"It seems that people are negating the impact of fine aerosols."


New WHO guidance suggests Covid-19 can be airborne inside

The World Health Organization is considering the idea the coronavirus can travel on small aerosols. Here's how that update may affect how we interact in public.

From the beginning of the pandemic, there's been no doubt that large droplets — like ones rocketed from the nose or mouth during a sneeze or cough spread the coronavirus — spread Covid-19. On Thursday, after a series of studies on talking, singing and yelling, and a letter signed by 237 scientists, the World Health Organization (WHO) also acknowledged that the virus can remain airborne while indoors.

It's a shift in thinking that goes from the highest levels of public health decision making all the way down to those of us eyeing the crowded outdoor bar.

In newly released guidance, the WHO states that the coronavirus outbreaks seen in "restaurants, bars, places of worship or places of work" (in other words indoor places where people speak, shout or sing in close quarters) could represent airborne transmission.

It further notes that "short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces over a prolonged period of time with infected persons cannot be ruled out."

The WHO's acknowledgment underscores the need to consider that the coronavirus may live on droplets of all sizes and adjust our behavior accordingly, explains Seema Lakdawala, an assistant professor at the University of Pittsburgh School of Medicine.

Aerosols are simply smaller droplets, much like the ones released during a cough or sneeze, Ladawal tells Inverse. They're all part of the same continuum.

"When you sneeze or cough or sing or talk or breathe, you are releasing aerosols of all size ranges," Lakdawala explains. "If you are infected with coronavirus, all of those sides ranges of aerosols will have the virus in them — most likely."

A video showing how droplets emerge during speech with and without a mask, published in the New England Journal of Medicine in April.

That said, we still don't know how efficient the small aerosol route of transmission is at spreading the virus. The WHO's updated guidance suggests that droplets and fomite-based transmission might also explain outbreaks in indoor spaces. (A fomite is an object that is likely to carry infection, like utensils.)

The guidance suggests that this could be the case if masks were not worn, or social distancing and hand hygiene were neglected.

Lakdawala says that the significance of airborne transmission has been underplayed:

"The reason these aero-biologists thought there was a need to push for this, is that it seems that people are negating the impact of fine aerosols."

What this news means for you – Though we've been trained to avoid people sneezing or coughing, we have to acknowledge that it's possible to get the virus through talking to another person at close range, explains Julian Tang, a consultant virologist at the University of Leicester Royal Infirmary. This is especially true if a mask isn't being worn by both parties

"Whilst sneezing and coughing look very dramatic, I think that most of the transmission occurs by simply breathing and talking whilst standing relatively closely," Tang tells Inverse.

Lakdawala adds that breathing tends to release these finer aerosols whereas talking or sneezing or coughing can release droplets of all sizes. Speaking can release as many as 1,000 droplets that can linger in the air between 8 and 14 minutes, according to a May study in the Proceedings of the National Academy of Sciences.

The fact that these can be infectious has hinted at in case studies. For instance, take the now infamous CDC case report documenting the spread of coronavirus amongst a choir in Washington. There, 32 members of the choir became infected likely due to one symptomatic individual (there was an additional 20 probable cases as well). The authors suggested that the act of singing "augmented" transmission.

In light of that Tang suggests wearing a mask whenever you are around someone not in your quarantine pod, especially indoors or in crowds. Masks are intended to at least help block some of those larger droplets from escaping — and thus protect others.

That said, for truly small aerosols non-medical masks and cloth face coverings don't help, says Lakdawala. In that case, N95 masks, which can filter out those smaller aerosols, are needed. This underscores why medical workers, in particular, need these masks.

For the rest of us, this ultimately drives home the importance of social distancing. For larger droplets, maintaining distance allows those droplets to fall to the ground. As for the smaller droplets, that space simply keeps you away from any droplets being expelled by another person.

Consider the crowded outdoor bar. As Tang notes, people will remove their masks to eat and drink. This means that tables need to be distanced at least six feet, he recommends. The more crowded that bar is, the harder that is to do, Lakdawala adds.

"Even in an outdoor enclosed space, if you are very physically close to another person, you are breathing in whatever they're expelling. You're not giving them enough time and space in order to dilute out the air," Lakdawala says.

A video from Tang's research showing how air flow is exchanged between talkers.

What small aerosols mean for reopening – Fortunately, this news doesn't fundamentally change the silver lining: transmission outdoors is still far less likely than transmission indoors.

That's because there's "more space outdoors to socially distance, as well as larger air volumes to dilute the virus, and sunlight to kill some of the virus and the wind to blow the virus elsewhere," Tang says.

That's not always true of indoor spaces. To take aerosolized transmission seriously, offices or restaurants with indoor seating will have to find creative ways to keep "diluting" the air, Lakdawala explains. That means opening a window (choose that over air conditioning) or turning on a fan.

For bigger office buildings, or schools, that will likely mean optimizing their ventilation rates, adds Tang. In some ways that's an engineering problem. In the Clinical Infectious Diseases letter, the authors suggest that buildings should consider installing high-efficiency air filtration systems or germ-killing ultraviolet lights.

In a press conference on July 7, Maria Van Kerkhove, the WHO's Covid-19 technical lead, noted that the letter sent by 237 scientists to WHO "adds to growing knowledge about the importance of ventilation."

For individuals the answer is simpler: there must be space for social distancing. Keeping distance from others, especially inside, will declaw aerosols' infectious potential.

"If you are next to another group of people and there's no physical distance, you have to realize that you are breathing in whatever they are expelling," Lakdawala says.

"Indeed, that is just the reality."

Related Tags