Is COVID-19 airborne? Over 200 scientists want WHO and policymakers to recognize lethal potential and act
The potential for Covid-19 to spread through airborne transmission needs to be recognized by the World Health Organization (WHO) and other relevant bodies, according to a group of over 200 scientists. In an open letter, which will soon be published in Clinical Infectious Diseases, researchers have called for greater acknowledgment of the role of airborne spread of Covid-19 and the subsequent need for countries and governments to take appropriate action and implement preventive measures against this risk.
According to the WHO, the new coronavirus is transmitted primarily between people through respiratory droplets and contact routes. The WHO says airborne transmission may be possible in specific circumstances and settings in which medical procedures like intubation or support treatments that generate aerosols are performed. The US Centers for Disease Control and Prevention (CDC) also says that the virus is thought to spread mainly from person-to-person, between people who are in close contact with one another (within about 6 feet), through respiratory droplets produced when an infected person coughs, sneezes, or talks. The CDC says it may be possible that a person can get Covid-19 by touching a surface or object that has the virus on it and then touching their mouth, nose or possibly their eyes, but this is not thought to be the main way the virus spreads.
Titled, “It is time to address airborne transmission of Covid-19,” the letter is authored by Lidia Morawska from the International Laboratory for Air Quality and Health, WHO Collaborating Centre, Queensland University of Technology, Australia and Donald K Milton, Institute for Applied Environmental Health, University of Maryland School of Public Health, US. It has been signed by 239 scientists from 32 countries. They say emerging evidence, including those from closed indoor settings and super spreading events, suggests that airborne transmission could be more important than the medical community and relevant bodies such as the WHO has acknowledged. According to the scientists, many studies have shown viruses are released during talking or coughing in microdroplets -- that are small enough to linger in the air and risk exposure at distances more than 3 feet to 6 feet from an infected person.
A recent analysis of a superspreader described how at choir practice, where one person who did not show any symptoms, led to 87% of others developing Covid-19 in Skagit County, Washington. “The act of singing itself might have contributed to the transmission through the emission of aerosols, which is affected by the loudness of vocalization,” it said. Skagit County (Washington). Research work on the same event, which is currently being peer-reviewed, says that “transmission by the airborne route is likely.”
“Airborne transmission is now strongly suspected to play a significant role in super spreading events under certain conditions. During respiratory disease pandemics, group singing indoors should be discouraged or at a minimum, carefully managed as singing can generate large amounts of airborne virus if any of the singers is infected,” the study suggests. The authors say while the high ventilation rate in hospital settings combined with other controls such as the use of isolation rooms and effective PPE is likely to mitigate transmission from a high viral shedder in the healthcare environment, in many community indoor spaces not dedicated to infection control, controlling airborne diseases transmission remains a great challenge during this pandemic.
Another study conducted by experts from the US National Institutes of Health (NIH) and the University of Pennsylvania, had said that thousands of droplets from the mouths of people who are talking loudly can stay in the air for between eight and 14 minutes before disappearing. Researchers had concluded that observations “confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments.”
In April, Morawska had also published a commentary on the airborne transmission of SARS-CoV-2, the virus that causes Covid-19. In it she wrote while hand washing and maintaining social distance are the main measures recommended by the WHO to avoid contracting Covid-19, they do not prevent infection by inhalation of small droplets exhaled by an infected person that “can travel distance of meters or tens of meters in the air and carry their viral content.” The authors, which also includes Junji Cao, Key Lab of Aerosol Chemistry & Physics (KLACP), Chinese Academy of Sciences, Beijing, said they strongly believe that the virus is likely to be spreading through the air. They argued that considering the many similarities between SARS and SARS-CoV-2 viruses and the evidence on virus transport in general, it is highly likely that SARS-CoV-2 also spreads by air.
The authors wrote back then that science explains the mechanisms of such transport and there is evidence that this is a significant route of infection in indoor environments, but no countries or authorities consider airborne spread of Covid-19 in their regulations to prevent infection transmission indoors. “It is, therefore, extremely important, that the national authorities acknowledge the reality that the virus spreads through the air, and recommend that adequate control measures be implemented to prevent further spread of the SARS-CoV-2 virus, in particular removal of the virus-laden droplets from indoor air by ventilation,” said the article published in Environment International. The analysis recommended taking all possible precautions against airborne transmission in indoor scenarios such as increased ventilation rate, using natural ventilation, avoiding air re-circulation, avoiding staying in another person’s direct airflow, and minimizing the number of people sharing the same environment.
University of Colorado Boulder atmospheric chemist Jose-Luis Jimenez, who is one of the signatories, has also released a pilot tool -- the Covid airborne transmission estimator -- where he explains many issues on airborne transmission. “There is a lot of fear about ‘airborne,’’ but the estimator allows one to see why different situations are very very different,” Jimenez told MEA WorldWide (MEAWW). The estimator calculates Covid-19 infection risk for many basic situations: college classrooms, choirs, taking a bus, being outdoors, participating in demonstrations.
“There are 3 ways of transmission: contact/fomite when a person shakes hands or touches an object that an infected person has touched (and the infected person had the virus in their hands from touching their nose, a cough etc.). However, this is thought to be unlikely, in particular, because enveloped viruses like SARS-CoV-2 only survive for about 5 minutes on hands. The second way is droplet transmission, when large ballistic droplets in a cough or sneeze land on the eyes, nostrils, or mouth of another person,” he writes. Jimenez further explains, “The third is aerosol (airborne) transmission when particles smaller than ~50 microns are inhaled by a healthy person. The pandemic is likely being driven by asymptomatic or pre-symptomatic spread, and those people don't cough, leaving airborne transmission as the main likely route. It is also likely being driven by super-spreading events, and those (like the Skagit choir) are very very hard to explain without airborne transmission.”
But we are not certain, so why should we take precautions against aerosol transmission? “Because of the precautionary principle. The cost of doing nothing, if aerosol transmission is happening, is much higher than the cost of doing something, if aerosol transmission ended up being unimportant. Many measures to reduce airborne transmission, such as reducing indoor crowding, masks, etc. also limit other ways of transmission. And remember that absence of (complete) evidence is not evidence of (complete) absence,” says Jimenez.
According to Jimenez, a problem with the WHO committee on disease transmission is that they “rely most strongly on evidence from hospitals.”
“ Cases from the community are much more difficult to analyze. They say that if transmission in hospitals is limited even when using surgical masks, therefore it must not go through the air. But hospitals have high ventilation, and everyone is wearing at least surgical masks,” he says. He says unlike community settings, in hospitals, patients are not talking loudly or singing, they are mostly just breathing. “Therefore it makes a lot of sense that aerosol transmission is limited in hospitals, but it doesn't disprove that it is major in the community,” he adds.
Lindsay Marr, professor of environmental engineering at Virginia Tech, who is also one of the signatories, explains the airborne transmission of SARS-CoV-2 and other viruses. She says that the virus can be released into the air through coughing, sneezing, talking, and breathing, some people release more than others. Marr says at close range, the concentration of airborne virus will be quite high and as a person gets farther from the source, the concentration falls off rapidly. “If you are close, you are much more likely to inhale the virus. You are also more likely to be sprayed by large droplets that land on your face, which doesn't happen if you're farther away,” she tweets.
“Viruses in small droplets can float around in air for many hours, but they will likely be quite diluted unless you're in a small confined space. You could inhale these, but it's much less likely than if you're close to the person. Airborne viruses will eventually settle on surfaces, like the floor and tables, and could be picked up if someone touches them. However, their ability to survive varies with temperature, humidity, and surface material. They decay gradually over hours or days,” says Marr.