Face-masks help reduce the spread of SARS-COV-2, the virus that causes covid-19. Several studies have reported the more surprising finding that, even if wearers do become infected, their disease is usually milder. Now Joseph Courtney and Ad Bax, a pair of researchers at the National Institutes of Health in Bethesda, Maryland, think they may have worked out why. As they report in the Biophysical Journal, it comes down to humidity, the immune system, and the protective powers of mucus.
口罩有助于减少导致新冠的SARS-COV-2(新型冠状病毒)的传播。一些研究报告了更为惊人的发现：即使口罩佩戴者被感染了，他们的症状通常也比较轻。目前，马里兰州贝塞斯达国家卫生研究院的两位研究员约瑟夫·考特尼和艾德·巴克斯认为他们可能已经找到了原因。他们在《生物物理学杂志》上发表的报告称，这归结为湿度、免疫系统和黏液的保护能力。
At first glance, there may not seem much of a mystery to unravel. Masks cut the number of infectious particles entering the nose and mouth. One might expect, therefore, that severe disease would be less likely. But it is not so. One vital factor which predicts disease severity is how far viral particles make it into a person's lungs. Cheap cotton face-masks struggle to block the smallest aerosols, which are the most likely to penetrate deeply. Dr Courtney and Dr Bax wondered if something else might explain their protective effect.
乍一看似乎没有什么神秘的事物需要揭开。口罩可以减少进入口鼻的传染性颗粒，因此人们可能会认为感染严重疾病的可能性会降低。但事实并非如此。预测疾病严重程度的一个重要因素是病毒颗粒进入人体肺部的距离。廉价的棉质口罩很难阻挡最小的悬浮微粒，而这些悬浮微粒最有可能渗透到深处。考特尼博士和巴克斯博士想探究是否有其他原因可以解释它们的保护作用。
One of the body's first lines of defence against airborne pathogens is known as the "mucociliary clearance mechanism". Sticky mucus in the nose and respiratory tract snares viruses and bacteria. Little hairs known as cilia push the mucus into the throat. From there it is swallowed and potent stomach acids destroy the invaders. But this mechanism relies on the relevant body parts staying moist.
人体抵抗空气传播病原体的第一道防线被称为“黏液纤毛清除机制”。鼻子和呼吸道里黏黏的粘液会吸引病毒和细菌，被称为纤毛的细小毛发会将粘液推入喉咙，粘液在喉咙处被吞下，之后强有力的胃酸摧毁入侵者。但这一机制依赖于一点：相关身体部位要保持湿润状态。
That is harder in winter, for as air gets colder, its capacity to hold waterfalls. Lower humidity tends to dry out the respiratory tract. This is one reason why many upper-respiratory viruses, such as influenza, flourish in the winter.
在冬天更难，因为随着空气变冷，它容纳水分的能力会减弱。湿度较低会使呼吸道变得干燥。这就是许多上呼吸道病毒(如流感病毒)在冬季肆虐的原因之一。
Dr Courtney and Dr Bax speculated that masks may help keep things damp. They reasoned that, as a person exhales, water vapour would condense on the inside of a mask. Then, upon inhalation, dry air passing through the mask would collect the deposited water and bring it back into the respiratory tract and lungs. That could be handing a mask-wearer's immune system a significant advantage.
考特尼博士和巴克斯博士推测，口罩可能具备保持湿润的功效。他们推断，当一个人呼气时水蒸气会在口罩内部凝结，被吸入后通过口罩的干燥空气会收集积水并将其带回呼吸道和肺部。这可能会给戴口罩者的免疫系统带来很大的好处。Keen to test their idea, the researchers checked several masks at 37°C, 22°C and 8°C. They breathed into a sealed box packed with sensitive instruments, and worked out
humidity levels in their upper respiratory tracts. They found that, although all masks increased humidity levels to some extent, the heavy cotton mask did best of all. In the hottest room it boosted the relative humidity of inspired air by over 50%, compared with breathing without a mask. In the cold room, that number rose to 300%. (The other masks recorded figures of between 150% and 225%.) That suggests that, besides filtering out at least some of the viral particles floating about in the air, masks help keep a person's snot levels healthily high.
为了验证他们的想法，研究人员在37°C、22°C和8°C的温度下检验了几种口罩。他们向一个密封的盒子里呼气，盒子里装满了敏感的仪器，然后计算出他们上呼吸道的湿度。他们发现，虽然所有的口罩都在一定程度上增加了湿度，但厚重的棉质口罩效果最好。在最热的房间里，戴棉质口罩呼吸与不戴口罩呼吸相比，它使吸入空气的相对湿度提高了50%以上。在寒冷的房间里这一数字上升到了300%。(其他口罩的记录数字在150%至225%之间。)这表明，口罩除了能过滤掉空气中漂浮的部分病毒颗粒外，还能让人的鼻涕保持在非常高的健康水平。
