Improving ventilation will help curb SARS-CoV-2

There has been a misunderstanding about how the virus spreads


ON JANUARY 24 TH 2020 three families, together numbering 21 people, came independently to eat lunch at a restaurant in Guangzhou. It was the eve of the Chinese New Year. Extra seating had been squeezed in to accommodate more patrons than usual, and these families were crowded onto neighbouring tables along one wall of the windowless room (see plan). The largest of them—a party of ten who had arrived the day before from Wuhan—sat around the middle table. Later that day, one of their number developed fever and a cough and, at a hospital, was diagnosed with covid-19. Within two weeks, ten of the 21 were confirmed as being infected with SARS-CoV-2.

去年 1 月 24 日,三个家庭共 21 人分头来到广州同一家餐厅吃午饭。当天是除夕。为接待比平日更多的食客,餐厅增设了桌椅,这三个家庭在一个无窗隔间里挤在三张靠墙相邻的餐桌上用餐(见平面图)。人数最多的一家共十人,前一天刚从武汉来到广州,围坐在居中的桌子边。当天晚上,其中一人出现发烧和咳嗽的症状,在医院确诊为新冠肺炎。接下来两周内,21 人中有 10 人确诊感染。

The families involved had never met and video footage showed they had no close contact during the lunch. An initial analysis by the Guangzhou Centre for Disease Control and Prevention proposed that the infection had spread via respiratory“droplets”。But medical lore has it that such droplets—defined as particles expelled while breathing that are more than five microns across—cannot travel more than a couple of metres after they have been exhaled. And some of those who became infected during the lunch were farther than that from the“index”patient.

这三个家庭之前从未有过交集,视频监控显示他们在午餐期间也没有过密切接触。广州市疾病预防控制中心初步分析后认为,感染是通过呼吸道「飞沫」传播的。但医学界一般认为,这种飞沫(即呼吸时排出的直径大于 5 微米的颗粒)在呼出后的飞行距离只有两三米。而当天午餐期间被感染的一些人和这个群组中的零号病人之间的距离要更远。

It made no sense. How could a single infected person transmit the virus to nine others in just an hour when there had been no direct contact between them?


Current thinking 气流认知

The outbreak at the Guangzhou restaurant was the first recorded“superspreading”event of the pandemic. Superspreading is loosely defined as being when a single person infects many others in a short space of time. More than 2,000 cases of it have now been recorded—in places as varied as slaughterhouses, megachurches, fitness centres and nightclubs—and many scientists argue that it is the main means by which covid-19 is transmitted.

广州餐厅的聚集性爆发是新冠疫情第一次有记录的「超级传播」事件。对超级传播的粗略定义是在短时间内一人感染许多人的情况。现在,已记录的超级传播事件达 2000 多例,发生的场所林林总总,有屠宰场、大型教堂、健身中心和夜总会等。许多科学家认为,超级传播是新冠肺炎传播的主要途径。

In cracking the puzzle of superspreading, researchers have had to re-evaluate their understanding of SARS-CoV-2’s transmission. Most documented superspreadings have happened indoors and involved large groups gathered in poorly ventilated spaces. That points to SARS-CoV-2 being a virus which travels easily through the air, in contradistinction to the early belief that short-range encounters and infected surfaces were the main risks. This, in turn, suggests that paying attention to the need for good ventilation will be important in managing the next phase of the pandemic, as people return to mixing with each other inside homes, offices, gyms, restaurants and other enclosed spaces.


It has taken a long time for public-health experts to acknowledge that covid-19 routinely spreads through the air in this way. Social distancing and mask-wearing were recommended with the intention of cutting direct, close-range transmission by virus-carrying droplets of mucus or saliva breathed out by infected individuals. The main risk of spreading the illness indirectly was thought to come not from these droplets being carried long distances by air currents, but rather by their landing on nearby surfaces, on which viruses they were harbouring might survive for hours, or even days. Anyone who touched such an infected surface could then transfer those viruses, via their fingers, to their mouth, eyes or nose. This makes sense if SARS-CoV-2 spreads in the same way as influenza—which was indeed the hypothesis in March 2020, when the World Health Organisation (WHO) declared the start of the covid-19 pandemic. Hence the advice to disinfect surfaces and wash hands frequently.

公共卫生专家经过了很长时间才承认,新冠肺炎经常在这种情况下通过空气传播。他们之前建议人们保持社交距离和戴口罩,目的是要切断受感染者呼出的带病毒的粘液或唾液飞沫导致的近距离直接传播。专家们过去认为间接传播病毒的主要风险不在于这些飞沫随气流长距离传播,而在于它们落在附近的物体表面上后,所携带的病毒可能在那里存活数小时甚至数天,任何触摸了这种受污染表面的人都可能将病毒通过手指转移到自己的口、眼或鼻部。如果新冠病毒与流感的传播方式相同(2020 年 3 月世卫组织宣布将新冠疫情定性为全球性大流行病时确实是这么假定的),那么这种想法就有其道理。他们因此建议人们勤消毒物体表面和勤洗手。

Doctors did know at the time that not all respiratory particles fall fast. Those smaller than five microns can become aerosols, staying aloft for hours and potentially travelling much farther than droplets, or simply accumulating in the air within a closed room. Anyone inhaling these aerosols could then become infected. But this was assumed not to matter, because aerosols were thought to be relevant only in specialist medical settings, such as when patients are attached to a ventilator in an intensive-care unit. Intubation, as this process is known, does indeed create aerosols, as the breathing tube is forced down a patient’s trachea. But a wider risk was not perceived. The WHO therefore played down the risks of aerosols, issuing guidance via its Twitter and Facebook pages at the end of March 2020 that the general public need not worry.“FACT: #COVID19 is NOT airborne,”it said, adding that any claims to the contrary were“misinformation”。

医生们当时确实知道并非呼吸道呼出的所有颗粒物都会快速沉降。那些直径小于 5 微米的颗粒可能会变成气溶胶,在空气中停留数小时,有可能比飞沫传播得远得多,或者在封闭房间内的空气里积聚。吸入这些气溶胶的人都可能被感染。但当时假定不需要担心这种情形,因为人们认为只有在专业的医疗环境下气溶胶才值得重视,例如患者在重症监护病房中连接呼吸机时。连接呼吸机的过程就是通常所说的「插管」,由于气管导管是被强行插入患者的气管,这个过程确实会产生气溶胶。但人们没有意识到一个更广泛存在的风险。因此,世卫组织淡化了气溶胶的风险,在 2020 年 3 月底通过其官方推特和 Facebook 主页发布的指南中告知公众无需担心。「事实:# 新冠不经空气传播。」指南如是说,并补充说任何相反的说法都是「错误信息」。

Physics envy 物理嫉妒

Researchers outside the medical world, however—especially those who study the physics of particles in the air—felt the evidence pointed in a different direction. The Guangzhou restaurant outbreak was an early warning. Around the same time, 1,300 km across the country in Ningbo, 23 of 68 passengers on a bus fitted with an air-recycling system had been infected during a one-and-a-half-hour journey. But the worst known case of superspreading early in the pandemic was American. This happened at a choir practice in Skagit Valley, Washington State, in March 2020. Of the 61 people present during a two-and-a-half-hour meeting, 53 became infected. In all these cases, investigation showed that those infected were not necessarily the people closest to the index patients, as might be expected if transmission had been by droplet or surface contact.

然而,医学界以外的研究人员、尤其是那些研究空气中颗粒物的物理学现象的人员认为证据指向了不同的方向。广州餐厅的聚集性感染是一次预警。大约在同一时间,在距广州 1300 公里的宁波,在一辆装有空气循环系统的大巴上,68 名乘客中有 23 人在一个半小时的车程中被感染。但已知晓的疫情初期最严重的超级传播案例发生在美国。2020 年 3 月,在华盛顿州斯卡吉特谷(Skagit Valley)的一次两个半小时的合唱团排练中,在场的 61 人中有 53 人被感染。在所有这些案例中,调查表明被感染者不都是与群组中的零号病人距离最近的人。如果病毒是通过飞沫或表面接触传播的话,那就应该是离得最近的人最易被感染。

None of this surprised Lidia Morawska, a physicist at the Queensland University of Technology, in Brisbane, Australia. She had spent much of her career studying how pollution caused by so-called particulate matter, such as dust and smog, affects air quality. After the original SARS outbreak, which happened in 2003, she began experiments to show how respiratory particles are generated in people’s throats and then transported through the air.

澳大利亚布里斯班的昆士兰科技大学(Queensland University of Technology)的物理学家莉迪亚・莫拉夫斯卡(Lidia Morawska)对上述种种都毫不意外。她职业生涯的大部分时间都在研究灰尘和雾霾等所谓的颗粒物造成的污染对空气质量的影响。2003 年的 SARS 疫情爆发之后,她开始通过实验来说明呼吸道颗粒物在人们的喉咙中产生继而经空气传播的过程。

She demonstrated that received medical wisdom is wrong. Because exhaled breath is a moist, hot, turbulent cloud of air, a five-micron-wide droplet released at a height of one and a half metres (about the distance above ground of the average mouth or nose) can easily be carried dozens of metres before settling. Also, the generation of respiratory particles is not restricted to medical settings. Liquid drops of all sizes—including those defined as aerosols—are continuously shed while people are breathing, talking, sneezing or singing (see chart).

她的实验证明公认的医学观点是错误的。由于人呼出的是潮热、涌动的气团,在一米半高度(也就是一般人的嘴或鼻子离地的距离)喷出的直径 5 微米的飞沫能很容易地飘散到几十米开外才落地。此外,不只是医疗环境下会产生呼吸道颗粒物。人们在呼吸、说话、打喷嚏或唱歌的过程中会不断产生各种尺寸的液滴,包括被定义为气溶胶的那些(见图表)。

In July 2020 Dr Morawska wanted to bring this work to the attention of public-health agencies. She assembled a group of 36 experts on aerosols and air quality to write an open letter outlining their evidence for infection by smaller liquid drops and calling on the WHO to change its tune on airborne transmission.“We appeal to the medical community and to the relevant national and international bodies to recognise the potential for airborne spread of coronavirus disease 2019 (covid-19),”they wrote in Clinical Infectious Diseases.“There is significant potential for inhalation exposure to viruses in microscopic respiratory droplets (microdroplets) at short to medium distances (up to several metres, or room scale), and we are advocating for the use of preventive measures to mitigate this route of airborne transmission.”More than 200 other researchers from 32 countries also signed the letter.

2020 年 7 月,莫拉夫斯卡希望让公共卫生机构注意到她的这项研究。她召集了 36 名气溶胶和空气质量专家共同撰写了一封公开信,概述了他们发现的较小液滴也能传播病毒的证据,并呼吁世卫组织改变对空气传播的说法。「我们恳请医学界及相关国家和国际机构认识到新冠病毒通过空气传播的可能性。」他们在《临床传染病》(Clinical Infectious Diseases)杂志上写道。「在短距离到中等距离(远至数米,或在房间范围内)吸入微小呼吸道飞沫(微飞沫)而感染病毒的可能性很大,我们呼吁采取预防措施来抑制这种空气传播。」来自 32 个国家的另外 200 多名研究人员也在这封信上署了名。

One signatory was Jose-Luis Jimenez, an atmospheric chemist at the University of Colorado, Boulder. He says that the confusion in health circles over whether or not airborne transmission of SARS-CoV-2 is important can be traced back to medical textbooks that still contain outdated descriptions of how respiratory particles are produced and move.

科罗拉多大学博尔德分校的大气化学家何塞-路易斯・希门尼斯(Jose-Luis Jimenez)是署名人之一。他说公共卫生界对空气传播是否是新冠病毒的重要传播途径意见不一,其原因可以追溯至医学教科书中仍然收录着对呼吸道颗粒物产生和运动过程的过时的说明。

But the widespread assertion, still stubbornly promulgated by the WHO, that droplets above five microns in diameter do not stay airborne, but rather settle close to their source, is a dodgy foundation on which to build public-health advice. According to Dr Jimenez, physicists have shown that any particle Less than 100 microns across can become airborne in the right circumstances. All of this matters because hand-washing and social distancing, though they remain important, are not enough to stop an airborne virus spreading, especially indoors. Masks will help, by slowing down and partially filtering an infectious person’s exhalations. But to keep offices, schools, hospitals, care homes and so on safe also requires improvements in their ventilation.

广泛传播的论断称,直径大于 5 微米的飞沫不会在空气中悬浮,而是会在靠近源头处沉降。世卫组织也仍在固执地宣传这一点。但以此为基础来制定公共卫生建议并不可靠。据希门尼斯说,物理学家已经证明,任何直径小于 100 微米的颗粒在适当条件下都可以在空气中悬浮。这一点意义重大,因为尽管勤洗手和保持社交距离仍然很重要,却并不足以阻止病毒经由空气传播,尤其是在室内。口罩会有帮助,它能减缓感染者呼出气体的流动速度,并能起到一定的过滤作用。但要保证办公室、学校、医院、护理院等地的安全,还需要改善室内通风。

Fan-tastic 风扇给力

Under pressure from physicists, the WHO recently acknowledged that better ventilation should be used to help prevent covid-19’s spread—and in March it published a“roadmap”to that effect. But the document fell far short of properly recognising the hazard of airborne transmission and, therefore, the need to control it. Despite overwhelming evidence that it happens, the agency still maintained that SARS-CoV-2“mainly spreads between people when an infected person is in close contact with another person”。

在受到来自物理学家的压力后,世卫组织于近期承认应该通过改善通风来帮助防止新冠肺炎的传播,并在今年 3 月发布了改善通风的「路线图」。但这份文件远未充分认识到空气传播的危害,因而也就没有认识到防控空气传播的必要性。尽管有大量空气传播的证据,但世卫仍坚持认为新冠病毒「主要是通过感染者与他人密切接触而实现人际传播」。

Others, though, are acting on the new knowledge. Martin Bazant, a chemical engineer, and John Bush, a mathematician, both at the Massachusetts Institute of Technology, have devised a way to calculate how long it would be safe to stay within a room that contains an infected person. The pair described their model in a paper in a recent issue of the Proceedings of the National Academy of Sciences.

不过,其他人正在根据新知识采取行动。麻省理工学院的化学工程师马丁・巴赞特(Martin Bazant)和数学家约翰・布什(John Bush)设计出了一种方法,可以计算出在一个有一名感染者的房间里待多久是安全的。两人在近期发表于《美国国家科学院院刊》的一篇论文中介绍了他们的模型。

Applied to a typical American school class of 19 pupils and a teacher, the safe time after an infected individual enters a classroom that is naturally ventilated (that is, how long before the risk of infection is unacceptably high) is 72 minutes. This period can, though, be extended in two ways. One is by mechanical ventilation of the room, which increases the safe time to 7.2 hours. The other is by everyone wearing masks. In the absence of mechanical ventilation, mask-wearing increases the safe time to eight hours. But the real benefit comes from combining these approaches. That pushes the safe time up to 80 hours—almost 14 days if a school day is six hours long. Add in intervening weekends and a class wearing masks in a school room with adequate ventilation would thereby be safe for longer than the time it takes to recover from covid-19, which is typically between one and two weeks. School transmissions would thus be rare.

将此模型应用于美国的学校里一个典型的班级——包含 19 名学生和一名教师,得出的结果是在一名感染者进入这样一个自然通风的教室后,安全时间(即感染风险高出可接受水平之前的时间)为 72 分钟。不过有两种方式可以延长这一时间。一种是机械通风,可将安全时间延长到 7.2 小时。另一种是人人戴口罩。在没有机械通风的情况下,戴口罩可将安全时间延长到 8 小时。但把这两种方式相结合才会取得真正可观的效果。这种方式把安全时间延长到了至多 80 个小时——如果每天上学 6 小时,就相当于近 14 天。加上穿插其间的周末,也就是说,对于在通风良好的教室里戴着口罩的师生来说,他们的安全时间超过了新冠肺炎患者康复的耗时(一般是一到两周)。结果是,校内传播将会变得很少见。

A caveat is that the modelling assumed a classroom with minimal talking, physical activity or singing by the pupils. But games lessons would usually be outdoors and singing lessons could be. As to too much talking, teachers might welcome an unimpeachable reason to tell pupils to keep quiet in class.


Infection risk will not always be distributed evenly around a room. Jiarong Hong, a mechanical engineer at the University of Minnesota, Minneapolis, therefore used computer models to study how aerosols would spread in a classroom, according to the location of an infected individual and the position of nearby fans or air filters. Assuming the teacher was infected, and so was releasing virus-laden aerosols at the front of the class, Dr Hong’s modelling shows that placing an air cleaner or extractor fan at the front of the room sets up an airflow which prevents the movement of such aerosols towards the pupils. An even better aerosol-cleansing effect is achieved when the fans and filters are elevated above the people in the room. This takes advantage of the rising air plumes created by body heat, which mean that exhaled aerosols tend to float upwards. Dr Hong’s modelling shows that even small, cheap box fans mounted in this way would do a good job of keeping classrooms safe and preventing aerosols from building up to dangerous levels.


Dr Hong has also modelled the air flow in the Guangzhou restaurant outbreak of January 2020. As the plan shows, he found that the movements of virus-laden aerosols around the three affected families of diners matched the seating positions of the people who eventually became sick. The outbreak occurred because there was no source of external fresh air and a nearby recirculating air conditioner redistributed aerosols from the infected person to the other tables, creating a contaminated bubble of air that was increasingly burdened with viruses over the course of the lunch.

洪家荣还模拟了 2020 年 1 月出现聚集性感染的广州餐厅里的空气流动。如餐厅平面图所示,他发现携带病毒的气溶胶围绕三个受影响的用餐家庭的运动轨迹与那些最终被感染的人的座位契合。出现这一聚集性感染是因为没有外部新鲜空气补充进来,而附近的一台再循环式空调将感染者呼出的气溶胶又吹到了另外两个餐桌,在那里形成了一个被污染的密闭「大气泡」,在午餐过程中那里头的病毒越积越多。

The risk, then, is real. But how can the occupants of a room know whether it is well-ventilated? Just because a room feels spacious and an air conditioner is operating does not mean the air inside it is clean.


Here, Dr Morawska has a suggestion. In a (non-scientific) experiment last year, she took a carbon-dioxide meter into a large, high-ceilinged, air-conditioned restaurant near her home. CO2 concentrations can be a useful proxy for clean air. Outdoor air contains around 400 parts per million (ppm) of the gas, and people’s exhaled breath contains around 40,000 ppm. Exhaling into a room therefore gradually raises its CO2 concentration unless the ventilation is good enough to remove the excess.

对此莫拉夫斯卡给出了一个参考。在去年的一项(非科学)实验中,她带着一个二氧化碳测量仪去了她家附近一家面积大、层高高且装有空调的餐厅。二氧化碳浓度可用作衡量空气清洁程度的有用指标。室外空气中的二氧化碳浓度约为 400 ppm(1 ppm 为百万分之一),在人体呼出气体中的浓度约为 40,000 ppm。因此,在房间里呼气会逐渐提高二氧化碳浓度,除非有足够的通风移除过量的二氧化碳。

According to experts on air quality, anything below 500 ppm in a room means the ventilation is good. At 800 ppm, 1% of the air someone is breathing has already been exhaled recently by someone else. At 4,400 ppm, this rises to 10%, and would be classed as dangerous. These sorts of levels are seen only in crowded spaces with poor airflow. To keep the risk of covid-19 low, CO2 levels should be well below 700 ppm.

根据空气质量专家的说法,房间内二氧化碳浓度低于 500 ppm 表明通风良好。浓度达到 800 ppm 时,一个人吸入的空气中有 1% 是他人呼出没多久的。浓度达到 4400 ppm 时,这个比例会上升到 10%,就会被列为危险水平。这么高的浓度只有在通风不佳的拥挤空间中才会出现。为了将感染新冠肺炎的风险保持在较低水平,二氧化碳浓度应远低于 700 ppm。

When Dr Morawska conducted her experiment, the restaurant had ten people in it—far fewer than would normally be allowed—and the CO2 concentration was already 1,000 ppm when she arrived. Within an hour it had jumped to 2,000 ppm.“We continued sitting during the dinner for another hour or so,”she says.“So if there was someone infected there, well this could have been a problem.”

莫拉夫斯卡做这项实验时,餐厅里有 10 个人,远少于正常情况下餐厅的接客量。她刚到餐厅时二氧化碳浓度就已经达到 1000 ppm,一小时内就已飙升至 2000 ppm。「我们在晚餐期间又多坐了一个小时左右,」她说,「所以,如果餐厅里有受感染者,那可能就要出问题了。」

Though anecdotal, that tale indicates a serious risk—and one which resonates beyond covid-19. All sorts of symptoms, from headaches, fatigue and shortness of breath to skin-irritation, dizziness and nausea, are linked to poor ventilation. It has also been connected with more absences from work and lower productivity.


The ventilation measures needed to deal with all this are not difficult, but existing regulations and design standards often have different objectives—particularly, these days, conserving heat and thus reducing energy consumption. That often means recirculating air, rather than exchanging it with fresh air from the outside world. (An exception is Passenger aircraft, which refresh cabin air frequently.)


In situations where it is not possible to reduce health risks by ventilation alone—for example, places like nightclubs, where there are lots of people crowded together, or gyms, where they are breathing heavily—air filtration could easily be incorporated into ventilation systems. Air could also be disinfected, using germicidal ultraviolet lamps placed within air-conditioning systems or near ceilings in rooms.


All change 全都要变

And then there is public awareness.“Before this pandemic it was completely socially acceptable to come to the Office coughing, sneezing, spreading viruses around,”says Dr Morawska.“No one would say anything—even people educated to understand how infections are transmitted.”


That insouciance must be corrected, she says. The WHO must acknowledge the need to control airborne pathogens and governments must agree and enforce comprehensive standards for indoor air quality that keeps people healthy. One way to ensure compliance might be to issue ventilation certificates for buildings, similar to the food-hygiene certificates which already exist for restaurants. Occupants should also be given information about air quality routinely, she adds, through the use of monitors and sensors that can display a room’s carbon-dioxide levels or other relevant measures.


For new buildings this should not cost much extra, though replacing exiting ventilation systems might be costly. But not as costly as covid-19 has been. And if improvements in indoor air quality also reduced absenteeism and improved productivity, those gains might cover that cost.“Although detailed economic analyses remain to be done,”wrote Dr Morawska in a recent edition of Science,“the existing evidence suggests that controlling airborne infections can cost society Less than it would to bear them.”


Published since September 1843 to take part in “a severe contest between intelligence, which presses forward, and an unworthy, timid ignorance obstructing our progress.”


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