In a tv drama, if you see an anti-trump who makes a long speech before he kills someone, 90% of the badguys are going to hang up soon. The spread of the virus is similar: the more compelling the voice, the easier it is to prevent it. For example, I sneezed, after a bit of unbearable not only to do a long whistling, but also with the shoulder strap elbow, elbow wrist, wristband hand blocking action, and make a loud noise. People with a few meters around can then make self-protective actions – stay away or cover their faces.
From a fluid mechanics point of view, sneezing is a drastic atomization process. A liquid membrane is attached to the human respiratory tract, mouth and nasal cavity. A slight breeze can blow a pool of spring water, but the wind deepens even more when the folds are more intense, until they break and leave the water to become mistdrops. The air flow of the sneeze can reach 50 m/s, which is comparable to a category 15 typhoon, and instantly wrinkles the gills and the mouth piece. Professor Lydia Bourouiba of MIT specializes in sneezing and is known as the “Queen of Sneezing”. This image shows the Queen’s team filming the droplet tracks after the sneeze. Green represents a large droplet of hundreds of microns, with a strong inertia and a range of up to two meters, but quickly settles. The red part is a few microns of small droplets, weak inertia, by the air viscosity to slow down, and instant evaporation into a micron-level particles, also known as droplet nuclei.
High-speed photographic images of sneezing (Bourouiba, etc.)
Sneezing and coughing are the most intense forms of breathing. In contrast, speaking is a less intense breathing activity, but still produces droplets (spitting stars) and droplet nuclei. It can be imagined that the study of the production of droplets for speech is more complex, not only the difference between language and volume, but also related to different syllable types such as blasting and smoothing. In addition, one of the difficulties of the study is the huge individual difference between man and man (heterogeneity). William D. The Ristenpart team found that the world did have “sprays” – more than ten times as many droplets they released when they spoke, and had nothing to do with the specific language spoken (Asadi et al.).
Small head, such as paint-like glue, can fly will float
Perhaps everyone has had this experience: walking in the corridor or even on the road, clearly around dozens of meters without anyone, but can still smell smoke. What we smell is a few hundred nanometers of particles formed by the burning of tobacco.
The smaller the particles, the more viscous the air becomes. Micron-sized particles are scattered in the air like sesame seeds in honey, barely sinking. Quantitatively speaking, the time of stay of small particles at the same height in the stationary air is inversely proportional to its surface area. For particles of 1 micron, the settling time in stationary air is more than 1 hour. And there is always wind and grass in the environment, so these particles almost never settle, always stay in the air. That’s why smokers may have disappeared long ago, but the aftertaste has lingered for a long time.
This small liquid or solid particle, which is stablely dispersed in the gas, is called aerosol. The reason translated as “glue” is about to take the grain and the media between sticky, difficult to give up.
The droplet nucleus, which contains the neo-coronavirus, is in the submicron to micron range, similar to the particle scale after tobacco combustion. As a result, the moth nucleus can be suspended in the air for a long time and drift into the distance, fuelled by turbulence in the air. Coronary viruses in the moth nucleus are protected by protein membrane shells and may remain active for a considerable period of time. If inhaled, it can lead to new coronavirus infection.
On February 1, 2020, Shenzhen Third People’s Hospital (The Second Affiliated Hospital of Southern University of Science and Technology) tested positive for neo-coronavirus nucleic acid (RNA) in the feces of some patients with new coronary pneumonia. Although RNA is not the same as an active virus, there is a possibility that this raises concerns about “dung spread”.
However, “dung transmission” is unlikely to be through eating, but mainly through aerosols and breathing. This is because, like breathing, excretion can also produce aerosols. Excretion is also a mechanical process involving soft matter and complex fluids (liquids and gases), which may even be more intense, so it is natural to produce aerosols.
In 2003, 321 people were infected with SARS virus in Hong Kong’s Taoda Garden E, killing 42 people, and it is considered highly likely that aerosol transmission is high. The excreta of the infected virus falls in a hundred-meter-high sewage pipe, and the interaction with the air flow forms a process of atomization. The mist droplets escaped through pipe cracks on the 8th floor and several occupants were not water-sealed U-tubes, eventually evaporating into a source of aerosol-style infection. It can be seen that public health is a systematic project, not short board.
In 2003, SARS virus spread schematics in Hong Kong’s Taobao Garden Seat E (where “water flower” is aerosol)
Aerosols may travel farther than we think. A related piece of research evidence is a horse flu transmission event involving 437 horse farms in Queensland, Australia (Davis et al.). The average distance between these stables is about 1 km and up to 13 km. At the beginning of the horse flu, the isolation policy was publicized and enforced, and there was no close contact between the horses and the horses, but many horses were still found to be infected. It turns out that the infected area is also closely related to the wind direction! These evidences suggest that horse flu may have the ability to spread kilometers in the condition of the aerosol in a fixed wind direction. China’s urban population density is very high, such as Shenzhen reached 10,000 people per square kilometer, so the long-distance transmission capacity of aerosols can not be ignored.
Mild breathing: the danger of invincibility
Compared with sneezing, coughing, and talking, the normal breathing of carriers of the virus is the mildest activity, but it is also the most defensive. This is because, while breathing, the lungs are doing a lot of long-term atomization, and the particles of atomization are minimal. The lungs are magical organs, like an upside-down tree, the trunk is divided into branches, bronchial branches in the lungs many times branched up to 25 levels, and finally formed the basic unit alveos, diameter and hair silk equivalent. There is a hypothesis that the alveoli is between a breath and one suction, the mucus is separated, like a small soap bubble ruptured, instantly producing very small droplets. These droplets have no inertia at all and will come out into the air with the exhaled air (Tellier et al.). In addition, pneumonia is a lower respiratory tract infection, which means that the lower respiratory tract has a higher level of virus. There are about 400 million alveoli in adults, with a total surface area of hundreds of square meters, and we breathe all the time. The RT-PCR measurement of 3 to 20 RNAs per minute is measured by RT-PCR, of which nearly 90% of the outbound particles are less than 1 micron in diameter (Fabian et al.). To add, about 15 minutes of breathing, the number of viruses can reach the dose of infection of the flu. That is to say, even if the carrier of the new coronavirus does not cough, do not sneeze, will quietly and uninterrupted release of the virus-containing aerosols. Therefore, confirmed and suspected patients should be firmly isolated.
Scientists have developed animal models to study the way viruses spread among mammals. Notable is the Guinea pig model. Guinean pigs infected with the virus have fever symptoms, but do not cough, which excludes the possibility of large droplets being transmitted (Lowen et al.). But the study found that Guinean pigs in two cages three feet apart could still be infected, which is strong evidence of a pattern of respiratory-induced aerosol infection.
Guinea pig with flu and no cough
Looking at the “Public Protection Guide for Pneumonia Of New Coronary Virus Infection” jointly published by the China Center for Disease Control and Prevention, the National Health and Health Commission and the People’s Health Press, we can understand why we should pay attention to the spread of aerosols. In all three modes of transmission, “direct transmission” (i.e. “droplet transmission”) can be effectively protected by wearing a mask, and sneezing is essentially a powerful hammer trade; “Contact communication” route can be cut by frequent hand washing, frequent disinfection. As early as 1982, Bean and others found that the influenza virus can survive for 24-48 hours on a hard, smooth surface, but within 5 minutes of the skin the infection of the virus is reduced by 100 to 1000 times; 9])。 This means that hand-to-mouth contact is unlikely to be the main route of transmission. In contrast, our understanding of the “aerosol propagation” pathway is weak.
Some scholars have reported that in the aerosol state, an hour after the virus may maintain the ability to infection. They believe that respiratory epithelial cells may fall off and become carriers of the virus leaving the body, providing conditions for the virus to remain active. And within an hour aerosol by virtue of the wind can be on the green clouds can also spread to a few kilometers away. Of course, we don’t have to worry too much, because long distances accompanied by a sharp drop in viral concentrations can be much lower than the dose of infection. But this leads to a number of meaningful and urgent scientific issues. For example, with crowded railway stations, infectious disease hospitals as the center, the spread of viral aerosol density within a few kilometers? Can the infection threshold be reached by high local viral concentrations caused by relatively constant wind direction? Is there a possibility that the gap in the building will cause the local concentration to rise? Does geomorphological mutations and the reflux generated around the building have a enrichment effect on aerosols? These studies may provide a quantitative basis for public health risk assessment in cities, and the corresponding results are even useful for terrorist attacks in the form of aerosols.
The new coronavirus transmission route announced by the official
Where is spring?
SARS is good, the flu is good, are in the winter, in the spring and summer at the turn of the death. This seasonality has long been known to humans, but the seasonal causes of influenza are still controversial, as the seasonality of the four distinct regions and the tropical regions cannot be explained by a uniform theory. The seasonal turnover corresponds to changes in temperature and humidity, and the researchers continue to delve into these two factors. But whether it is temperature, humidity, or relative humidity (the ratio of actual humidity to saturated humidity) or absolute humidity remains uncertain. Dissenting articles have been published in Science.
A recent paper points out that there is also the possibility that particles grow into infectious lying sizes in supersaturated water vapor. The trachea is a very humid environment, and the cold air, will make the temperature inside the trachea reduced, water vapor condensation on the moth core, grow up to become very suitable for deep into the lower respiratory tract size, increase the probability of infection. On the other hand, high humidity and high temperatures in the tropical environment, the inhaled moist air will also make the moth nucleus grow. If you do not understand, one is strange I did not say clearly, and the other is because the spread of the virus is a very complex, across the long time and space of the complex problem, need to cross the discipline of sincere cooperation, from medicine, biology, fluid mechanics and other angles of collaboration to give a complete answer.
For me personally, this outbreak is also an opportunity to reflect on my scientific attitude and direction. During my Ph.D., i published in two major journals, Aerosol Science and AeroSol Science and Technology, both of which had only about 3 factors and were less than one zone in the Chinese Academy of Sciences. After his ph.D. graduation, the United States went through the Iraq war, counter-terrorism and subprime crisis, research funding became more intense, the atmosphere of scientific research unknowingly changed. As an assistant professor at a state university, the most “fair” and brainless way to show impact is to chase the influence factors of an article, while many journals of materials science have much higher influence factors and are published as quickly and smoothly. The same study of aerosols and droplets, can serve aerosols, can also serve for material handling (printing, spraying), so go to the hot spots of calcium titanium ore, and gradually away from aerosols. When I wrote this science essay to review the literature, I regained his familiarity a few years ago, feeling warm at the same time guilty and sweaty. Many traditional, simple disciplines are gradually giving way to a fashionable, cool direction. It is too difficult for basic disciplines to resemble the position and situation of the real economy, but it is likely that the building will be dumped after the basic hollowing out. I hope that, after the outbreak, our scientific soil will nourish the unpretentious discipline, which will give birth to a spring full of flowers and trees, and that there will be enough preparation and variety to cope when the next storm comes.
Writing . . . Deng Wei (Professor, Department of Mechanics and Aerospace Engineering, Southern University of Science and Technology)