In the movie and TV series, if you see a villain make a long speech before harming others, 90% of the villain will die soon. Viral transmission is similar: the more compelling the momentum, the easier it is to prevent it.
For example, when I sneeze, I have to not only roar to the sky, but also cooperate with the blocking action of shoulder elbow, elbow wrist and wristband hand, and make a lot of noise. People a few meters around can then do self-protection actions - away from or cover their faces.
From the point of view of hydrodynamics, sneezing is a violent atomization process. The human respiratory tract, oral cavity and nasal cavity are all attached with liquid membrane. A slight breeze can wrinkle a pool of spring water, but if the wind is stronger, those wrinkles will deepen until they are broken and leave the water surface to become droplets. The air speed of sneezing can reach 50 meters / second, which is comparable to the typhoon of level 15, so it instantly wrinkled the cheek and broke the saliva.
Professor Lydia bourouiba of MIT specializes in sneezing and is known as sneezing Queen. Heres a picture of the Queens team taking pictures of the droplets after a sneeze. Green represents a large drop of hundreds of microns, with strong inertia and a range of up to two meters, but it settles quickly. The red part is a few microns of small droplets, with weak inertia, which is slowed down by the air viscosity, and evaporated into micron sized particles in an instant, also known as the droplet core.
High speed photographic image of sneeze (source: bourouiba et al )
Sneezing and coughing are the most violent forms of respiratory activity. Speaking, by contrast, is a less intense breathing activity, but it still produces droplets (spittle stars) and nuclei. It can be imagined that the research on the generation of droplets in speech needs to be more complicated, not only with the differences of language and volume, but also with different syllable types such as blasting and smoothing.
In addition, one of the difficulties of the study is the huge individual difference between people. WilliamD. Ristenparts team found that there are really speechsprayers in the world - they release more than ten times more droplets when they speak than ordinary people, and have nothing to do with the specific language they speak (Asadi et al.
Its small, like paint and glue. It can fly and float
The smaller the particles, the more viscous the air is. Micron sized particles in the air are like sesame seeds scattered in honey, hardly sinking. Quantitatively speaking, the residence time of small particles at the same height in still air is inversely proportional to their surface area. For 1 micron particles, the settling time in still air is more than 1 hour. And there is always wind and grass in the environment, so these particles almost never settle, and always stay in the air. Thats why smokers may have disappeared long ago, but the aftertaste is still lingering.
The small liquid or solid particles, which are stably dispersed and suspended in the gas, are called aerosols. The reason why it is translated as glue is that it means that particles and media are sticky and inseparable.
The size of the droplets containing the new coronavirus is in the range of submicron to micrometre, which is similar to the particle size after tobacco combustion. As a result, the droplets can be suspended in the air for a long time, and drift far away under the help of turbulence in the air. The coronavirus in the core of the droplets is protected by the protein membrane shell, and may remain active for a long time. If inhaled, it may lead to infection with the new coronavirus.
On February 1, 2020, the third peoples Hospital of Shenzhen (the Second Affiliated Hospital of South University of science and Technology) detected positive RNA in the feces of some patients with new crown pneumonia. Although the detection of RNA is not the same as the active virus, but the possibility is there, which makes fecal oral transmission cause concern.
However, fecal oral transmission is not likely to be transmitted through eating, but mainly through aerosols and breathing. This is because, like respiratory activity, excretory activity can also produce aerosols. Excretion is also a mechanical process involving soft matter and complex fluid (liquid and gas). These processes may even be violent, so it is also natural to produce aerosols.
In 2003, 321 people in tower e of Taoda garden in Hong Kong were infected with SARS virus, and 42 people died. It is believed that the most likely cause is aerosol transmission. The excreta of infected virus fall down in the sewage pipe of hundreds of meters high, and the interaction with air flow also forms an atomization process. The droplets escaped through cracks in the eighth floor of the pipeline and U-shaped pipes that had not been sealed by water in several households, and eventually evaporated to become a source of infection in the form of aerosols. It can be seen that public health is a systematic project, which cannot be short board.
Schematic diagram of SARS virus spreading in tower e, Taoda garden, Hong Kong in 2003 (among which water flower is aerosol)
The spread distance of aerosols may exceed our imagination. One relevant research evidence is the spread of equine influenza in 437 horse farms in Queensland, Australia (Davis et al ). The average distance between these horse farms is about 1 km, with a maximum distance of 13 km. At the beginning of the horse flu, the propaganda and implementation of the quarantine policy were in place. There was no close contact between horses, but many horses were found infected. It turns out that the infected area is also closely related to the wind direction! These evidences suggest that the equine influenza may have the ability of transmission over a long distance in kilometer scale under the action of fixed wind direction.
Chinas urban population density is very high, for example, Shenzhen has 10000 people per square kilometer, so the aerosol with long-distance transmission ability can not be ignored.
Gentle breathing: an overwhelming danger
Scientists have developed animal models to study how the virus spreads among mammals. It is worth noting that the guinea pig model of H1N1 is a new swine model. Guinea pigs infected with H1N1 have fever symptoms, but do not cough, which eliminates the possibility of large droplets infection (Lowen et al ). But the study found that guinea pigs in two cages three feet apart can still be infected, which is strong evidence of an aerosol transmission pattern caused by breathing.
Guinea pig with flu and no cough
We can understand why we should pay attention to aerosol transmission by looking at the transmission route proposed in the public protection guide for pneumonia caused by new coronavirus jointly published by China Center for Disease Control and prevention, the National Health Commission and the peoples Health Publishing House. Among the three routes of transmission, direct transmission (i.e. droplet transmission) can be effectively protected by wearing masks, and sneezing is basically a one-off business with great momentum; and the frequency of coughing is usually low (flu patients cough about twice an hour). Contact transmission can be cut off by frequent hand washing and frequent disinfection. As early as 1982, bean et al.  found that the influenza virus can survive for 24-48 hours on a hard and smooth surface, but the infectivity of the virus on the skin will be reduced by 100-1000 times within 5 minutes; the typical infection probability of influenza through the mouth and hands is only 1 / 10000-1% (nicas & best ). In other words, hand mouth contact is unlikely to be the main route of virus transmission. In contrast, our understanding of aerosol transmission is still weak.
It has been reported by some scholars  that in the aerosol state, the virus may maintain its infection ability one hour later. They believe that respiratory epithelial cells may shed and become vectors for the virus to leave the body, providing conditions for the virus to remain active. In this hour, the aerosol can travel to the blue cloud or several kilometers away depending on the wind. Of course, we dont have to worry too much, because long distance with the sharp drop of virus concentration may be far lower than the infection dose.
But this will lead to a lot of significant scientific problems to be studied. For example, what is the spread of viral aerosols within a few kilometers in densely populated railway stations and infectious disease hospitals? Can the higher local virus concentration caused by relatively constant wind direction reach the infection threshold? Is there any possibility of local concentration rise due to building clearance? Does the abrupt change of landform and the backflow around the building enrich the aerosol? These studies may provide quantitative basis for risk assessment of urban public health, and the corresponding results can be used for reference even for terrorist attacks in the form of aerosols.
Transmission of guanxuans new coronavirus
SARS and flu are all raging in winter and dying out at the turn of spring and summer. This seasonal feature has been recognized by human beings for a long time, but the cause of influenza seasonality is still controversial , because the seasonality of areas with four distinct seasons and tropical areas cannot be explained by a unified theory. The change of seasons corresponds to the change of temperature and humidity, and the researchers continue to study these two factors. However, whether it is temperature, humidity, or relative humidity (the ratio of actual humidity to saturated humidity) or absolute humidity is still uncertain . Articles with different opinions have been published to science .
A recently published paper  pointed out that there is another possibility that particles grow up in supersaturated water vapor to a highly infectious size. The trachea is a very humid environment, and when breathing in cold air, the temperature in the trachea will be reduced, and the water vapor will condense on the droplet core, and grow up to a size suitable for penetrating into the lower respiratory tract, increasing the probability of infection. On the contrary, the high humidity and temperature in the tropical environment make the droplets grow up. If you dont understand, first, Im not clear about it. Second, because the spread and infection of virus is a very complex problem that spans a long time and space. It requires sincere cooperation across disciplines. Only through cooperation from multiple perspectives, such as medicine, biology, and fluid mechanics, can a complete answer be given.
Personally, this epidemic is also an opportunity for me to reflect on my research attitude and direction.
During my Ph.D. study, my articles were mainly published in Journal of aerosols science and aerosols science and technology, two major magazines in the direction of aerosols. Their influencing factors are only about 3, and they are not in the first area of CAS. After graduation, the U.S. experienced the Iraq war, anti-terrorism and subprime mortgage crisis one after another. The funding for scientific research became more intense, and the atmosphere for scientific research changed unconsciously. When I was an assistant professor in a State University, the most fair and brainless way to show influence was to pursue the influence factors of articles; many magazines of materials science had a lot of high influence factors, and the publication seemed to be short and smooth. The same research on atomization and droplets can serve for aerosols, as well as for material processing (printing and spraying), so it can rub away the hot spots of perovskite, and gradually away from aerosols.
When I wrote this popular science essay and looked up the literature, I got familiar with it several years ago. I felt warm and guilty at the same time. Many traditional and simple subjects are gradually giving way to fashionable and cool directions. The status and situation of basic subjects and real economy are similar, which is too difficult; but the foundation hollowing out is likely to usher in a general trend. I hope that, after the outbreak, our scientific research soil can nourish the simple discipline, give birth to the spring with flowers, grass and trees in full bloom, and have sufficient and diverse preparations to deal with the next storm. The author introduces Deng Weiwei, Professor of mechanics and Aerospace Engineering Department of South University of science and technology, whose research field is the hydrodynamics process and application of micro droplets. Source: Mr. Sai, editor in charge: Zhang Zutao, nt5054
Author brief introduction
Deng Weiwei is a professor in the Department of mechanics and aerospace engineering, South University of science and technology. His research field is the hydrodynamics process and application of micro droplets.