Under social distancing, we’re all doing our best to stay sane, and one of the best ways to maintain sanity is to go out for some nice fresh air. But venturing outside can be stressful if you’re worried that the very air is full of virus particles just waiting to infect you.
So, how worried should you be that any time you go outside, you’ll contract coronavirus from a fellow pedestrian, runner, or cyclist who happens to exhale as they pass by?
The answer is, you probably don’t need to freak out about it. As long as you’re maintaining at least 6 feet of distance from other people and you’re not in a high-risk group, you’re engaged in a very low-risk activity, particularly if you and others are wearing masks.
In April, Belgian and Dutch engineers publicized some findings that went viral online and gave people the opposite impression. The engineers used a spray nozzle particle generator to simulate the spread of droplets we generate as we walk, run, or bike. Since the particles hit a trailing athlete who was farther than 6 feet away, the engineers concluded we need to maintain more distance than that to avoid the risk of contracting Covid-19.
They recommended staying 16 feet behind someone who’s walking, 33 feet behind someone who’s running or biking slowly, and 65 feet behind someone who’s biking hard. Those sorts of distances are almost impossible to maintain in big cities. So, as the findings made the rounds online, lots of people panicked.
But this research — despite being branded as a “study” in a much-shared Medium post aiming to summarize it — contained no input from epidemiologists or virologists and was not peer-reviewed. Its logic is deeply flawed.
“I think we should be very careful with making assumptions about transmission based on that ‘study,’ since it didn’t account for any variables related to transmissibility,” said Angela Rasmussen, a virologist at Columbia University, adding, “It’s important to understand that infections are started with a minimum infectious dose of virus.”
In other words, the “study” failed to consider two key questions: How easy is it for particles traveling in the air outdoors to infect you? And how many particles containing infectious virus would you have to inhale to become infected?
The engineers simply concluded that any exposure was too much. But that’s not the case; the truth is more reassuring. Let’s break down each of the two questions in turn.
How easy is it for particles traveling in the air outdoors to infect you?
To start with, we should get clear on something that has proven confusing to lots of people: Is the coronavirus “airborne”?
The confusion stems from the fact that there’s an everyday meaning of the word “airborne” — carried in the air — and a technical scientific meaning, which is based on the size of the virus particles and how long the virus can linger in the air before losing its infectivity.
Public health experts like to refer to bigger particles that are heavy and thus fall fast as “droplets,” and tiny particles that evaporate faster than they can fall as “aerosols.” When a virus is transmissible as aerosols, they say it’s “airborne.”
We know for sure that the coronavirus moves as droplets. But what about aerosols?
“There’s no debate at all as to whether or not aerosols are generated (they are [at least in hospitals where medically invasive procedures are performed]), whether or not they contain virus (they do [at least at first]), and whether or not a particularly violent sneeze can propel them across a room (it can),” said Jennifer Kasten, a pathologist with training in infectious disease epidemiology and global health.
Research by MIT’s Lydia Bourouiba has shown that coughs and sneezes can release turbulent clouds of particles in droplet and aerosol form alike. The particles from a cough can travel as far as 16 feet and particles from a sneeze can travel as far as 26 feet.
That, and a handful of other studies, have left some people wondering if maybe we do need to stay much farther apart from one another than the recommended 6 feet.
But if we focus solely on the aerodynamics of particle dispersion, we obscure more crucial unanswered questions: How hardy is the virus when it’s in aerosol form? How long can it linger in the air before its protective coat of moisture dries up and it falls apart, ceasing to be infectious?
We’ve seen Covid-19 case clusters, like the one originating in a choir practice in Washington state, that seem to indicate that asymptomatic people can generate aerosols that make other people sick — at least when they’re closely packed together in an enclosed space, for a few hours, singing, which generates higher pressures than does breathing or speech.
Fortunately, increasing your distance, decreasing the duration of your exposure, and improving the ventilation of the air around you can all lower your risk. And being outdoors generally helps you do all three.
“The risks of virus transmissibility in the air outdoors is likely quite low in those contexts, although this risk hasn’t been definitively measured,” Rasmussen said. “Outside, things like sunlight, wind, rain, ambient temperature, and humidity can affect virus infectivity and transmissibility, so while we can’t say there’s zero risk, it’s likely low unless you are engaging in activities as part of a large crowd (such as a protest). Solitary outdoor exercise is likely low-risk.”
Rasmussen and Kasten both noted that a perfect sequence of events has to happen for a virus to jump from an infected passerby outdoors to you. The particles — enough of them to be able to kickstart an infection — have to spray out of the passerby with enough force to make their way over to you. The virus inside the particles has to survive while sunlight, humidity, wind, and other forces work to decay and disperse them. The particles have to land right in your upper throat or respiratory tract — or on your hands, which you then use to touch your eyes, nose, or mouth — and they have to get past all the barriers to infection in the respiratory system, like nose hairs and mucus. Then they have to dock up with your cells’ ACE-2 receptors and use them to enter the cells.
This is a pretty arduous sequence to execute properly, and it’s even more difficult for the virus if everyone involved — say, both you and the runner in front of you — is wearing a mask. You can see why, if you’re standing outdoors several feet away from an infected person, the virus might have a hard time making its way over to you at a high enough dose to actually infect you.
How many particles do you have to inhale to launch an infection?
One thing we’d all love to know about Covid-19 is what constitutes the infectious dose — that is, how many live virus particles you need to inhale at once before they kickstart an infection. Unfortunately, scientists just don’t have the answer yet.
“As animal model data continues to come out, we’ll have a much better idea about estimated ranges of infectious dose,” Rasmussen said.
In the meantime, some experts are estimating the infectious dose for Covid-19 from previous coronaviruses that have infected humans, like Middle East Respiratory Syndrome (MERS).
To develop a MERS infection, the number of virus particles you need to inhale is somewhere in the thousands, perhaps as high as 10,000. Willem van Schaik, a professor of microbiology at the University of Birmingham, estimates that to develop Covid-19, that number is lower, perhaps in the high hundreds or low thousands.
That’s just an educated guess, but it’s reasonable to assume it takes fewer particles to launch an infection in the case of Covid-19 than MERS, because Covid-19 is much more transmissible. Each person with Covid-19 infects two or three others on average, while for MERS that number is less than one.
Although we don’t know exactly how many particles it takes — 900? 1,500? — the point to bear in mind here is that you’re not going to contract Covid-19 if a single particle falls on you. One of the problems with the Belgian-Dutch exercise “study” is that its recommendations seem to be based on the idea that any exposure is too much.
Obviously, you don’t want to walk through someone’s fresh sneeze or cough cloud, and if this accidentally happens you should go home, change, and shower. But if you’re worrying about how many particles a passerby is generating with their exhalations, it may help to know that a study managed to quantify how many virus particles were detectable after patients who had Covid-19 and who were not wearing masks coughed five times into a petri dish at a distance of roughly 8 inches.
The scientists detected 363 virus particles on average per 1 ml of petri dish. That seems like a lot, but again, that was at a distance of 8 inches. If you’re 6 feet away (72 inches), much of that will disperse before you can inhale it — particularly if you’re outdoors, where factors like increased distance, decreased duration of exposure, and improved air ventilation are all working in your favor.
It may also help to know about some studies suggesting that most Covid-19 transmission happens indoors, not outdoors. In China, a study of 318 outbreaks found that transmission occurred outdoors in only one of them. In Japan, a study found that “the odds that a primary case transmitted Covid-19 in a closed environment was 18.7 times greater compared to an open-air environment.” Note, however, that both of these are preprint papers (not yet peer reviewed).
An understanding of infectious dose, combined with an understanding of transmissibility as laid out above, should also help calm excessive levels of worry about picking up the virus from objects like mail or groceries. Remember that to kickstart an illness, you have to have enough virus particles to get an infection going, and it’s got to be live, infectious virus, not just dead RNA (genetic material that won’t harm you). The latter will simply fall apart after enough time on inert objects — up to 24 hours on cardboard and up to three days on plastic and stainless steel.
“Viral RNA does not imply the presence of infectious virus,” Kasten explained. “The virus, without host cells and a bit of moisture to keep it temporarily going, can fall apart, leaving bits of its RNA lying around like bleached bones in the sun. A researcher can come along with PCR [polymerase chain reaction, a common method in molecular biology] and detect the RNA, but that doesn’t necessarily mean they detected infectious virus.”
So, when you see a headline that seems frightening — for example, one saying that researchers found coronavirus on a cruise ship 17 days after passengers disembarked — just remember that doesn’t necessarily mean it was live, infectious virus.
“The good news is that we do know that while the virus can persist in the environment on different surfaces and in different environments, it does lose infectivity over time,” Rasmussen said. “So if you inhale a large number of total [virus particles] but only a small number of them are infectious, you are at much lower risk of actually getting infected.”
None of this is to say you should be cavalier when you venture into the outside world. Washing your hands, avoiding touching your face, being diligent about physical distancing, wearing masks in public, and disinfecting communal surfaces — all these things likely reduce transmission risk, and we should keep doing them, Rasmussen said. But she also said she feels fine about taking a walk with her husband outdoors.
Psychologically, different people have different levels of tolerance for risk. For some people, any risk that can be minimized, should be, no matter how small. For others, the recommended 6-foot distance, with masks, and the known decay of both the amount and the infectiousness of the virus — that’s good enough.
“The people who are 26-footers should know, though, that the 6-footers are not being foolhardy or endangering others unnecessarily,” Kasten said.
Asked if she could offer a way to assess the risk of various activities in terms we can easily wrap our minds around, Kasten said to consider the difference in the risk between taking a stroll through the park on an even path versus climbing up a steep cliff face.
“Sure, you could slip, fall, strike your head, and die on that path in the park. Likewise, you could free-solo successfully to the top of El Capitan. But most of us would accept the risk of the stroll and not accept [the risk of] dangling from the cliff,” she said. “Breathing in someone’s sneeze cloud, close by, without a mask — that’s the cliff face. Jogging several feet away, or getting the mail — that’s the park.”
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