I am a scientist who studies infectious diseases, specializing in severe respiratory infections. I am also a member of my church security team.
Over the past few weeks, as states began to ease restrictions, we have been discussing whether and how to safely restart services. But the coronavirus is far from gone. As we try to figure out how to organize services while protecting our members, one question is of particular concern: How often is the virus spread by air?
How to spread a virus
Respiratory infections are generally spread in three possible ways: by direct contact, by droplets, and by airborne particles.
Contact transmission occurs when a person touches an object with a living virus – called fomitis – on it and becomes ill.
Droplets are small particles of mucus or saliva that come out of a person’s mouth or nose when they cough or speak. Their size ranges from 5 microns to hundreds of microns in diameter – a red blood cell to a grain of sand. Most droplets, especially the larger ones, fall to the ground within seconds and usually travel no more than 1 or 2 meters. If someone coughed on you and you got sick, it would be droplet transmission.
Airborne transmission occurs from airborne particles known as droplet nuclei. Droplet nuclei are pieces of mucus or saliva that are less than 5 microns in diameter. People produce droplet nuclei when speaking, but they can also form when small droplets evaporate and shrink. Many of these droplets shrink so much that they begin to float before hitting the ground, becoming aerosols.
People produce thousands of these droplet nuclei per second when speaking, and the aerosolized particles can contain live viruses and float in the air for hours. They are easy to inhale and if they contain a live virus they can make people sick. The ability of droplet nuclei to transmit coronavirus has a huge impact on whether and how places like my church can reopen.
At the start of the pandemic, experts from the Centers for Disease Control and Prevention and the World Health Organization were most concerned about the transmission of the coronavirus from surfaces and large droplets.
But the more research is done on SARS-CoV-2, the more evidence there is that airborne transmission occurs despite being controversial. The CDC and WHO now recommend that the general population wear masks, but for people going about their lives and wondering how to reopen public spaces around the world, the question remains: how important is transmission. airborne?
Longevity in air in the laboratory
To be infected, a person must come into contact with a live virus. 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 use special equipment to create an aerosolized virus and keep it 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 initial study by researchers at the National Institute of Health kept the virus in the air for four hours and found a virus alive all the time. A subsequent pre-print study I participated in found that the coronavirus can stay alive for up to 16 hours in the air.
Neither the original study nor the one I participated in measured the impact 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 SARS-CoV-2 viruses in saliva on surfaces or in aerosols within seven minutes. These studies suggest that the virus would be quickly inactivated outdoors, but the risk of transmission indoors would remain.
Real world proof
Laboratory studies can provide valuable information, 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 at very low levels in the ventilation systems of hospitals where COVID-19 patients have been treated. The report from China also found evidence of the virus at the entrance to a department store. So far, this sampling has been done using polymerase chain reaction assays that look for pieces of viral DNA, not live virus. They can’t tell researchers if what they find is infectious.
For direct evidence of the risks of airborne transmission, we can look at a few case studies in the United States and abroad.
A study followed how a single infected person at a call center in South Korea infected 94 other people. There is also the widely reported case of an infected person at a restaurant in Guangzhou, China, spreading the virus to nine other people due to the airflow created by an air conditioning unit in the room.
Perhaps most striking, especially for me as we reflect on how to reopen our church, is the example of the church choir in Skagit County, Washington. A single individual singing during a choir rehearsal infected 52 other people. Singing and loud vocalization in general can produce a lot of aerosol, and evidence shows that some people are super aerosol emitters even during normal speech. It is likely that some infections in this incident were due to droplets or direct contact, but the fact that one person could have infected so many people strongly suggests that airborne transmission was the determining factor in this outbreak.
An article published last week compared the success of mitigation measures – like social distancing or wearing a mask – in trying to determine how the virus is spreading. The authors concluded that aerosol transmission was the dominant route. This conclusion is hotly debated in the scientific community, but this study and others show the effectiveness of masks in slowing the spread of COVID-19.
What does this mean for the reopening and for individuals?
Evidence strongly suggests that airborne transmission occurs easily and is likely a major driver of this pandemic. It should be taken seriously as people start to venture out into the world.
Fortunately, there is a simple, if not perfect, way to reduce airborne transmission: masks. Since people can spread the virus when they are pre-symptomatic or asymptomatic, wearing the universal mask is a very effective and inexpensive way to slow the pandemic.
Since the main risk is indoors, increasing ventilation rates and not recirculating air inside buildings would clear the virus from the indoor environment more quickly.
My church has decided to reopen, but we are only planning to allow a limited number of people in the church and distribute them throughout the sanctuary to maintain social distancing. And at least for now, everyone is required to wear masks. Especially while singing.
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