New Study Explains How Coronavirus Stays Airborne Longer Than Previously Thought

At the risk of stepping on Lambert’s toes, since he’s been relentless in covering the considerable, successful efforts by scientists to have aerosols recognized as a means of Covid transmission, some new studies shed more light on how this happens. And by extension, they offer support for the notion that Covid stays airborne much longer than earlier, long-standing conventional wisdom held possible. They also explain why it seems way too easy to contract the common cold.

These studies serve to explain how developments that surprised experts likely came about. One eye-opener was the set of early Omicron cases in Oslo. Those of you who follow Covid news likely recall that the patients extended beyond those who were at the party, ie, potentially speaking to or very near the guests who’d returned from South Africa:

And 30 additional people were reportedly infected AFTER the party was over because they later entered the same space where it had been held.#COVIDisAirborne https://t.co/RPrA9wL3nD

— Leah McElrath ?️‍? (@leahmcelrath) December 5, 2021

This tweet warrants some additional detail. From a peer-reviewed study in Eurosurveillance:

The closed event was held in a separate room (ca 145 m2) in a restaurant in Oslo from 18:00 to 22:30, after which the venue was opened to the public from 22:30 to 03:00. A pre-party had been arranged for the Christmas party attendees at a separate venue, after which they were transported by private buses to the restaurant…

After detection of the outbreak on 30 November, all attendees at the party were requested by the municipality doctor in Oslo to self-quarantine at home for 10 days and to immediately take a PCR test. Those who tested positive were required to remain in isolation for at least 7 days [3]. In addition, a public message was released on 1 December asking anyone who had been at the venue from 22:30 on 26 November to 03:00 on 27 November to get tested by PCR as soon as possible, regardless of symptoms.

The company has a private room to itself from 6:30 PM till 11:30 PM…and had had a preparty before that. So the odds of anyone from the company party staying on were not high.

In other words, if stale Omicron-y air is what made the new arrivals sick, it had to have hung in the air for a while. Yet the old prevailing belief was viral nasties in aerosol form, even if they remained in the air, dried out pretty pronto and became harmless. But is that correct?

A new article in MedicalXpress (hat tip Robert M) recaps a new study in International Communications in Heat and Mass Transfer by scientists at the Department of Energy’s Pacific Northwest National Laboratory (as in not by medical scientist….due say to intellectual capture?). Sadly I can’t find it online, but the writeup is thorough, so we’ll rely on that and look for the Internet versions to surface. The key finding, and it has implications well beyond Covid, is that mucus coatings greatly increase how long airborne viruses remain viable.

So this finding also serves as a reminder that Covid deserves a lot more respect than many, starting with most public health officials, give it. From the MedicalXPress article:

A modeling study raises questions about how far respiratory droplets, like those that transmit the virus that causes COVID-19, can travel before becoming harmless. Can the airborne particles that carry the virus remain infectious not just for a few feet but rather more than 200 feet, farther than the length of a hockey rink?…

The PNNL team took a long look at the mucus that coats the respiratory droplets that people spew from their lungs. Scientists know that mucus allows many viruses to travel further than they otherwise would, enabling them to journey from one person to another.

Conventional wisdom has been that very small, aerosolized droplets of just a few microns, like those produced in the lungs, dry out in air almost instantly, becoming harmless. But the PNNL team found that mucus changes the equation.

The team found that the mucus shell that surrounds respiratory droplets likely reduces the evaporation rate, increasing the time that viral particles within the droplets are kept moist. Since enveloped viruses like SARS-CoV-2 have a fatty coating that must be kept moist for the virus to be infectious, the slower evaporation allows viral particles to be infectious longer.

The team estimates that droplets encased in mucus could remain moist for up to 30 minutes and travel up to about 200 feet.

The article then discusses earlier work by one of the co-authors of the PNNL study, Carolyne Burns, who with different co-authors had studies published in Indoor Air and Building and Environment. They focuses on how contagion would spread from one room in a multi-room building. Again from MedicalXpress:

A team led by Alex Vlachokostas and Burns measured droplet levels in two adjoining rooms with controlled building ventilation….

The scientists found that both low and high levels of filtering were effective at reducing levels of respiratory droplets in all rooms. Filtration quickly cut down the levels of droplets in the adjoining rooms—within about three hours, to one-third the level or less without filtration.

The team also found that increasing ventilation rapidly reduced particle levels in the source room. But particle levels in the other connected rooms jumped immediately; levels spiked 20 to 45 minutes later with vigorous air changes increasing the spike. Ultimately, after the initial spike, levels of droplets in all the rooms gradually dropped after three hours with filtration and after five hours without it.

The scientists say that increased air exchange for crowded spaces may be beneficial in certain situations, like large conferences or school assemblies, but in normal work and school conditions, it may actually increase transmission rates throughout all rooms of a building.

As I read this, the scenario is a setting with limited external ventilation. So intuitively, this finding makes sense. Moving air around in a closes system moves a virus around. Maybe if the total cubic volume in the room with virus is small relative to the total cubic volume of the building, you might get some dilution. But that isn’t what the authors posit. They appear to be using only three similar sized rooms (the source room and two adjoining rooms). So even diluting the virus across 2X as much original cubic volume does not provide a net reduction in contagion risk. It would seem to take even more de facto dilution, either by air exchange to the outside or a lot more uncontaminated internal air space. And how can you be confident that you actually have that?

So don’t forget those Corsi-Rosenthal boxes! Personally, I think they look cute in a geeky way.

Update 4:00 AM EST. How embarrassing. Reader BH pointed out that the MedicalXpress story provided a full name for each article and a DOI descriptor, and when you put that into a search engine, the article pops up. Just so you know, I had spent about ten minutes on various searches using “International Communications in Heat and Mass Transfer” (in quotes) plus other words and got only the MedicalXpress article. So I had assumed the original piece was well paywalled.

Here are the listings, thanks to BH, and again, use the address starting with DOI to find them:

Leonard F. Pease et al, A missing layer in COVID-19 studies: Transmission of enveloped viruses in mucus-rich droplets, International Communications in Heat and Mass Transfer (2021). DOI: 10.1016/j.icheatmasstransfer.2021.105746

Leonard F. Pease et al, Investigation of potential aerosol transmission and infectivity of SARS-CoV-2 through central ventilation systems, Building and Environment (2021). DOI: 10.1016/j.buildenv.2021.107633

From the first paper:

Yet, the Wells model based on water droplets divides respiratory droplets into either quickly evaporated aerosolized particles termed droplet nuclei (<10 s) or liquid droplets that fall to the nearest surface, leaving no physical mechanism for airborne transmission of fully infective enveloped viruses over large distances (greater than a few meters). Yet, the role of mucus layers on evaporation times has not been considered even though the formation of mucus shells around liquid cores of respiratory droplets has been shown experimentally. Here we show that mucus shells increase the drying time by orders of magnitude so that enveloped virions may remain well hydrated and, thus, fully infective at substantial distances. This provides a mechanism by which infective enveloped virus particles can transmit as aerosols within buildings and between buildings over extended distances. This analysis is important because public health agencies typically follow the Wells model to establish health policies including social/physical distancing guidelines.

And the paper includes Lambert’s aerosol transmission hit parade:

Field studies in the built environment suggest that enveloped viruses spread much farther than social distancing guidelines, challenging the dichotomy of fomite transmission via hydrated droplet on surfaces or dry virus containing particles suggested by Wells [4,7]. Yu, et al., [8] tracked the spread of SARS in the tall Amoy Gardens apartment complex to show that spread seemed to follow wind patterns (e.g., wind at 2 m/s between buildings ~60 m apart), which would not be the case if all droplets that did not hit the floor had completely dried out (whether by coughing or by sewer-formed droplets the same inadequacy exists). Similarly, Lu, et al., [2] suggest that infectivity of SARS-CoV-2 in an air-conditioned restaurant followed air currents, which would not be feasible if all of the droplets containing enveloped viruses dried within a fraction of a second. Also, Hamner, et al., [9] report 52 choir members became ill from one index patient, suggesting aerosol spread substantially beyond the nominal 6 ft of social distancing in the U.S. Therefore, recent research has begun to focus on mechanisms that would extend the hydrated status of droplets. For example, Xie, et al., [10] revisited the work of Wells to include the influence of droplet evaporation and humidity on viral spread. Increasing humidity decreases the minimum size of liquid droplets that land on surfaces to enable fomite transmission and extends the time to evaporation into 10’s of seconds. Bourouiba, et al., [11] suggest that some small droplets circulate within a cloud that may decrease their evaporation rate. Liu, et al., [12] include the influence of turbulence on spread to show some marginal increase in the droplet distribution pattern due to decaying turbulent eddies. While each of these observations extends the range of infectivity relative to that proposed by Wells, none of these fully explain the apparent infectivity of SARS-CoV-1 between buildings.

So as an economists might say, it’s good to see what works in practice work in theory.