (2013).
3) 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.

The above is a machine translation of the JAAST’s statement on wearing masks for new coronaviruses and hay fever. Please refer to the original Japanese statement to preserve the correct expression. | JAAST | Statement (21 Feb 2020) |

[Additional information]

A personal-activity video on Mask (made by an aerosol reseacher in Japan) …how commercially available masks or “handmade (paper towel or handkerchief)” masks collect particles effectively. キッチンペーパーやハンカチで作ったマスクも、同様に小さな粒子を捕集できる効果があります。それを研究者が検証した動画です。