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有關口罩嘅二三事

2020/2/3 — 23:58

【文:Edwin Lo(中文大學通識基礎課程講師)】

睇咗兩日有關口罩嘅 paper ,將記得嘅嘢寫出嚟。

  1. 重用同消毒口罩係有文出過(不過淨係講有關N95)。基本上暫時都係冇一篇文夠膽確認重用嘅方法,但的確有啲方法似乎係可以考慮(所有實驗都係喺實驗室度試,冇試過喺實際用嘅情況下做研究。暫時亦冇住何公認嘅 guideline 支持重用即棄口罩)
  2. 基本上冇(未有)一份文支持 Cloth Mask 能夠有效保護戴嘅人減低減染風險。但有少量文支持布口罩 Cloth Mask 能夠過濾 aerosol 。有少量文提及 Cloth Mask 喺資源不足下可能係冇辦法之中嘅辦法。
  3. 口罩唔係淨係睇 Filtration Efficiency ,因為側邊嘅 face leak 都會係 aerosol /病源體入侵嘅方法,而佢哋係會行 Path of the least resistance 。所謂 99% BFE, PFE, VFE 係冇計到 face leak 。
    即係話,就算係 99% PFE/BFE/VFE 都好,如果有 10 幾 20% 由旁邊 leak 入去,而呢啲 leak 入去嘅嘢根本就連 filter 都唔洗過,即係話根本唔係擋到 99% 。
    另外,所以如果個口罩越焗(越大 pressure drop),口罩內就越大 negative pressure ,反而會越多 aerosol 喺側邊入侵。亦因為咁, N95 口罩其實唔做 fit test ,效果可能同 surgical mask 差唔多。
  4. 口罩最初並唔係用嚟保護戴嘅人,而係令醫生做手術時唔好噴到啲菌去病人度。所以本身用處係用嚟隔絕個 source ,防止戴嘅人播菌播毒,保護佢身邊嘅人;而唔係保護戴嘅人。
  5. 澄清一點,雖然口罩 (Surgical Mask) 最初發明出嚟唔係用嚟保護戴嘅人,但 N95 型口罩呢類稱為 Respirator 嘅口罩嘅就真係發明嚟保護戴嘅人。Surgical Mask 同 Respirator 兩樣嘢嘅性質同目的都唔同。
  6. 不過,近年嘅發現亦有唔少研究指出 surgical mask 能夠減低醫護人員 (Health care worker, HCW) 受感染嘅機會 [Protected from clinical respiratory illnesses (CRI) and Flu] 。雖然唔同研究之間會有啲差異,但普遍嚟講都會覺得 surgical mask 應該或多或少保護到戴嘅人。
    即係話,雖然口罩設計原意唔係用嚟保護戴嗰個,但又佢的確有少少保護作用。
  7. 又澄清一點,同 (3) 有關,雖然喺實驗室度做實驗,我哋往往睇到某某surgical mask/respirator 係能夠有 X% 嘅過濾效果,但喺實地研究口罩能否減低感染嘅研究入面,大多係做唔返咁明確嘅結果。亦即係話,喺真正使用口罩時,係有多好多環境因素影響到究竟佢有冇用。
  8. 又再澄清一點,似乎好多有關口罩嘅研究都係睇佢喺 hospital setting 入面保唔保護到啲 HCW ,又或者有啲文係睇 household setting (即係屋企入面如果有個人病,其他人戴口罩有冇用...etc)。至於社區入面人人戴口罩究竟有幾多用,好似冇乜文講(其實諗吓都知道係幾難做呢種實驗)。
  9. 口罩有冇用,亦好視乎隻病毒係點傳播。有啲係飛沫傳播 (droplets) :有啲可以靠 Aerosol 傳播,呢啲我哋會叫做 Air-borne 。
    前者係指病毒靠一啲大粒嘅 droplet (>5 um) 傳播,呢啲 droplet 因為較重,所以唔會懸浮喺空氣,會跌落地,因此傳播距離會比較短。
    譬如 SARS 就係靠 droplets 傳播。 [erratum: 多謝 Calvin Chong 指正 SARS 可以靠 airborne 傳播]
    而 Aerosol (通常< 5 um) 則係一啲講嘢或咳漱時產生嘅微粒,佢哋會浮喺空中,可以飄到好遠,留存較耐,穿透較高,所以較麻煩。口罩對前者係最有效,對後者則可能有啲效。N95如果做咗fit test,理論上係可以隔絕到大部份aerosol。流感似乎可以靠 droplet 同 aerosol 去傳。
  10. 暫時武漢肺炎 (2019-nCoV) 係估計靠飛沫。我都希望佢真係靠飛沫。
  11. 講返轉頭,通常會話口罩呢啲已經係最後嘅防線嚟。第一層防線係 crowd control (即係封關隔離之類),之後就靠 cough etiquette 同 hand hygiene 等等嘅方法,最後先係 personal respiratory protection 。因為 personal respiratory protection 就算講緊最好、最有效嘅情況下,其實都唔係好有效... 所以都係嗰句,出少啲街,見少啲人。
  12. 膠樽笠頭係好可能有啲用。有份文 show 過戴 face shield 本身可以隔到一大堆大粒嘅 influenza droplets 。
  13. 有唔少文其實已經估計過流感大爆發 (Pandemics) 時口罩不敷應用嘅情況。即係話口罩唔夠用其實係「預咗」嘅事嚟。今次某程度上係畀我哋感受到呢個壓力...(而家未算 pandemics ,因為暫時未係全球大規模爆發,如果之後變成呢個狀況,啲口罩會更唔夠用..)
  14. 叉開講,今時今日呢種 Polypropylene 口罩其實係 1960 年後嘅產物嚟,1890-1950 年其間其實醫護真係用紗布做嘅口罩,而且會重用。呢啲口罩嘅 efficiency 係冇而家啲即棄口罩咁好。
  15. 1918 年流感大爆發 (Spanish Flu, 死超多人嗰次) 試過全民戴罩,但效果好似係近乎冇用...(當然因為 i) 當時唔係而家呢種防水即棄口罩,而係布罩,ii) 流感可靠 airborne, iii) 口罩要配合其他 preventive measure 。)。而後來好似好一段時間都冇呢種全民戴罩嘅例子,直到 2003 SARS(?)。但即使係 SARS 時期,好似都冇研究顯示到究竟戴罩本身係咪有用(因為當時除咗戴罩仲同時做咗好多其他嘅 measures ,所以好難分辨出入面邊種方法有效 & 有幾有效。)[呢句係指 community-setting 嘅情況。]
  16. 又叉開講,口罩最主要嘅過濾作用係將一啲細菌/病毒/微粒等等經撞擊 (impaction) 而痴實咗喺啲口罩物料度(i.e. 啲 polypropylene fiber)。佢會 van der waals force 或其他 electrostatic force 痴實咗,基本上一痴住就好難甩出嚟。不過如果呢啲嘢慢慢 build up ,佢就會令過濾網啲窿越細,然後令呼吸更難, pressure drop 就越大,之後 face leak 嘅問題就會越嚴重。而且當越嚟越多嘢疊上疊噉痴埋一齊,佢哋就有機會甩出嚟。

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睇過(但唔限於此)嘅文,大家有心想搵返啲料可以自己睇:

1. Effectiveness of N95 respirators versus surgical masks in protecting health care workers from acute respiratory infection: a systematic review and meta-analysis.
2. Face mask decontamination and reuse: is it ok?
3. Evaluation of Control Measures Implemented in the Severe Acute Respiratory Syndrome Outbreak in Beijing, 2003
4. The Evolution of the Surgical Mask: Filtering Efficiency versus Effectiveness
5. Protecting healthcare workers from pandemic influenza: N95 or surgical masks? [review]
6. An investigation into the efficiency of disposable face masks
7. The efficiency of surgical face masks
8. Surgical Mask vs N95 Respirator for Preventing Influenza Among Health Care Workers A Randomized Trial
9. Workers against Respiratory Infections: Are We
10. A century after their introduction, are surgical masks necessary?
11. REUSABILITY OF FACEMASKS DURING AN INFLUENZA PANDEMIC
12. N95 Respirators or Surgical Masks to Protect Healthcare Workers against Respiratory Infections: Are We There Yet?
13. In vivo protective performance of N95 respirator and surgical facemask.
14. The Relationship of Fabric Properties and Bacterial Filtration Efficiency for Selected Surgical Face Masks
15. Surgical mask vs N95 respirator for preventing influenza among health care workers: a randomized trial.
16. Surgical mask to prevent influenza transmission in households: a cluster randomized trial.
17. How does breathing frequency affect the performance of an N95 filtering facepiece respirator and a surgical mask against surrogates of viral particles?
18. Potential demand for respirators and surgical masks during a hypothetical influenza pandemic in the United States.
19. Surgical mask versus N95 respirator for preventing influenza among health care workers: a randomized trial.
20. Effects of Ultraviolet Germicidal Irradiation (UVGI) on N95 Respirator Filtration Performance and Structural Integrity.
21. The efficacy of medical masks and respirators against respiratory infection in healthcare workers.
22. Surgical mask filter and fit performance
23. Influenza virus aerosols in human exhaled breath: particle size, culturability, and effect of surgical masks.
24. Filtration efficiency of surgical face masks: The need for more meaningful standards
25. A Randomized Clinical Trial of Three Options for N95 Respirators and Medical Masks in Health Workers
26. Masks, barriers, laundering, and gloving: Where is the evidence?
27. A cluster randomised trial of cloth masks compared with medical masks in healthcare workers
28. A cluster randomized clinical trial comparing fit-tested and non-fit-tested N95 respirators to medical masks to prevent respiratory virus infection in health care workers.
29. Face mask use and control of respiratory virus transmission in households.
30. Airborne transmission and precautions: facts and myths
31. Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?
32. A Study of Disposable Surgical Masks
33. The efficiency of surgical masks of varying design and composition (1975)
34. Simple respiratory protection—evaluation of the filtration performance of cloth masks and common fabric materials against 20–1000 nm size particles
35. Effectiveness of surgical masks against influenza bioaerosols
36. Approach to Prioritizing Respiratory Protection When Demand Exceeds Supplies During an Influenza Pandemic: A Call to Action
37. Proper use of surgical n95 respirators and surgical masks in the OR.
38. Evaluation of five decontamination methods for filtering facepiece respirators.
39. A pandemic influenza preparedness study: Use of energetic methods to decontaminate filtering facepiece respirators contaminated with H1N1 aerosols and droplets
40. Performance of an N95 filtering facepiece particulate respirator and a surgical mask during human breathing: two pathways for particle penetration.

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