- The novel coronavirus can spread via droplets, aerosols, and fomites, but the CDC and WHO insist on droplets as the main way people catch COVID-19.
- Some researchers have stressed the importance of airborne spread for months, urging officials to acknowledge the risks.
- Scientists from the Netherlands attempted to map the behavior of droplets using lasers and concluded that coronavirus actually doesn’t spread efficiently via aerosols.
- The researchers still say that airborne transmission via droplets is the main way the virus spreads, and they warn people to continue respecting health measures and ventilating indoor spaces.
The novel coronavirus spreads in three distinct ways, but ultimately the same principle applies to all of them. Droplets or aerosols are inhaled through the mouth or nose before entering the respiratory system, where the virus starts spreading. The other way involves touching the nose, mouth, or eyes with a hand that touched a contaminated surface or object. That’s why health officials advise the same safety measures over and over. Face masks can block droplets and aerosols. Social distancing further reduces the risk, as the droplets and aerosols can only travel so far and are affected by gravity and ventilation. Frequent hand hygiene allows you to kill or remove any live virus that may be present on your fingers.
Droplet transmission is seen as the main way COVID-19 spreads. These are large saliva particles that contain active virus. They’re ejected while speaking, coughing, or sneezing and they can land on surfaces or be inhaled by nearby people. Public health agencies like the WHO and the CDC have acknowledged this airborne transmission, but they say aerosol spread occurs only in certain cases. Aerosols are different — they’re water-free particles that can contain live virus, and linger in the air for longer periods and travel farther because they’re so much smaller. We already saw plenty of studies showing what happens when we speak, cough, and sneeze, but researchers haven’t stopped studying the matter. A new study now indicates that the virus might not actually be able to spread as efficiently via aerosols as we think it does.
Published in Physics of Fluids, the study comes from the Netherlands, where physicists and doctors studied the way aerosols behave in the air and attempted to assess the risk of COVID-19 spread via airborne aerosol transmission.
They concluded that the aerosols hold a more limited quantity of virus than droplets, which makes infection less likely. It’s important to note, however, that science has yet to answer a major question about COVID-19 spread. We have no idea what kind of viral load in droplets or aerosols is enough to get someone sick. The researchers attempted to estimate the viral loads in aerosol particles compared to droplets, and then compared those with the observed viral loads in infectious patients.
The team used a laser diffraction model to measure the size of the particles and their distribution. They looked at speech and coughing, and they estimated that only one in 2,000 aerosol particles could contain the virus. They also looked at how long these microdroplets would need to reach the ground and found that the smaller the particle, the longer it would spend in the air.
They offered an example of potential spread. “The highest probability of infection occurs when a person enters a poorly ventilated and small space where a high emitter has just coughed and inhales virus-carrying droplets,” the researchers explain. “We model coughing in our 2 × 2 × 2 m3 [6 x 6 x 6 cubic feet] unventilated space that could represent, e.g., a restroom.”
“Our dynamic modeling of transmission of SARS-CoV-2 in confined spaces suggests that aerosol transmission is not a very efficient route, in particular from non-symptomatic or mildly symptomatic individuals that are likely to have low virus content in their saliva,” the researchers wrote, pointing out that the risk remains real. “Highly infected people having a large viral load in their saliva and superspreaders producing lots of aerosols are likely far more dangerous. Comparing aerosol transmission to other transmission routes, it is useful to realize that the large droplets that are believed to be responsible for direct and nosocomial infections may contain about 500 virus particles per droplet and are thus likely to also be very important in a mixed transmission model.”
They also noted that their “results do not completely rule out aerosol transmission. It is likely that large numbers of aerosol droplets, produced by continuous coughing, speaking, singing, or certain types of aerosol-generating medical interventions, can still result in transmission, in particular in spaces with poor ventilation.”
The researchers say the virus isn’t as effective at spreading via the air as the measles, and that droplet and fomite transmission “are relatively more important ways of transmission than airborne transmission” for COVID-19.
Real-life environments and physics experiments aren’t the same things, however. Crowded places where multiple people speak, cough, and sneeze aren’t the same as controlled environments where scientists study the particles that a single person emits. If anything, the study underscores the importance of social distancing and face masks, especially in poorly ventilated indoor places where people might gather. The air in an indoor setting can contain a combination of potentially infectious droplets and aerosols. The higher the proportion of infected individuals in the room, the higher the risk of transmission.
“The public should, of course, continue social distancing and hand cleaning, and for the aerosols to critically look at the ventilation capacity of their working and living environment,” lead researcher Daniel Bonn told ZME Science. “Too many active (speaking, singing, shouting) people in ill-ventilated environments pose a problem, and so this should be avoided this winter. If you want to minimize the risk of infection, you need to not only keep the 6 feet, or 1.5 meters but also make sure the room you are in is well ventilated. And wash your hands.”
The study is available in full at this link.