立場新聞 Stand News

新型冠狀病毒疫情系列之十-口罩

2020/3/27 — 14:20

立場新聞圖片

立場新聞圖片

口罩問題-如何使用口罩?應否使用口罩?什麼口罩有效?,已成為人們關注的口罩,連特朗普也呼籲人們使用消毒液、重複用外科口罩。

3 月 17 日《新英倫》期刊有文章討論了新冠狀病毒的傳播途徑。(註一) 它比較了新型冠狀病毒與 2003 年沙士的傳播,發現兩者在這方面十分相似,其在塑料、不锈鋼表面上殘留期較長,理論上可達3天 (由於病毒隨時間減弱,未必帶來危害)。

世衛的早期報告指新冠狀病毒很難在乾燥的空氣殘存 (註1.1),但後來研究發現,它可以在空氣中 (霧化) 存在 3 小時。這可能誘使人們對口罩的需求。

廣告

霧化

根據定義,霧化 (氣溶膠) 是固體或液體顆粒在空氣中懸浮,由於其沉降速度低,只要足夠小,就可以長時間在空氣中懸浮。100 微米大小的顆粒沉降 3 米距離的時間為 10  秒,對於 20 微米為 4 分鐘,對於 10 微米為 17 分鐘,對於 5 微米為 62 分鐘。直徑小於 3 微米的顆粒基本上不會沉降。咳嗽或打噴嚏會產生大量顆粒,其中許多顆粒的直徑 <5–10微米 (註1.2)。

直徑大於6微米的顆粒容易留在上呼吸道;一般地,大於 10–20 微米的顆粒不會沉積在下呼吸道中。從理論上講,流感病毒可以通過霧化傳播。人的頭髮一般為 50 微米。

廣告

新冠狀病毒的平均大小為 125 納米 (註1.3)。1 微米=1000 納米。因此,這裡的討論不是一顆新冠狀病毒的傳播,而是有效的可感染的或有一定數量的新冠狀病毒的液體顆粒。已知的能夠通過空氣傳播的疾病包括:結核、水痘和麻疹。但霧化的概念並不統一 (註1.5),市民常用的外科口罩只對咳嗽或打噴嚏會產生大量顆粒有效。在霧化的情況需要 N-95 口罩,並嚴格依照配帶手法。

外科口罩

各國對巿民使用外科口罩的建議不一 (註二):

世衛只建議照顧者使用外科口罩;

中國建議巿民一般地使用,但在很低風險的可以使用布口罩;

香港建議巿民在乘搭公共交通和人口密集地方使用;

新加坡建議有感冒跡象患者使用口罩;

日本認為口罩只可以防止因對方咳嗽、口沫帶來的感染,只建議巿民在空氣混濁的地方使用;

美國不建議沒有受感染跡象的巿民帶口罩,美國衛生及公共服務部呼籲巿民停止囤積口罩;

英國認為口罩只在探病時有用,巿民沒有廣泛使用的必要;

德國認為人們使用口罩反而有可能導致對其他衛生措施,如洗手等疏忽;而世衛在如何使用口罩上意見分歧。

《柳葉刀》的一篇在3月3日發表的文章被廣泛引述,支持人們常戴口罩,它甚至指在外科口罩缺乏的情況下,布口罩可以幫助外科醫生。(註三)

但是,一篇在 2015 年發表的文章已指出布料可積存大量病毒和細菌,它們約有 95% 可穿透布口罩,而外科口罩的穿透率為 44%。(註四)

無論如何,新型冠狀病毒的播毒期可以達 24 天,大部份患者在受感染的 5 天後才出現病癥,戴口罩還是一個良好習慣。日本在 2014 年對一萬名學生做了一項研究,發現常戴口罩可減少染上流感。(註五)

西班牙向北約求助,要求 150 萬個外科口罩(註六),全球的口罩的一半以上在中國大陸生產,中國現正解封,恢復生產,它可能對全球的口罩荒有幫助。全民帶口罩是否必需,這將是西方社會面對的問題。

備註

註一

Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV

We found that the stability of SARS-CoV-2 was similar to that of SARS-CoV-1 under the experimental circumstances tested. This indicates that differences in the epidemiologic characteristics of these viruses probably arise from other factors, including high viral loads in the upper respiratory tract and the potential for persons infected with SARS-CoV-2 to shed and transmit the virus while asymptomatic.3,4 Our results indicate that aerosol and fomite transmission of SARS-CoV-2 is plausible, since the virus can remain viable and infectious in aerosols for hours and on surfaces up to days (depending on the inoculum shed). These findings echo those with SARS-CoV-1, in which these forms of transmission were associated with nosocomial spread and super-spreading events,5 and they provide information for pandemic mitigation efforts.

註1.1

How does COVID-19 spread?

People can catch COVID-19 from others who have the virus. The disease can spread from person to person through small droplets from the nose or mouth which are spread when a person with COVID-19 coughs or exhales. These droplets land on objects and surfaces around the person. Other people then catch COVID-19 by touching these objects or surfaces, then touching their eyes, nose or mouth. People can also catch COVID-19 if they breathe in droplets from a person with COVID-19 who coughs out or exhales droplets. This is why it is important to stay more than 1 meter (3 feet) away from a person who is sick.

WHO is assessing ongoing research on the ways COVID-19 is spread and will continue to share updated findings.

註1.2

Review of Aerosol Transmission of Influenza A Virus

In theory, influenza viruses can be transmitted through aerosols, large droplets, or direct contact with secretions (or fomites). These 3 modes are not mutually exclusive.

By definition, aerosols are suspensions in air (or in a gas) of solid or liquid particles, small enough that they remain airborne for prolonged periods because of their low settling velocity. For spherical particles of unit density, settling times (for a 3-m fall) for specific diameters are 10 s for 100 μm, 4 min for 20 μm, 17 min for 10 μm, and 62 min for 5 μm; particles with a diameter <3 μm essentially do not settle. Settling times can be further affected by air turbulence (10,11).

The median diameters at which particles exhibit aerosol behavior also correspond to the sizes at which they are efficiently deposited in the lower respiratory tract when inhaled. Particles of >6-μm diameter are trapped increasingly in the upper respiratory tract (12); no substantial deposition in the lower respiratory tract occurs at >20 μm (11,12). Many authors adopt a size cutoff of <5 μm for aerosols. This convenient convention is, however, somewhat arbitrary, because the long settling time and the efficient deposition in the lower respiratory tract are properties that do not appear abruptly at a specific diameter value. Certainly, particles in the micron or submicron range will behave as aerosols, and particles >10–20 μm will settle rapidly, will not be deposited in the lower respiratory tract, and are referred to as large droplets (10–12).

註1.3

https://microbewiki.kenyon.edu/index.php/Coronavirus

The virion shape is spherical, with an average size of 125 nm

註1.4

Airborne Precautions.

註1.5

Recognition of aerosol transmission of infectious agents: a commentary

註二

Rational use of face masks in the COVID-19 pandemic :March 20, 2020

Recommendations on face mask use in community settings

WHO

If you are healthy, you only need to wear a mask if you are taking care of a person with suspected SARS-CoV-2 infection.

China

People at moderate risk * of infection: surgical or disposable mask for medical use. 

*People at moderate risk of infection include those working in areas of high population density (eg, hospitals, train stations), those have been or live with somebody who is quarantined, and administrative staff, police, security, and couriers whose work is related to COVID-19.•

People at low risk † of infection: disposable mask for medical use. 

†People at low risk of infection include those staying in areas of high population density (eg, supermarket, shopping mall), who work indoors, who seek health care in medical institutions (other than fever clinics), and gatherings of children aged 3–6 years and school students.•

People at very low risk ‡ of infection: do not have to wear a mask or can wear non-medical mask (such as cloth mask). 

‡People at very low risk of infection include those who mostly stay at home, who do outdoor activities, and who work or study in well-ventilated areas.

Hong Kong

Surgical masks can prevent transmission of respiratory viruses from people who are ill. It is essential for people who are symptomatic (even if they have mild symptoms) to wear a surgical mask.•

Wear a surgical mask when taking public transport or staying in crowded places. It is important to wear a mask properly and practice good hand hygiene before wearing and after removing a mask.

Singapore

Wear a mask if you have respiratory symptoms, such as a cough or runny nose.

Japan

The effectiveness of wearing a face mask to protect yourself from contracting viruses is thought to be limited. If you wear a face mask in confined, badly ventilated spaces, it might help avoid catching droplets emitted from others but if you are in an open-air environment, the use of face mask is not very efficient.

USA

Centers for Disease Control and Prevention does not recommend that people who are well wear a face mask (including respirators) to protect themselves from respiratory diseases, including COVID-19.

  • US Surgeon General urged people on Twitter to stop buying face masks.

UK

Face masks play a very important role in places such as hospitals, but there is very little evidence of widespread benefit for members of the public.

Germany

There is not enough evidence to prove that wearing a surgical mask significantly reduces a healthy person's risk of becoming infected while wearing it. According to WHO, wearing a mask in situations where it is not recommended to do so can create a false sense of security because it might lead to neglecting fundamental hygiene measures, such as proper hand hygiene.

註三

Mass masking in the COVID-19 epidemic: people need guidance :March 03, 2020

Disposable surgical masks and their technical specifications were designed specifically for the protection of health-care workers during occupational exposures. Cloth masks were used by surgeons successfully during operations before disposable masks were available. In real life, most people in all seriously affected areas are reusing their disposable masks.

註四

A cluster randomised trial of cloth masks compared with medical masks in healthcare workers

2015 Apr 22

. An analysis by mask use showed ILI (RR=6.64, 95% CI 1.45 to 28.65) and laboratory-confirmed virus (RR=1.72, 95% CI 1.01 to 2.94) were significantly higher in the cloth masks group compared with the medical masks group. Penetration of cloth masks by particles was almost 97% and medical masks 44%.

註五

Experts urge authorities to broaden the use of face masks to curb the spread of COVID-19

While a non-peer reviewed study found that the incubation period for the coronavirus could be as long as 24 days, other experts and researchers have estimated that most infected people will develop symptoms about five days after infection.

In a 2014 study in Japan of over 10,000 students, it was shown that face masks do reduce the risk of catching the flu.

A 2016 analysis of 25 studies conducted between 1999 to 2014 on the usage of face masks during the Muslim pilgrimage, commonly known as hajj (haji), concluded that masks could provide “significant protectiveness” from respiratory infections.

註六

Spain asks NATO for urgent medical supplies to fight coronavirus epidemic

In a statement, NATO said Spain's military had asked for "international assistance", seeking medical supplies to help curb the spread of the virus both in the military and in the civilian population.

The request specified 450,000 respirators, 500,000 rapid testing kits, 500 ventilators and 1.5 million surgical masks.

 

發表意見