Digging some wild vegetables, and poof, I got COVID-19?!

There have been cases of suspected aerosol transmission leading to COVID-19 infection in many places, and some residents were even infected when they were digging wild vegetables outdoors without wearing masks. What is aerosol transmission? What are its characteristics? What should we pay attention to? Let's take a look at the interpretation.

Compiled by New Media Editor Fang Yongzhen

Recently, many places including Ningbo, Zhejiang, Zaozhuang, Shandong, and Xi'an, Shaanxi have issued reports of "aerosol transmission" leading to new coronavirus infection, involving scenarios such as underground parking lots and digging wild vegetables outdoors.

"Aerosol transmission" has once again attracted public attention. Is "aerosol" a gas or a glue? What exactly is aerosol transmission? Let's learn more about it.

▲Data map (Source: China Economic Weekly)

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Aerosols are small and ubiquitous

Maybe everyone has had this experience: walking in the corridor or even on the road, there is no one around within dozens of meters, but you can still smell the smell of cigarettes. What we smell is actually the particles formed by the burning of tobacco.

The smaller the particles, the more obvious the viscosity of the air. The small liquid or solid particles that are stably dispersed and suspended in the gas are called aerosols. The reason why it is translated as "glue" is probably because the particles and the medium are sticky and inseparable.

Conceptually, the essence of aerosol is particles that can be suspended in the air. It refers to a dispersed system of solid or liquid particles suspended in a gas medium. The particle diameter of aerosol is between 0.001 and 100 microns.

Aerosols are everywhere. Take a deep breath, and even if the air seems fresh, you are inhaling millions of solid particles and liquid particles, and these ubiquitous substances in the air are aerosols.

▲Aerosol morphology and its main formation characteristics (Image source: Materials People)

Aerosols can be produced naturally, such as clouds, fog, haze, and dust floating in the air, which are all aerosols.

Aerosols can also be formed artificially. Many human behaviors can produce aerosols, such as breathing, coughing, talking, spitting, vomiting, urinating (flushing with water), etc. The droplets produced by these behaviors are large and small. Large droplets quickly fall to the ground or the surface of objects, while tiny droplets can be suspended in the air for a period of time. The latter are aerosols.

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Aerosols can fly and spread far

According to the diagnosis and treatment plan for COVID-19, aerosol transmission and respiratory droplets are both routes of transmission of the new coronavirus, and both are forms of airborne transmission.

Aerosol transmission is the process of droplets forming droplet nuclei in the air and then spreading to farther places in the form of aerosols. Compared with general air transmission, aerosols can stay suspended longer and float farther.

▲Comparison of aerosol transmission and droplet transmission (Source: Xiamen CDC)

Because of their large diameter, droplets can generally only travel a distance of no more than 2 meters before falling to the ground. However, aerosols have a small diameter and can travel more than 10 meters or even hundreds of meters in windy conditions, far exceeding the distance of droplets. In closed spaces and spaces with poor air circulation, they can remain longer.

The new coronavirus, which is about 0.1 microns in diameter, can attach to dust, droplets, or droplet nuclei that have lost moisture, and spread through the air in the form of aerosols, or aerosol transmission. If other people inhale aerosols containing high concentrations of virus particles, they are easily infected and cause the spread of the epidemic.

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How long can the virus survive in aerosols?

There is no definitive answer to this question. When the droplets evaporate to form droplet nuclei, the virus mortality rate will be higher due to increased salt concentration and other reasons. However, when the droplet nuclei are formed after evaporation, the virus attached to the droplet nuclei will die more slowly.

Some scholars believe that in the aerosol state, the virus may remain infectious after one hour. They believe that respiratory epithelial cells may fall off and become a carrier for the virus to leave the human body, providing conditions for the virus to remain active. During this hour, the aerosol can rise to the sky with the wind and can also spread to places several kilometers away.

▲The change of droplet concentration with distance, microbial survival rate with time, and infection risk with distance. (Picture from the Internet)

Studies have shown that the new coronavirus can survive in droplets for more than three hours, and the survival rate of the new coronavirus in droplets will be further increased in low humidity and low temperature environments. Laboratory studies have also confirmed that the new coronavirus can remain active in aerosols for up to three hours.

Another study published in the New England Journal of Medicine showed that the new coronavirus can survive stably in aerosols and on surfaces for hours to days. The study was jointly conducted by scientists from the National Institutes of Health, the Centers for Disease Control and Prevention, the University of California, Los Angeles, and Princeton University.

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Is the aerosol transmission caused by Omicron more severe?

Most of the transmission routes of confirmed cases reported in the past were direct contact or droplet transmission. Recently, cases of aerosol transmission have appeared. Does this mean that the aerosol transmission caused by the Omicron variant is more severe?

Li Dongzeng, chief physician of the Department of Infection at Beijing You'an Hospital affiliated to Capital Medical University, told the media that there is a certain relationship, but it is not the main one. The Omicron virus is highly contagious mainly because it can more easily invade and replicate in the upper respiratory tract of humans, and the virus can be easily expelled when the infected person talks or coughs, making it highly contagious.

Moreover, the Omicron variant has the ability to escape immunity, and the protective effect after vaccination has also decreased, so it spreads faster. Even before the virus mutated in 2020, there was aerosol transmission, including some seasonal influenza, which has aerosol transmission, so the increase in cases is not closely related to the virus mutation. It is just that because Omicron is highly contagious and more people are infected, there are more cases of aerosol transmission.

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In which environments should we pay special attention to aerosol transmission?

The National Health Commission stated in the sixth edition of the epidemic guidance plan that there is a possibility of aerosol transmission in a relatively closed environment and under long-term exposure to high-concentration aerosols. The seventh edition added the statement that "because the new coronavirus can be isolated in feces and urine, attention should be paid to feces and urine pollution causing aerosol or contact transmission."

Disease control experts also remind that aerosols are particularly likely to appear in small and confined spaces, such as elevators, carriages, toilets, cinemas, etc. When the virus forms an aerosol, it will survive for a longer time. If you stay in these places for too long, the risk of infection is greater.

For example, when an infected person takes an elevator, there will be virus aerosols in the elevator, and due to poor air circulation, if a healthy person enters the elevator afterwards, the risk of infection will increase. Therefore, it is recommended that everyone who enters the elevator wears a mask, and they cannot not wear one just because there is only one person in the elevator.

Aerosol transmission is usually rare in outdoor environments, but when the viral load is high, aerosol transmission may also occur if people gather or are close to each other (less than 1 meter). An example is the incident in which residents in Zaozhuang, Shandong, were infected while digging wild vegetables outdoors.

In addition, virus-containing aerosols may enter the room through relatively closed circulation systems such as central air conditioning systems and sewer systems. Special attention should be paid to central air conditioning with full air systems, where air in different rooms will cross-flow, which can easily cause cross-contamination. This type of central air conditioning is generally used in shopping malls, airports, stadiums, auditoriums, theaters and other places, so it should be stopped during the epidemic.

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Be vigilant, but don’t worry too much

Although aerosols are easy to form, it is not easy for them to infect people. Generally speaking, only when the threshold of an extremely high order of magnitude is reached can some viruses enter the human body through the mucous membrane. Although viruses can be transmitted over long distances through aerosols, the virus concentration will drop sharply as the distance increases, reaching a level far below the infectious dose.

(Photo source: Shangguan News)

According to a study by the Massachusetts Institute of Technology, when people cough or sneeze, droplets can travel about 2 meters, while aerosols can travel about 6-9 meters. However, once the distance is far, the concentration of aerosols will continue to decrease, and the virus particles contained in them will also become smaller, and the risk of infection will also decrease.

Moreover, as long as we take certain measures in our daily lives, we can avoid infection through aerosol transmission.

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What should individuals do to prevent aerosol transmission?

The currently prevalent Omicron variant has increased transmission ability, and there are no symptoms or relatively mild symptoms after infection, which makes it highly hidden and there is a risk of aerosol transmission. Experts remind that while actively cooperating with nucleic acid testing, remember to take personal protection. Specifically, you can do this:

(Photo source: People's Daily Weibo)

1. Wear masks properly. Aerosol particles are generally large, usually larger than 10 microns, and at most larger than 50 microns. Ordinary medical masks can block them. Masks can physically isolate aerosols with larger particle diameters. Masks with melt-blown layers can absorb small molecule aerosols through electrostatic effects. Wearing medical surgical masks properly can effectively prevent them.

2. Frequent ventilation and disinfection. For particularly small aerosol particles (radius less than 0.1 micron), they are light in weight and mainly distributed in the high altitude (close to the ground from the soil). They will drift away with the wind and are unlikely to be breathed in by people. Moreover, timely ventilation and disinfection can minimize aerosol concentration and virus density.

3. Wash your hands frequently and avoid gathering. Only a very small proportion of the virus suspended on clothing and skin in the form of aerosols can enter the eyes, mouth and nose through hand contact. With such a large amount of virus, the possibility of causing disease is not high. However, experts recommend that you try not to go to crowded places on a daily basis and keep a certain distance from others. Generally speaking, 2 meters is a safe distance for prevention.

▲Sewage discharge produces aerosols, and the chimney effect causes airflow to transport aerosols vertically upwards, according to a research paper by the Beijing Center for Disease Control and Prevention. (Photo source: Red Star News)

In addition, excretion activities can also generate aerosols. Excretion activities are mechanical processes involving soft matter and complex fluids (liquids and gases), which may even be quite violent, so it is natural to generate aerosols. To prevent aerosol transmission through sewers, try to flush the toilet with the lid closed when using it.

Finally, I would like to remind everyone that although the new coronavirus has the risk of being transmitted through aerosols, it is not that scary. The so-called "virus is floating everywhere" is not rigorous unless you happen to be near a major pollution source. Ordinary people should continue to take preventive measures, such as frequent ventilation, frequent hand washing, wearing masks, and less gathering.

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(The content of this article is compiled from Science and Technology Daily, China News Network, Xinwen Lianbo, the website of the Central Commission for Discipline Inspection and the National Supervisory Commission, Beijing Science and Technology Daily, Jiemian News, Red Star News, etc.)

Produced by: Science Central Kitchen

Produced by: Beijing Science and Technology News | Beijing Science and Technology Media

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