People are getting sick from coronavirus spreading through the air – and that's a big challenge for reopening
Coughing, sneezing, speaking, and even just breathing can create particles in the air that SARS-CoV-2 can spread. Stanislaw Pytel / Digital Vision via Getty Images
I am a scientist who studies infectious diseases and I specialize in serious respiratory infections. But I'm also a member of my church's security team.
In the past few weeks, as states began to relax restrictions, we have discussed whether and how services can be started safely. But the corona virus is far from gone. As we try to figure out how to keep services while protecting our members, one question is of particular importance: how often does the virus spread in the air?
How to spread a virus
Respiratory infections are generally transmitted in three ways: by direct contact, by droplets, and by particles in the air.
The contact transfer takes place when a person touches an object on which there is a living virus - called Fomite - and becomes sick.
Droplets are small particles of mucus or saliva that come out of a person's mouth or nose when they cough or speak. Their sizes range from 5 micrometers to hundreds of micrometers in diameter - red blood cells to a grain of sand. Most of the droplets, especially the large ones, fall to the ground within seconds and usually do not move more than 1 or 2 meters. If someone coughed at you and you got sick, it would be a droplet transmission.
Airborne transmission occurs due to airborne particles known as droplet cores. Droplet cores are pieces of mucus or saliva with a diameter of less than 5 micrometers. Humans produce droplet kernels when they speak, but they can also be formed when small droplets evaporate and become smaller. Many of these droplets shrink so much that they start to hover before hitting the ground, becoming aerosols.
Humans produce thousands of these droplet nuclei per second when they speak, and the aerosolized particles can contain live viruses and hover in the air for hours. They are easy to inhale and can make people sick if they contain live viruses. The ability of droplet cores to transmit the corona virus has a massive impact on whether and how places like my church can be reopened.
At the beginning of the pandemic, experts from the Centers for Disease Control and Prevention and the World Health Organization were most concerned that the coronavirus would spread from surfaces and large droplets.
The more studies are done on SARS-CoV-2, the more evidence that there is airborne transmission, although this is controversial. Both the CDC and the WHO now recommend that the general population wear masks. However, for people who live their lives and wonder how to reopen public areas around the world, the question remains: how important is airborne transmission?
Longevity in the air in the laboratory
To become infected, a person must come into contact with living viruses. If the virus dies before a person can inhale it, they will not get sick.
To test how well SARS-CoV-2 can live in the air, researchers are using special devices to generate aerosolized viruses and keep them in the air for long periods of time. Researchers can then take samples of the virus and see how long it stays alive in an aerosol. An early study by researchers at the National Institute of Health kept the virus in the air for four hours and found live viruses all the time. A subsequent pre-print study that I was involved in showed that the coronavirus can stay alive for up to 16 hours in the air.
Neither the first nor the first study in which I was involved measured the influence of temperature, humidity, ultraviolet light or pollution on the survival of the virus in aerosols. There is evidence that simulated sunlight can inactivate 90% of the SARS-CoV-2 viruses in saliva on surfaces or in aerosols within seven minutes. These studies suggest that the virus quickly inactivates outdoors, but the risk of transmission indoors remains.
Evidence from the real world
Laboratory studies can provide valuable insights, but real-world scenarios indicate the real risk of airborne transmission.
Reports from China, Singapore and Nebraska have found the virus in patient rooms and in very small amounts in the ventilation system of hospitals where COVID-19 patients were treated. The report from China also found evidence of the virus at the entrance to a department store. Previously, this sampling was done using polymerase chain reaction tests that looked for viral pieces of DNA rather than live viruses. You can't tell researchers if what they find is contagious.
To get direct evidence of the risks of airborne transmission, we can look at some case studies in the US and abroad.
One study followed how a single infected person in a call center in South Korea infected 94 other people. There is also the widespread case of an infected person in a restaurant in Guangzhou, China, who is transmitting the virus to nine other people due to the airflow generated by an air conditioner in the room.
Perhaps the most striking example is the church choir in Skagit County, Washington, especially for me when we think about how we can reopen our church. A single person singing in a choir practice infected 52 other people. Singing and loud vocalization in general can produce many aerosols, and there is evidence that some people are super-emitters of aerosols even during normal speech. It is likely that some infections in this incident were caused by droplets or direct contact. However, the fact that one person infected so many people strongly suggests that airborne transmission was the driving factor behind this outbreak.
An article published just last week compared the success of mitigation measures - such as social distancing or wearing masks - to determine how the virus spreads. The authors concluded that aerosol transmission was the dominant route. This conclusion is hotly debated in the scientific community, but this and other studies show the effectiveness of masks in slowing the spread of COVID-19.
What does this mean for reopening and for individuals?
The evidence strongly suggests that airborne transmission is easy and is likely to be a major driver of this pandemic. It has to be taken seriously when people start to venture out into the world again.
Fortunately, there is a simple, if not perfect, way to reduce airborne transmission: masks. Since people can spread the virus if they are pre-symptomatic or asymptomatic, wearing a universal mask is a very effective and inexpensive way to slow the pandemic.
Since the main risk is indoor, increasing ventilation rates and no air circulation in buildings would remove the virus from the interior faster.
My church has decided to reopen, but we are only planning to allow a limited number of people into the church and to spread them throughout the sanctuary to maintain social distance. And at least for the moment everyone has to wear masks. Especially when singing.
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This article has been republished by The Conversation, a nonprofit news site dedicated to the exchange of ideas from academic experts.
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Douglas Reed receives funding from NIH and DOD as well as from CEPI.
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