Wherever and whenever people are up in each other’s faces, laughing, shouting, cheering, sobbing, singing, greeting, and praying

Friday, April 24th, 2020

In 1899, a German bacteriologist named Carl Flügge proved that microbes can be transmitted ballistically through large droplets emitted at high velocity from the mouth and nose:

His method for proving the existence of these “Flügge droplets” (as they came to be known) was to painstakingly count the microbe colonies growing on culture plates hit with the expelled secretions of infected lab subjects. It couldn’t have been pleasant work. But his discoveries saved countless lives. And more than 12 decades later, these large respiratory droplets have been identified as a transmission mode for COVID-19.

Flügge’s graduate students continued his work into the 20th century, experimenting with different subjects expelling mucosalivary droplets in different ways. Eventually they determined, as a 1964 report in the Proceedings of the Royal Society of Medicine put it, that the quantity of expelled Flügge droplets varies markedly based on the manner of respiration: “Very few, if any… droplets are produced during quiet breathing, but [instead, they] are expelled during activities such as talking, coughing, blowing and sneezing.” A single heavy cough, it is now known, can expel as much as a quarter teaspoon of fluid in the form of Flügge droplets. And the higher the exit velocity of the cough, the larger the globules that can be expelled.

Yet if Flügge were with us today, he might be surprised by how little his science has been usefully advanced over the last few generations. As Lydia Bourouiba of the MIT Fluid Dynamics of Disease Transmission Laboratory recently noted in JAMA Insights, the basic framework used to represent human-to-human transmission of respiratory diseases such as COVID-19 remain rooted in the tuberculosis era. According to the binary model established in the 1930s, droplets typically are classified as either (1) large globules of the Flüggian variety—arcing through the air like a tennis ball until gravity brings them down to Earth; or (2) smaller particles, less than five to 10 micrometers in diameter (roughly a 10th the width of a human hair), which drift lazily through the air as fine aerosols.

In a fascinating paper published on March 26th, Turbulent Gas Clouds and Respiratory Pathogen Emissions: Potential Implications for Reducing Transmission of COVID-19, Bourouiba shows that analyzing a human sneeze is unusually difficult, even by the standards of fluid dynamics (whose mathematics I once modeled in my former capacity as an engineer and computer programmer). That’s because those mucosalivary droplets we emit are cocooned within a warm, moist enveloping gas cloud—Bourouiba calls it a “puff”—that protects the droplets from evaporation and allows even small globules to travel much farther than one might otherwise predict. The binary distinction between large and small droplets remains fundamental: Eventually, the big particles fall while the smaller ones don’t. But during those first fractions of a second when a sneeze (or cough, or shout) is expelled, Bourouiba shows, the enveloping gas sheath allows smaller particles to act, ballistically speaking, as if they were larger.

The science here is mind-bogglingly complex, because modeling the puff’s behaviour requires that Bourouiba and her team model not only the dynamics of the puff as it travels and dissipates, but also the biophysical and thermodynamic processes unfolding within the gas cloud. But the overall upshot is that such a puff “and its payload of pathogen-bearing droplets of all sizes” can travel seven to eight meters—about four times the length of the six-foot social-distancing buffer zone we’ve all been taught to enforce since mid-March.

Bourouiba’s research hits squarely on a blind spot in our knowledge of COVID-19. On one hand, scientists have an intimate molecule-by-molecule knowledge of the virus’s structure, its full genome having been sequenced months ago. On the other hand, the scientific and lay literature is bursting with epidemiological reports from just about every corner of the planet. But the nitty-gritty mechanics of actual disease transmission doesn’t take place on the microscopic scale of nucleic acids or on the gargantuan scale of whole nations. It takes place on the everyday face-to-face scale of inches and feet, as Flügge showed 121 years ago.

And it is on this crucial scale that our knowledge is thinnest. Despite the passage of four months since the first known human cases of COVID-19, our public-health officials remain committed to policies that reflect no clear understanding as to whether it is one-off ballistic droplet payloads or clouds of fine aerosols that pose the greatest risk—or even how these two modes compare to the possibility of indirect infection through contaminated surfaces (known as “fomites”).

Super-spreader events fit a pattern:

These parties, funerals, religious meet-ups and business networking sessions all seem to have involved the same type of behaviour: extended, close-range, face-to-face conversation—typically in crowded, socially animated spaces. This includes the many people infected by a bartender while being served at a raucous après ski venue in Austria, and party guests in Brazil greeting “each other with two kisses on the cheek [a local custom], hugs and handshakes.” The funerals in question are generally described as highly intimate and congested scenes of grieving among close friends and relatives. In the case of the SSE funeral in Albany, Georgia that devastated the local population, “people wiped tears away, and embraced, and blew their noses, and belted out hymns. They laughed, remembering. It was a big gathering, with upward of 200 mourners overflowing the memorial chapel, so people had to stand outside.”

With few exceptions, almost all of the SSEs took place indoors, where people tend to pack closer together in social situations, and where ventilation is poorer. (It is notable, for instance, that the notorious outbreak at an Austrian ski resort is connected to a bartender and not, say, a lift operator.) But generalizations in this area are complicated by the fact that some of the religious festivals described herein were mixed indoor/outdoor affairs. (Moreover, the February 19 SSE at San Siro stadium in Milan is also ambiguous, since that stadium has a roof over the seating area, but not over the field—and thousands of the fans spent hours bouncing around bars in and around Milan.)

The media accounts of these SSEs are full of descriptions in this vein. At a February 15 festival in Gangelt, a town in Germany’s tiny Heinsberg district, “beer and wine flowed aplenty as approximately 350 adults in fancy dress locked arms on long wooden benches and swayed to the rhythm of music provided by a live band. During an interval in the programme, guests got up to mingle with friends and relatives at other tables, greeting each other as Rhineland tradition commands, with a bützchen, or peck on the cheek.” Since that time, more than 40 Germans from the Heinsberg district have died. It’s been called “Germany’s Wuhan.”


When do COVID-19 SSEs happen? Based on the list I’ve assembled, the short answer is: Wherever and whenever people are up in each other’s faces, laughing, shouting, cheering, sobbing, singing, greeting, and praying. You don’t have to be a 19th-century German bacteriologist or MIT expert in mucosalivary ballistics to understand what this tells us about the most likely mode of transmission.

It’s worth scanning all the myriad forms of common human activity that aren’t represented among these listed SSEs: watching movies in a theater, being on a train or bus, attending theater, opera, or symphony (these latter activities may seem like rarified examples, but they are important once you take stock of all those wealthy infectees who got sick in March, and consider that New York City is a major COVID-19 hot spot). These are activities where people often find themselves surrounded by strangers in densely packed rooms—as with all those above-described SSEs—but, crucially, where attendees also are expected to sit still and talk in hushed tones.


  1. Bob Sykes says:

    We have about 4% of the World’s population. Yet we have one third of all the World’s COVID cases (927,00) and one fourth of its deaths (54,000). China, with 20% of the World’s population had only 84,000 + cases and fewer than 5,000 deaths. One might add that China was taken by surprise. We had weeks of warning.

    So, which country is totally FUBAR? Which government is a total failure? Which country takes care of its people? Which country has a crashed stock market? Which country has 30% unemployment?

    The US is plainly undergoing a sociological, economic, political, and military collapse. Russia and China need to do nothing. Just sit by, enjoy the spectacle, and pick up whatever pieces they want.

  2. Harry Jones says:

    Bob, 87% of statistics are made up.

    The remainder are misunderstood or misconstrued to lead to a moral panic.

    Stay away from the dihydrogen monoxide. That stuff’ll kill you.

  3. Dave NYC says:

    Bob, as much as it might pain you to admit it, the crisis is over. The virus has been floating around coastal cities in the U.S. since at least December. Most of the population outside rural areas has been exposed. We are close to herd immunity, and further lockdown is preventing it at this point. The elderly and immunocompromised should continue to stay at home, and let the rest of us get on with our lives.

    We are an open society, and despite our obvious flaws, the hysteria generated by the media being one of them, we are far superior to deeply regimented and conformist societies like China. If you feel that China is more your speed, you are welcome to apply for citizenship and build a new life there.

    The problem now becomes rescuing the economy and the mental stability of half the population that bought into the hysteria, and shutting down sociopathic governors like Cuomo and Newsom and their dictatorial mandates. The cure was, and is, most certainly worse than the disease.

  4. Christopher says:

    China numbers are not believable. They claim a death rate much lower than South Korea, Russia, all the European countries (200 times lower than Belgium), and 70+ other countries.

    China’s numbers were not credible when their epidemic was raging in January and February; they were not credible when they reported flat 0 new cases for weeks on end in March and April; and they are not credible now.

    And even assuming there were a scintilla of truth to a comparison between China numbers and other countries, they would have to account for the number of tests done in each country, and the criteria for classifying a death as coronavirus-related.

  5. Graham says:

    I’ve always taken the view that war, plague and rebellion are legitimate occasions for the exercise of rather extreme state power [not that they necessarily all should be exercised at the same time, or in every given crisis, or without scope for challenge, just that this is what government is for, if for anything] and that this is a valid approach within both constitutional government, free society, and ‘conservatism’. Those things don’t amount to a libertarian suicide pact.

    So I’ve been dismayed by just how much some kind of libertarian mentality has driven ‘conservative’ responses in the US and Canada in the past two months. We’re well short of totalitarianism here, or indeed any dictatorship.

    That doesn’t mean there aren’t criticisms. If cops are harassing you as a healthy person walking on the street 6 ft from anyone else as much as possible, jogging in the empty parks, or grocery stores insisting on you wearing low protection-value masks when they have none to sell you if you have none, or officious busybody citizens are harassing you at any time, there’s room for aggressive retort. It is also possible that any specific aspect of government measures is wrong. All fine.

    But too many people have been complaining that cops enforcing lockdown are preventing them from doing some trivial unnecessary thing that they would normally do. Self-government is just that- self government requires self-discipline as well as enjoyment of freedom. I feel like Canadians or Americans a few generations ago knew all that already.

    I’m not sure myself where this is going. I’m 49, overweight, diabetic [not too badly], have hypertension [not too badly], have had pneumonia nearly 20 years ago, was briefly and I think wrongly diagnosed asthmatic [I think it was more likely something environmental hit me overseas and took a couple years to work its way out of my lungs] 10 years ago. The DB and BP are well medicated. I have not quite seen details on whether or not it being controlled is beneficial or makes no diff.

    All that to say, I’ve got several identified or formerly-identified possible comorbidities. The ones I have, especially at the modest level I have them, are shared with truly huge numbers of Canadians and Americans, not all of whom yet know they have these conditions. Some, not all, public discussion seems not always aware of how many people in North America are not healthy by the standards apparently required to shake off COVID19 as mere trifle.

    I’ve been teleworking through this but also obliged to go to the office 3 days/week, in theory just afternoons but in practice longer. Buses here are on relaxed schedules, run fairly empty, and I have taken them, but now weather mostly permits me to take the 3 km walk each way every time. This, and doing similar walks on other days, is improving my health and habits and my chances of being within 6 feet of anyone at work or on the street for very long or often is low. So I haven’t been much bothered.

    OTOH, I am thinking seriously about how to operate once lockdown ends and people flood the streets again, the buses get crowded, work fills up again, and so on. Or as next winter comes and we still have no vaccine or sure treatment, and maybe this time COVID starts when it’s far too cold and or snowy/icy here to walk 6 k a day, so I’m on those buses.

    I’ve always been fairly rigid in my habits of public behaviour so I don’t expect of others what I can’t deliver myself. But next winter the first person who coughs in my direction or spits phlegm anywhere near me, practices I already loathe, is going to get an earful of my wishes for their grisly death.

    That said, I’m actually sympathetic to the serious arguments- the economic ones. Very sympathetic. This is not something the modern, information driven, hyperlinked, financial-services dominated economy has really had to deal with, and we’re seeing how robust it is, or isn’t. The small business side, I can’t say. Such businesses have always existed and dominated the economy, and probably went under in some past pandemics too, without any bailouts to hand. In the past they probably benefited from more community support and forbearance, and were more necessary as the only alternative for their customers. And their customers didn’t know as much about how infection works, and had higher risk tolerance. So a different picture.

    OTOH, we do have some better idea of infectious disease, and greater expectation of avoiding it and not dying from it by going about our business. Perhaps a mixed blessing, but I’m in favour of medical improvement and safer living in this area. So we don’t go shopping as much.

    I don’t know what the answer is, but for all those reasons and probably others, our economy is wildly more fragile than it once was. I get it for individuals or families without income, or small businesses seeing the last of custom drain off to Amazon. But when I first heard arguments that ‘the economy’ writ large could not survive, or even rebound, from a couple of months lockdown, my only possible reaction was to think that a) it should be able to do that and b) if it can’t, it’s built wrong.

    This won’t be the last pandemic. And there will be other such challenges. Whether you think the solution is carbon reduction [the left version] or the implementation of new and better technologies that preserve Western economic power [more of the right version] or even if you are of the view that climate change is entirely natural and not of our doing, we’re going to be building new equipment, new industries, moving industries around, altering or bolstering the built environment, and controlling water movements on a large scale in the next century. There’d better be some fresh attention to robustness in the marketplace. And less ‘stock market freaks out on Day 1 of Crisis!’.

    I appreciate your various forbearances on this rant. I’ve been away a couple weeks.

  6. Graham says:

    On the plus side, the comment about movies and concert halls being worse vectors because they oblige people to sit still and speak in hushed tones really struck me. It seems to imply those are bad things.

    I’m hoping fear of infection drives folks to do what I want of them in these venues, sit still, unmoving, and in perfect silence for two hours so I can enjoy the entertainment.

    Must be a personal or hereditary choice- I inherited from my father either by genes or by teaching the idea that there should not be talking in the theatre. Now that he’s old he developed the impulse to talk a bit more and I always just give him the look. He grins.

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