Collective Masks Anti-Epidemic Capability (集體口罩抗疫力)

There are three effective in infection controls that employed in Hong Kong. They are frequency handwashing, social distancing (at least 1.5m) and wearing disposable surgery face mask. I try to propose a new term named Collective Masks Anti-Epidemic Capability (集體口罩抗疫力) to explain the positive control results from Asia people’s wearing mask practice through my following studies.

Introduction

In early March 2020, UK proposed “Herd Immunity” that once enough people get Covid-19, it will stop spreading on its own.  However, they haven’t considered the cost that will be devastating! (Ref. 1) The follow picture showed how herd immunity can stop a virus but it has an estimated fatality rate per infection somewhere around 1% (some region and special situation in hospital is much higher).

It verified not working now based on the trend of Covid-19 inflected in the world (Upto 2^nd Apr 2020). The only way to achieve herd immunity is through Covid-19 vaccine!

The nature of size distribution of viral aerosol and mask filtration effectiveness:

Why most Western people and experts think wear surgery face masks (not N95) are not effectively to protect people?  For example, US CDC recommends that healthy persons should not wear masks at all, only the sick ones.  Even though surgical masks, and improperly worn N95 respirator masks, do not offer perfect protection, the fact that wearing masks in Asian countries obtains a good result for controlling the inflection.

William G. Lindsley, etal. (2012) studied size distribution of cough-generated aerosol particles.  The average number of particles expelled per cough varied widely from patient to patient, ranging from 900 to 302,200 particles/cough while subjects had influenza and 1100 to 308,600 particles/cough after recovery.  When the subjects had influenza, an average of 63% of each subject's cough aerosol particle volume in the detection range was in the respirable size fraction (SD 22%), indicating that these particles could reach the alveolar region of the lungs if inhaled by another person. Most bio-aerosol size is below 1μm. (Ref. 2)

The below image shows the size of the coronavirus, relative to other small molecules like a red blood cell, or the often talked about PM2.5 particle size.

The follow table showed the different filtration effectiveness of different mask type. Most surgery face masks brought in market by citizen are ASTM F2100 Level 1 that BFE & PFE exceeds 95% or EN 14683 Type I that BFE exceeds 95%.

1.      BFE (bacterial filtration efficiency) measures how well the mask filters out bacteria when challenged with a bacteria-containing aerosol. ASTM specifies testing with a droplet size of 3.0 microns containing Staph. aureus (average size 0.6-0.8 microns). In order to be called a medical/surgical mask, a minimum 95% filtration rate is required. Moderate and high protection masks have bacterial filtration rates of 98% to greater than 99%.

2.      PFE (particulate filtration efficiency) measures how well a mask filters sub-micron particles with the expectation that viruses will be filtered in a similar manner. The higher the percentage, the better the mask efficiency. Although testing is available using a particle size from 0.1 to 5.0 microns, ASTM F2100-07 specifies that a particle size of 0.1 micron be used. When comparing test results it is important to note the size of the test particles used, as use of a larger particle size will produce a misleading PFE rating.

Recently, Sui Huang wrote an article named “COVID-19: Why we should all wear masks – There is new scientific rationale” on 27 Mar 2020. He said that small aerosols are carried by ventilation or by winds and thus can travel across rooms. The latest biological findings on SARS-Cov-2 viral entry into human tissue and sneeze/cough-droplet ballistics suggest that the major transmission mechanism is not via the fine aerosols but large droplets, and thus, warrant the wearing of surgical masks by everyone. (Ref. 3)

Only droplets below 10 micrometer diameter can reach the alveolae. The large spray droplets get stuck in the nose and throat (the naso-pharyngeal space) and in the upper air ducts of the lung, trachea and large bronchia.

Even with respect to the small aerosols we must not forget that the partial filtering provided by surgical masks is better than nothing.  The the particles leaked through mask and into environment was showed in the following diagram.  

On 31 Mar 2020, Ka Hung Chan and Kwok-Yung Yuen published an article in International Journal of Epidemiology (Ref. 4). They discussed three key epidemiological questions as follows:

Question 1: Can infected individuals reduce the risk of spreading the virus to others by wearing facemasks?

The answer to this is unambiguous.

Question 2: Can uninfected people reduce the risk of infection by wearing facemasks?

This is at the centre of the controversy.

Question 3: Can widespread use of facemasks in a population can facilitate the control of an epidemic?

The indirect impact of widespread use of facemasks must not be overlooked. It might increase the public’s risk awareness and improve their personal hygiene behaviours, many of which have been proven to be effective in infection control (e.g. handwashing, social distancing). The widespread use of facemasks may also reduce other droplet-transmitted infectious diseases, thus alleviating some burden on a highly-stressed health care system during an epidemic.

Preliminary Conclusion:

After study those paper and articles, I make a preliminary conclusion that the more people in a given population are wearing surgery face mask, the lesser probability for an individual to contact with that infectious agent through breaking.  It is because the inflection transmission link was broken or weaken both from suspended people breathing out and health people breathing in.  The following diagram is modified by herd immunity based on basic reproduction number (R0) equal to 4 and then reduce to R_n = 1 (Ref. 5).

I believe most of citizen wear surgery face mask could reduce the viral aerosol suspended in public area and indoor area so as to reduce the probability of Covid-19 inflection.  Therefore, I proposed a new term named “Collective Masks Anti-Epidemic Capability (CMAEC)” (集體口罩抗疫力) to describe Hong Kong, mainland China and Asia people practice of wearing face mask.  The further study on the model of CMAEC is valuable and verified by Asia practical successful cases. 

Some Additional Information:

My previous studies information about aerosol. (Ref. 6)

The curve A is experimental deposition velocities of aerosol showed the average is ~5x10^-2cm/s.

If man is 170cm height and his generated aerosol through cough will be deposited to ground after {(170/0.05)/60}min ~ 57min ~ 1hr! (Turbulence effect has not considered.)

Another study I performed was for indoor viable aerosol and its size distribution. (Ref. 7)

It was found that indoor airborne microbes most in the range of 1.1 to 2.1μm.  So that the mask of BFE could filter most of indoor airborne microbes >3μm; mask of BFE & PFE could filter all most all indoor airborne microbes (0.1 to 5μm)

Since aerosol suspends in the air as long as 1hr and indoor airborne microbes from the range of 1.1 to 7μm and above, wearing disposable face mask could help to remove at least bacteria level aerosol.

Reference:

1)      Antonio Regalado (2020) “What is herd immunity and can it stop the coronavirus?” MIT Technology Review - https://www.technologyreview.com/s/615375/what-is-herd-immunity-and-can-it-stop-the-coronavirus/

2)      William G. Lindsley, etal. (2012) Quantity and Size Distribution of Cough-Generated Aerosol Particles Produced by Influenza Patients During and After Illness”,  J Occup Environ Hyg. 2012; 9(7): 443–449. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4676262/pdf/nihms742694.pdf

3)      Sui Huang (2020) COVID-19: Why we should all wear masks – There is new scientific rationale - https://medium.com/@Cancerwarrior/covid-19-why-we-should-all-wear-masks-there-is-new-scientific-rationale-280e08ceee71

4)      Yuen, K.Y. & Chan, K.H. (2020). COVID-19 epidemic: disentangling the re-emerging controversy about medical facemasks from an epidemiological perspective. International Journal of Epidemiology, Published Online. dyaa044, https://doi.org/10.1093/ije/dyaa044

5)      Edward Nirenberg (2019) Herd Immunity: How Does It Work? https://medium.com/@edwardnirenberg/herd-immunity-how-does-it-work-28eeeeed2dfa

6)      Kim Hung, Lotto LAI (1997) “Detailed investigations on the inorganic chemical constituents of atmospheric aerosol and various deposition in Hong Kong” MPhil Thesis, City University of Hong Kong.

7)      Kim Hung Lotto LAI (2001) “A study of the characteristic of Indoor Air Quality in the multi-purpose premises” MSc Dissertation, Hong Kong Polytechnic University.