Respiratory disease transmission routes

Droplet transmission, contact transmission, and aerosol transmission, among which the risk proportions of the three types of transmission vary greatly in different scenarios, and there is currently no unified quantitative conclusion.

Last week, half of a class in the elementary school downstairs in my home had the flu. During the influenza investigation, we found that the child who got sick first was sitting in the front row of the classroom. He couldn't help but cough several times in class last Thursday without covering his mouth. The eight children sitting in the two rows in front and behind him that day developed fever symptoms one after another the next day. This is the most typical droplet transmission - respiratory droplets larger than 5 microns are emitted when the patient coughs, sneezes, or talks, and carries the pathogen and is directly inhaled into the respiratory tract by others within 1 to 2 meters, completing the infection. You can think of these droplets as large droplets of mosquito repellent toilet water sprayed in summer. They fall on the ground or the surface of clothing not far after spraying. Therefore, wearing a medical surgical mask at a normal social distance can block most of the risk of droplet transmission.

Many people think that it is absolutely safe if I am two meters away from the patient, but this may not be the case. During the epidemic two years ago, our community encountered a cluster infection in a chess and card room. The first confirmed patient stayed in the chess and card room for an hour and then left. The four customers who came in in the next three hours did not meet him in the whole process and eventually became positive. Later, it was found that aerosol transmission was the cause. Unlike large droplets, aerosols are respiratory particles with a diameter of less than 5 microns. They are light enough to be suspended in the air for several hours. If they are in a closed space with poor ventilation, they can even float to upper and lower floors along flues and sewers. Previous studies have detected live influenza viruses in the air of corridors in centralized isolation points. This is the reason.

However, regarding the actual risk of aerosol transmission, the public health field and the clinical field still have somewhat different views. Most public health scholars believe that aerosol transmission accounts for less than 10% in ordinary daily scenarios. As long as you are not staying in a closed, densely populated place for a long time, there is no need to be overly protective. Some even think that ordinary people wearing N95 every day is "overly defensive."” ； However, many respiratory doctors we have contacted feel that in autumn and winter, there are many closed spaces with poor ventilation such as schools and office buildings, and the risk of aerosol transmission is actually underestimated. Especially for families with elderly people and children with low immunity, it is not superfluous to open windows for ventilation and wear a mask when entering the elevator. My usual habit is not to wear a mask outdoors in open spaces, but I still wear a mask when entering small spaces such as subways and elevators. This is not because I am anxious, but because I know that once a patient has been in such a place, aerosols can float for half an hour, so it never hurts to take precautions.

There is another transmission route that is most easily overlooked, that is, you may be infected even if you are not in the same space as the patient. Last month, my best friend got the flu. I went to deliver medicine to her. I wore an N95 the whole time, and I didn’t talk to her at close range. I put down my things and left. As a result, I also had a fever the next day. Later I figured out that it was me who rang her doorbell. She had rang the doorbell when she had a fever before, and there was respiratory secretion on her hand. After I rang it, I didn’t. You take off your mask when you wash your hands, rub your nose, and bring the virus in. This is contact transmission, which includes direct contact such as kissing and shaking hands with patients, and more indirect contact. After touching door handles, elevator buttons, and tabletops contaminated by patients, and then touching your own mouth, nose, and eye mucosa, you will be infected. The simplest way to prevent this kind of transmission is to wash your hands frequently, which is more effective than anything else.

Of course, not all respiratory diseases are spread only through these three routes. Special pathogens such as tuberculosis may also be spread through inhalation of contaminated dust. However, this kind of situation is too rare and we basically don’t encounter it in our daily life, so we don’t need to take it to heart. In fact, to understand these transmission routes is not to make everyone suspicious every day and want to wear a three-layer mask when going out, but to pay more attention to small details: cover coughs and sneezes with your elbows, not your hands. ； Don’t touch your face after touching public facilities, wash your hands first ； Ventilate your home and office twice a day for 10 minutes each time. Doing this can block at least 80% of the risk of respiratory infections, which is much more reliable than stocking up on cold medicines.