Virus World

The US Centers for Disease Control and Prevention updated guidance on its website to say coronavirus can commonly spread "through respiratory droplets or small particles, such as those in aerosols," which are produced even when a person breathes. "Airborne viruses, including COVID-19, are among the most contagious and easily spread," the site now says. Previously, the CDC page said that Covid-19 was thought to spread mainly between people in close contact -- about 6 feet -- and "through respiratory droplets produced when an infected person coughs, sneezes or talks." The page, updated Friday, still says Covid-19 most commonly spreads between people who are in close contact with one another, and now says the virus is known to spread "through respiratory droplets or small particles, such as those in aerosols, produced when an infected person coughs, sneezes, sings, talks or breathes." These particles can cause infection when "inhaled into the nose, mouth, airways, and lungs," it says. "This is thought to be the main way the virus spreads."

"There is growing evidence that droplets and airborne particles can remain suspended in the air and be breathed in by others, and travel distances beyond 6 feet (for example, during choir practice, in restaurants, or in fitness classes)," the page now says. "In general, indoor environments without good ventilation increase this risk." The CDC also added new measures to its information about protecting yourself and others. Previously, CDC suggested maintaining "good social distance" of about 6 feet, washing hands, routinely cleaning and disinfecting surfaces and covering your mouth and nose with a mask when around others. Now, it says "stay at least 6 feet away from others, whenever possible," and continues to direct people to wear a mask and routinely clean and disinfect. However, it also now says people should stay home and isolate when sick, and "use air purifiers to help reduce airborne germs in indoor spaces." Masks, it notes, should not replace other prevention measures. The update also changed language around asymptomatic transmission, shifting from saying "some people without symptoms may be able to spread the virus" to saying "people who are infected but do not show symptoms can spread the virus to others."

For months, scientists have noted the likelihood of coronavirus transmission through viral particles in the air, and pushed health agencies to acknowledge it. In April, a prestigious scientific panel told the White House in a letter that research showed coronavirus can be spread not just by sneezes or coughs, but also just by talking, or possibly even just breathing. "While the current [coronavirus] specific research is limited, the results of available studies are consistent with aerosolization of virus from normal breathing," according to the letter, written by Dr. Harvey Fineberg, former dean of the Harvard School of Public Health and chair of the NAS' Standing Committee on Emerging Infectious Diseases and 21st Century Health Threats. "Currently available research supports the possibility that [coronavirus] could be spread via bioaerosols generated directly by patients' exhalation," the letter said. And in July, 23 nine scientists published a letter that urged the World Health Organization and other public health organizations to be more forthcoming about the likelihood that people could catch the virus from droplets that were floating in the air. "The current guidance from numerous international and national bodies focuses on hand washing, maintaining social distancing, and droplet precautions," scientists wrote in the letter, published in the journal Clinical Infectious Diseases. "Most public health organizations, including the World Health Organization, do not recognize airborne transmission except for aerosol-generating procedures performed in healthcare settings. Hand washing and social distancing are appropriate, but in our view, insufficient to provide protection from virus-carrying respiratory microdroplets released into the air by infected people," they added...

These simple experiments illustrate how wearing a mask can stop the spreading of germs. In the study petri plates containing media to grow bacteria are exposed to an individual wearing a surgical mask or not. Conditions tested include coughing, sneezing, talking and singing. The results are striking. Masks block the ejection of droplets that contain bacteria. Exposed petri plates appear full of bacterial colonies when exposed to an individual who is not wearing a mask.

Experiments also confirm the effect of social distancing, and demonstrate how the individual does not spread bacteria to plates exposed at a distance of 6 feet away from the source. Although these experiments evaluate bacterial contamination, the droplets that carry these germs can also transport viruses such as SARS-CoV-2. 

Aerosols and droplets generated during speech have been implicated in the person-to-person transmission of viruses, and there is current interest in understanding the mechanisms responsible for the spread of Covid-19 by these means. The act of speaking generates oral fluid droplets that vary widely in size, and these droplets can harbor infectious virus particles. Whereas large droplets fall quickly to the ground, small droplets can dehydrate and linger as “droplet nuclei” in the air, where they behave like an aerosol and thereby expand the spatial extent of emitted infectious particles. We report the results of a laser light-scattering experiment in which speech-generated droplets and their trajectories were visualized...

When a person spoke through the open end of the box, droplets generated during speech traversed approximately 50 to 75 mm before they encountered the light sheet. An iPhone 11 Pro video camera aimed at the light sheet through a hole (7 cm in diameter) on the opposite side of the box recorded sound and video of the light-scattering events at a rate of 60 frames per second. The size of the droplets was estimated from ultrahigh-resolution recordings...

We found that the number of flashes increased with the loudness of speech; this finding was consistent with previous observations by other investigators.3 In one study, droplets emitted during speech were smaller than those emitted during coughing or sneezing. Some studies have shown that the number of droplets produced by speaking is similar to the number produced by coughing.

N.ENgland. J. Med. (April 15, 2020):

https://doi.org/10.1056/NEJMc2007800

When researcher Josh Santarpia stands at the foot of a bed, taking measurements with a device that can detect tiny, invisible particles of mucus or saliva that come out of someone's mouth and move through the air, he can tell whether the bedridden person is speaking or not just by looking at the read-out on his instrument. "So clearly the particles that that person is putting out are being breathed in by someone that is five feet away from them, at the foot of their bed," says Santarpia, who studies biological aerosols at the University of Nebraska Medical Center. "Do they contain virus? I don't know for sure."

He and his colleagues are doing their best to find out. Already, using another contraption that looks like a fancy dustbuster, they've sucked up air samples from 11 isolation rooms that housed 13 people who tested positive for COVID-19 infection, all of whom had a variety of mild symptoms. In those air samples, researchers found the genetic fingerprint of the virus. "It was more than half of the samples that we took. It was fairly ubiquitous," says Santarpia, "but the concentrations were really pretty low." Finding the genetic material doesn't necessarily mean that there's viable virus that could potentially make someone sick, he cautions. Some preliminary evidence indicates that this might be the case, but the team wants to do more work "and try and be as certain as we possibly can whether or not certain samples had infectious virus in them or not." They want to know that with a high degree of confidence because the question of whether or not the coronavirus can be "airborne" is extremely contentious right now — and it's a question that has real implications for what people should do to avoid getting infected....

Dr. Martin Fischer, from Duke University, developed a simple, low-cost technique to visualize the effectiveness of different face coverings on droplet emissions during normal wear. Testing several face coverings, the researchers found that the particles can be blocked by some, but not all recommended face coverings.

N95s without valves, and surgical, and polypropylene masks worked best. Cotton face coverings provided some coverage. Bandanas and neck fleece didn’t block the droplets very much, and indeed the "filter" smaller droplets that remained longer airborne. The experiments clearly shows the importance of wearing masks to prevent spreading of droplets that cary the COVID-19 virus, and also suggest that some face covers don 't do the job at all.

See also original study in Science Advances (August 7, 2020):

https://doi.org/10.1126/sciadv.abd3083

Respiratory infections occur through the transmission of virus-containing droplets (>5 to 10 μm) and aerosols (≤5 μm) exhaled from infected individuals during breathing, speaking, coughing, and sneezing. Traditional respiratory disease control measures are designed to reduce transmission by droplets produced in the sneezes and coughs of infected individuals. However, a large proportion of the spread of coronavirus disease 2019 (COVID-19) appears to be occurring through airborne transmission of aerosols produced by asymptomatic individuals during breathing and speaking (1–3). Aerosols can accumulate, remain infectious in indoor air for hours, and be easily inhaled deep into the lungs. For society to resume, measures designed to reduce aerosol transmission must be implemented, including universal masking and regular, widespread testing to identify and isolate infected asymptomatic individuals.

Humans produce respiratory droplets ranging from 0.1 to 1000 μm. A competition between droplet size, inertia, gravity, and evaporation determines how far emitted droplets and aerosols will travel in air (4, 5). Respiratory droplets will undergo gravitational settling faster than they evaporate, contaminating surfaces and leading to contact transmission. Smaller aerosols (≤5 μm) will evaporate faster than they can settle, are buoyant, and thus can be affected by air currents, which can transport them over longer distances. Thus, there are two major respiratory virus transmission pathways: contact (direct or indirect between people and with contaminated surfaces) and airborne inhalation.

In addition to contributing to the extent of dispersal and mode of transmission, respiratory droplet size has been shown to affect the severity of disease. For example, influenza virus is more commonly contained in aerosols with sizes below 1 μm (submicron), which lead to more severe infection (4). In the case of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), it is possible that submicron virus-containing aerosols are being transferred deep into the alveolar region of the lungs, where immune responses seem to be temporarily bypassed. SARS-CoV-2 has been shown to replicate three times faster than SARS-CoV-1 and thus can rapidly spread to the pharynx from which it can be shed before the innate immune response becomes activated and produces symptoms (6)....

We identified seasonal human coronaviruses, influenza viruses and rhinoviruses in exhaled breath and coughs of children and adults with acute respiratory illness. Surgical face masks significantly reduced detection of influenza virus RNA in respiratory droplets and coronavirus RNA in aerosols, with a trend toward reduced detection of coronavirus RNA in respiratory droplets. Our results indicate that surgical face masks could prevent transmission of human coronaviruses and influenza viruses from symptomatic individuals.

A study of 246 individuals with seasonal respiratory virus infections randomized to wear or not wear a surgical face mask showed that masks can significantly reduce detection of coronavirus and influenza virus in exhaled breath and may help interrupt virus transmission...