การใช้หน้ากากอนามัยและเครื่องปกปิดใบหน้าของประชากรเป็นกลยุทธ์สำคัญในการลดการแพร่กระจายของโรคซาร์ส-2. ในขณะที่โรคระบาดดำเนินไป, การสวมหน้ากากเป็นสิ่งที่หลีกเลี่ยงไม่ได้! ในความเป็นจริง, หลักฐานการใช้หน้ากากเพิ่มขึ้น.
The symbol of the pandemic era is a face mask – a visual metaphor for the tiny, unseen viral foe that could be lurking around any corner. Some opt for a scarf wrapped around their face, others make do with a t-shirt yanked up over their mouth. The more creative hook colourful homemade varieties around their ears, while a lucky few wear distinctive surgical masks or, rarer still, N95 respirators.
A while back, I mean before February 2020, anyone wearing a mask in public would have drawn stares in many countries unused to this behaviour, they are now a reminder of the strange times we live in. And as governments around the world start to ease their lockdowns to allow their citizens out to mingle in the wider world again, growing numbers of people are opting to wear face masks in public.
But there is still debate about whether members of the public should be encouraged to wear face masks at all.
In the early days of the pandemic, many governments warned the public against wearing face masks for fear demand would leave frontline health workers without vital supplies and that it may lull people into a false sense of security. Some – such as the US – have since reversed that advice. The state of Utah has said it will provide a free face mask to any citizen who requests it. And other countries such as the Czech Republic, สโลวาเกีย, ออสเตรีย, โมร็อกโก, Turkey and Germany have all made wearing masks in public mandatory. It is likely others will follow their lead as they ease restrictions.
But can face masks really make a difference in our battle against Covid-19?
“A key point is that the countries that flattened the curve used masks in public,” says Chris Kenyon, head of the sexually transmitted diseases unit at The Institute of Tropical Medicine in Antwerp, who has examined whether face masks may have played a role in limiting the spread of Covid-19 in certain countries. “These were mostly Asian countries. ด้วยเหตุผลบางอย่าง, until very recently European experts – Czechia (สาธารณรัฐเช็ก) excluded – were unable to learn from what worked in Asia.”
To understand why face masks might work, it is important to look at how the virus that causes Covid-19 spreads in the first place.
Once it has infected someone, the Sars-CoV-2 virus responsible for the disease hijacks their cells to replicate itself. As it multiplies, these new virus particles then burst out of the cells and become suspended in the bodily fluids in our lungs, mouth and nose. When an infected person coughs, they can send showers of tiny droplets – known as aerosols – filled with the virus into the air.
A single cough can produce up to 3,000 droplets. There are fears the virus can also be spread simply through speaking. One recent study showed that we spray thousands of droplets invisible to the naked eye into the air just by uttering the words “stay healthy”.
Once out of our mouths, many of the larger droplets will quickly settle onto nearby surfaces while smaller ones remain suspended in the air for hours, where they can be breathed in. While the behaviour of the virus-filled droplets in rooms with air conditioning and outside environments are less well understood, they are thought to settle on surfaces more quickly in disturbed air. There are also some reports that the coronavirus can spread through ventilation systems in buildings. (Read more about how long coronavirus survives on surfaces.)
When an infected person coughs, they can send showers of tiny droplets – known as aerosols – filled with the virus into the air
The Sars-CoV-2 virus has been found to survive in these aerosol droplets for at least three hours, according to one study by virologist Neeltje van Doremalen and her colleagues at the US National Institute of Allergy and Infectious Diseases, แฮมิลตัน, Montana. But a more recent, but as yet unpublished study, has found that the Sars-CoV-2 virus is still infectious for more than 16 hours after being suspended in aerosol droplets. It found the virus was “remarkably resilient in aerosol form” compared to other similar coronaviruses they studied.
ด้วยกัน, they suggest that in the right conditions, the virus can linger in the air for several hours and still infect people if breathed in. And in indoor environments, they seem to be particularly prone to spreading through the air.
An unpublished analysis of 318 outbreaks of Covid-19 in China showed that it was most commonly transmitted in indoor environments, particularly in people’s homes, but also on public transport, in restaurants, cinemas and shops. They found just one example where the virus appeared to have been transmitted while people were outside.
Genetic material from Sars-CoV-2 has also been detected in the air in toilets and rooms used by people infected with Covid-19. One study of a cluster of cases that occured in a restaurant in Guangzhou, จีน, suggests that in poorly ventilated spaces the virus can spread to people sitting in the immediate vicinity through airborne aerosol droplets.
“Face masks could help to reduce transmission in the community particularly if used in public transport and crowded areas,” says Ben Cowling, head of epidemiology and biostatistics at the University of Hong Kong.
He and his colleagues recently published a study looking at the effectiveness of face masks to prevent the spread of the virus from infected people. They found a standard surgical face mask was enough to considerably reduce the amount of virus escaping in the breath and coughs of people infected with different respiratory viruses, including a mild type of coronavirus, influenza and a rhinovirus that causes the common cold.
“One of the proposals for lifting lockdowns is that we use mass testing along with contact tracing and quarantine, to get ahead of infections in the community,” says Cowling. “If you are identified as an infected person, the health department can trace your family members, your social contacts and your occupational contacts, but it is very difficult to trace who you were sitting next to on the bus or train.
“If we can limit transmission in these kinds of locations, it could really be a big help.”
One of the reasons widespread, public face mask wearing is so important with Covid-19 has to do with the prevalence of asymptomatic carriers who can still spread the virus to others. It is estimated that anywhere from 6% to almost 18% of those infected can carry the virus without developing symptoms. Add to this an incubation period of around five days, but up to 14 days in some cases, before symptoms develop and even those who do go on to show signs of being contagious can spread the virus to a lot of people before they start to fall ill.
“This makes it particularly difficult to suppress transmission in the community,” says Cowling. “But if everybody is wearing face masks, that would mean infected and asymptomatic people are also wearing masks. That could help to reduce the amount of virus which gets into the environment and potentially causes infections.”
Even wearing a damp homemade mask can reduce the number of droplets each of us emit while we speak, according to a study by researchers at the US National Institutes of Health, Bethesda, แมริแลนด์.
If the general public buy up all of the supplies of these masks, it will leave health workers, who are the most likely to be exposed, unprotected and vulnerable
ดังนั้น, while face masks might help those already carrying the virus from passing it to others, can they also protect the uninfected from breathing it in?
Certainly the ability of specialised, disposable masks like the N95 respirator and the equivalent FFP-2 respirator mask in Europe to filter out particles from the air is high. They are designed to passively filter out 95% และ 94% of airborne particles respectively – down to the size of 0.3 micrometres across – as the wearer breathes.
Their performance with blocking viruses from entering, อย่างไรก็ตาม, is more mixed. Some viruses can be as small as 0.01 micrometres, while researchers have reported the coronavirus that causes Covid-19 is 0.07-0.09 micrometres in size. Respiratory viruses, อย่างไรก็ตาม, tend to be suspended in aerosol droplets, which can range in size from 0.1-900 micrometres, so blocking these is often more important.
Some older studies have suggested that smaller viruses than might be expected can slip through the N95 filter, but they have been found to be effective at blocking the influenza virus.
And there is some research to suggest these respirator masks are effective when it comes to protecting people against Covid-19. One analysis of health workers in China showed that those who wore N95 respirators did not become infected with the virus, despite caring for highly-contagious patients. This is one of the reasons why these masks have been deemed so important for frontline healthcare workers.
The concern is that, if the general public buy up all of the already short supplies of these masks, it will leave these crucial workers, who are the most likely to be exposed, unprotected and vulnerable. The World Health Organization has urged the general public not to wear these masks in order to secure supplies for health workers, and this has also been the rationale behind the reluctance of many governments to encourage the public to wear masks.
While there is now some evidence that respirator masks can be disinfected for reuse, it is far from a perfect solution.
“We really need to ensure we have sufficient supplies of facemasks for healthcare workers,” adds Cowling. Such is the shortage of Personal Protective Equipment (PPE) like N95 respirator masks, that some scientists have investigated building alternatives from materials that can be found lying around in hospitals.
Another potential issue with asking the public to wear these masks is that they require training to fit properly. If they are not correctly fitted, the seal around the mouth and nose can still allow viral particles to slip around the side. Facial hair can also affect their performance as it disrupts the seal. The US Centers for Disease Control and Protection (CDC) has published a helpful guide for anyone sporting facial hair and looking to wear face masks – a neat “soul patch”, a pencil moustache or Zappa-style trim should be fine. Those sporting designer stubble, a flamboyant Dali or full Garibaldi might need to consider a shave.
But there are simpler alternatives available too. One recent, but yet to be peer-reviewed, study found a 3M surgical mask – the kind worn by surgeons in operating theatres – could keep out almost 75% of particles down to a size of 0.02 micrometres. While far less effective than an N95 respirator, a surgical mask can still help to reduce the number of particles breathed in. But by cutting a hole in the end of a stocking and wearing that over the mask, it was possible to improve the mask’s ability to cut out particles to 90%.
“Surgical masks, unlike N95 respirators, are designed to fit loosely,” says Loretta Fernandez, an environmental chemist at Northeastern University, in Boston, แมสซาชูเซตส์, who was one of those involved in the study. “This allows some air to go around the mask to the breathing zone rather than through the mask material.” They found that the addition of “nylons” on the outside of the mask helped to reduce this.
A cotton bandana was the least effective, followed by a woolen scarf, but a 600-thread-count pillowcase folded four times could filter out around 60% of the particles
But with demand for surgical masks also high, many members of the public are being forced to make their own alternatives. There are already a wide range of designs available online for do-it-yourself mask makers, with most using cotton fabric to create a pocket that a filter of some kind can be placed in. Some suggest using vacuum cleaner bags, others add coffee filters between two bandanas or insert folded pillow cases.
Yang Wang, an environmental engineer at Missouri University of Science and Technology, and one of his students built a few examples of these masks and found that using fabrics with a higher thread count worked better. A cotton bandana was the least effective, followed by a woolen scarf, but a 600-thread-count pillowcase folded four times could filter out around 60% of the particles.
Masks made from allergy-reducing air conditioning filters and vacuum cleaner bags were found to work best, almost matching the performance of an N95 respirator.
“There is a lot of potential for fabric masks and particularly masks which incorporate non-woven materials like those used in some dust mops,” says Cowling. “There is no reason why surgical masks should be the ‘best’ type of masks to wear in the community, but some fabrics have too many ‘holes’ and would not perform well.”
Fernandez and her collaborator Amy Mueller, an engineer at Northeastern University, also examined the effectiveness of different homemade masks. The most effective used multiple fabric layers, although they fell some way short of N95 and surgical masks. Adding a nylon on top to clinch the mask onto the face, อย่างไรก็ตาม, did increase the effectiveness to the point where some homemade designs were able to keep out 80% of the particles.
Other researchers have found that while pulling a t-shirt up over your nose and mouth will block less than half of the aerosols coming towards it, doubling and even tripling the layers of a cotton t-shirt fabric can dramatically improve things in an emergency. Other fabrics such as silk and polyester were also found to be surprisingly effective. Cotton flannel, felted wool and quilting cotton have been found to be particularly good at blocking ultrafine particles, and one study suggested that a single sock, when flat and pressed tightly against the nose and mouth, could also serve as a good emergency mask substitute.
Much like the disposable N95 and surgical masks, homemade varieties like these are really only good for one use before they need to be disinfected if you want to maximise their potential. The US CDC recommends routinely washing homemade masks. Hot water alone may not be enough – one recent study found the Sars-CoV-2 virus can survive temperatures of at least 60C. โชคดี, the oily envelope that encases coronaviruses can be pulled part by soap and household detergent.
But Mueller warns that all these alternatives cannot be seen as replacements for an N95 mask. “There is a very important question – for health officials to interpret from the data that we are collecting – about what level of particle filtration is ‘safe enough’? It is unfortunate but true that in some cases people may be choosing between multiple imperfect options.”
But even with these imperfect options, wearing them in public could make a difference by helping to keep infection rates down as people come out of lockdown and start mingling again. ตัวอย่างเช่น, researchers at University College London have warned that the pavements in the UK’s busiest city, much like many others around the world, may not be wide enough to allow people to maintain a safe distance from each other. In confined spaces, such as on public transport, it is even harder.
Tests on homemade masks have shown they can still significantly reduce the spread of other viral infections such as influenza. They can also help to reduce the dispersal of the virus onto nearby surfaces when people cough.
Provided enough people wear masks when venturing out in public, it could have a dramatic impact on how quickly the coronavirus starts to spread again, particularly if combined with other measures such as social distancing and handwashing. One unpublished study by scientists at Arizona State University found that if 80% of people wore only moderately effective masks, it could reduce the number of deaths in New York by 17-45% over a two month period. Even wearing masks that were just 20% effective could cut mortality by 24-65% in Washington and 2-9% in New York, if enough people wore them.
What Wearing Face Masks Study Reveals
Study of Personal Protective Measures and Risk of SARS-CoV-2 Infection, ประเทศไทย (Doung-ngern, พฤศจิกายน 2020). This study with 211 COVID-19 cases and 839 controls evaluated the effectiveness of personal protective measures against SARS-CoV-2. Wearing masks at all times during contact was independently associated with a lower risk of SARS-CoV-2 infection compared with no masks; wearing masks occasionally during contact did not reduce the risk of infection. The type of mask wear was not independently associated with infection. Keeping >1 m away from a person with COVID-19 who had close contact for <15 minutes and frequent handwashing were independently associated with a lower risk of infection. Contacts who always wore masks were also more likely to practice social distancing.
Global Projections of Potential Lives Saved from COVID-19 through Universal Mask Use (Gakidou, ตุลาคม 2020). This study measures the reduction in transmission associated with the use of cloth or paper masks in the general population using Bayesian meta-regression from 40 studies. The use of plain masks can reduce transmission of COVID-19 by 40% (95% uncertainty interval [UI], 20% ถึง 54%). Use of universal masks globally would reduce mortality by 815,600 (95% UI, 430,600 ถึง 1,491,000) between August 26, 2020, and January 1, 2021, the difference between the projected 3 million deaths (95% UI, 2.20 ถึง 4.52 ล้าน) in the reference scenario and 2.18 million deaths (95% UI, 1.71 ถึง 3.14 ล้าน) in the universal mask scenario during this time period.
In countries where masks are in short supply, some researchers have suggested that prioritising them for the elderly could also be effective.
While face masks might bring a little discomfort, and make it harder to spot the facial expressions of those we are talking to, those things are a small price to pay for keeping the people around us safe and well.
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