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Guidance for health-care workers is, therefore, clear: wear masks. Advice for members of the public has been more variable.
因此，对医护工作者的指导方针很明确：戴口罩。对公众的建议则更加多变。
Since the start of the outbreak the WHO and the CDC have advised people to avoid the use of masks
自疫情爆发以来，WHO和CDC就建议大家不要使用口罩，
unless they are in direct contact with symptomatic covid-19 patients. This is because such masks are a scarce resource.
除非他们和有症状的covid-19病患有直接接触。这是因为这类口罩是一种稀缺资源。
At the end of February Jerome Adams, America's surgeon-general, beseeched his fellow citizens on Twitter to "seriously people—STOP BUYING MASKS!"
二月底，美国卫生部长杰罗姆·亚当斯在推特上恳求他的同胞们“严肃点——不要再买口罩了!”
because a run on them might cause supply problems for health-care workers.
因为对它们的持续购买可能会导致医护工作者的口罩供应问题。
A month or so later, the advice seems to be changing. The CDC now encourages the populace to wear homemade coverings for their mouths and noses.
大约一个月后，这个建议似乎改变了。CDC现在鼓励民众戴自制的口罩。
Dr Adams himself starred in an official video demonstrating how to use rubber bands around folded pieces of cloth to make DIY masks.
亚当斯医生本人在一段官方视频中担任主角，演示如何用橡皮筋缠在折叠的布片上自制面具。
Recently, the Czech Republic and Slovakia started requiring anyone stepping out into certain public places to use nose and mouth coverings.
最近，捷克共和国和斯洛伐克开始要求任何进入某些公共场所的人都要遮住自己的鼻子和嘴巴。
The same rule applies in Lombardy, in northern Italy, which was the centre of the covid-19 outbreak in that country.
意大利北部的伦巴第也采取了同样规定，那里是该国covid-19疫情的中心。
And Austria now requires masks in places such as supermarkets and pharmacies.
奥地利现在要求在超市和药店等地方戴口罩。
The changing advice is testament to an evolving understanding of SARS-COV-2 itself,
不断变化的建议证明了对SARS-COV-2本身的不断的理解，
and also to continuing debate among scientists about how far previous research on the effectiveness of masks can usefully be brought to bear on the current crisis.
也证明了科学家们对以往关于口罩有效性的研究在多大程度上能够有效地应对当前危机的争论仍在继续。
It might seem intuitively obvious that having people cover their noses and mouths in public would be useful.
让人们在公共场合捂住鼻子和嘴似乎是很有用的。
In fact, the science of the matter is not clear-cut.
事实上，这个问题的科学依据并不明确。
Extrapolating to the laity the research which shows that masks and respirators are effective for those who work in health care is actually problematic.
表明口罩和防毒面具对那些在医护工作者是有效的研究实际上是有问题的。
One reason is that doctors and nurses are better-trained than others in how to wear these devices.
一个原因是医生和护士在如何佩戴这些设备方面比其他人更专业。
For example, surgical masks work less well when they become moist, and so need to be replaced regularly throughout the day.
例如，当口罩变得潮湿时，效果就不太好了，因此需要在一天中定期更换口罩。
Health-care workers will do this routinely. A member of the public might not.
这是医护工作者常规做法。公众可能不会。
As to N95 respirators, they are notoriously difficult to fit in a way that seals them properly to a user's face.
至于N95口罩，它们是出了名的难戴，很难把它们正确地密封在使用者的脸上。
Yet not fitting them correctly can negate their benefits—as can touching the front of a mask, or taking it off in the wrong way and so contaminating your hands.
然而，错误的佩戴可能会抵消它们的好处——就像触摸口罩的正面，或以错误的方式摘下口罩，从而污染你的双手一样。
Nor do masks and respirators protect people's eyes from virus particles. Only close-fitting goggles can do that.
口罩和防毒面具也不能保护人们的眼睛免受病毒颗粒的侵害。只有贴身的护目镜才能做到这一点。
Finally, as Susan Michie, a health psychologist at University College London, observes, people might feel a false sense of security when wearing masks,
最后，伦敦大学学院的的健康心理学家Susan Michie观察到，人们戴口罩时可能会产生一种虚假的安全感，
and thus pay less attention to other important behaviours such as social distancing and handwashing.
从而对社交距离和洗手等其他重要行为关注较少。
This does not, of course, show that masks for the public are of no use. A better way to think about them in this context might be that,
当然，这并不能说明公众戴口罩是没有用的。在这种情况下，更好的思考方法可能是，
rather than helping healthy people shield themselves from infection,
与其帮助健康的人保护自己免受感染，
they could be used to stop those who are already infected broadcasting the virus into the air around them.
不如用口罩来阻止那些已经感染病毒的人向周围的空气传播病毒。
Given that much transmission of SARS-COV-2 is suspected to occur before someone who has the virus actually shows any symptoms,
鉴于我们怀疑SARS-COV-2的大量传播是发生在患者真正表现出任何症状之前，
encouraging everyone to wear masks in public regardless of whether they are symptomatic could be a useful way to break the chain of transmission.
因此鼓励所有人在公共场合戴口罩，不管他们是否有症状，这可能是打破病毒传播链条的一种有效方法。
A study published in the current edition of Nature Medicine backs this idea up.
发表在最新一期《自然医学》期刊杂志上的一项研究支持了这一观点。
Ben Cowling, an epidemiologist at Hong Kong University, measured the amount of virus shed, in half an hour of breaths and coughs,
香港大学的流行病学家Ben Cowling检测了参与者在半小时内呼吸和咳嗽中排除的病毒数量，
by participants infected with a variety of respiratory viruses, including influenza, rhinovirus and coronaviruses (though not SARS-COV-2).
这些参与者感染了各种呼吸道病毒，包括流感、鼻病毒和冠状病毒（虽然不是SARS-COV-2） 。
In the case of those with coronaviruses, 30% of droplets and 40% of aerosol particles exhaled by participants without a surgical face-mask on contained virus particles.
在对冠状病毒参与者的检测中，没有戴医用口罩的参与者呼出的30%的飞沫和40%的气溶胶颗粒含有病毒颗粒。
When they wore masks, that dropped to zero.
当他们戴上口罩时，这个数字降为零。
译文由可可原创，仅供学习交流使用，未经许可请勿转载。
