6 feet away isn’t enough. Covid-19 risk involves other dimensions, too.

Visitors on the boardwalk in Long Beach, New York, on May 21. City officials had closed the boardwalk on March 26 over worries of overcrowding on the city’s 2.2-mile beachfront walkway. | Al Bello/Getty Images

Distance, time, activity, environment: 4 ways to think about Covid-19 risk as states reopen.

When states had strict stay-at-home orders and lockdowns in place, many decisions about the risk of getting the coronavirus were simple. People didn’t have to think about whether dining in a restaurant is safe if the restaurant was closed.

Now, as states open up — perhaps prematurely, and with varying degrees of precautions and adherence in place — individuals will need to weigh some risks on their own.

It isn’t easy; information about what’s safe, and what’s not, can be contradictory and confusing. A state may allow restaurants to reopen and concerts to resume: but should you really go? Is it safer if people are only allowed to dine outside?

John Moore/Getty Images A bartender serves drinks at L’escale restaurant in Greenwich, Connecticut, on May 20.

The hunger for guidance is clear: On May 6, infectious disease expert Erin Bromage posted a blog post summarizing the evidence of coronavirus transmission risks, and 17 million people have since read it, he says.

As Bromage conveys, the scientific understanding of how the virus transmits in public is improving. Contact tracing studies around the world have taken a magnifying glass to the “superspreading” events, where one person ends up infecting dozens of others. These studies shine a light on the key risk factors that create dangerous situations.

From these studies, one thing is clear: The main way people are getting sick from SARS-CoV-2 is from respiratory droplets spreading between people in close quarters. The risk of catching the coronavirus, simply put, “is breathing in everybody’s breath,” says Charles Haas, an environmental engineer at Drexel University. Droplets fly from people’s mouths and noses when they breathe, talk, or sneeze. Other people can breathe them in. That’s the main risk, and that’s why face masks are an essential precaution (they help stop the droplets from spewing far from a person’s mouth or nose).

The Centers for Disease Control and Prevention (CDC) recently updated its guidelines, emphasizing the risk of close contact over other modes of transmission. “The virus does not spread easily in other ways,” the CDC writes. It’s still possible that a person can catch it from touching a contaminated surface (more on that below.). But it’s “not thought to be the main way the virus spreads,” the CDC states.

As Bromage put it in his piece, “We know most people get infected in their own home,” from housemates or family members who caught the virus in the community.

So how can we assess the risk of going places outside the home?

The story is a little more complicated than the simple “stay 6 feet away” guidelines. Coronavirus risk is simply not one-dimensional. We need to think about risk in four dimensions: distance to other people, environment, activity, and time spent together.

Let’s walk through them.

A simple suggestion: Imagine people are smoking, or farting really bad, and try to avoid breathing it in

It’s easy to get into the weeds talking about the risk of catching and spreading the coronavirus as people reenter communal spaces in society. We can talk about the number of viral-laden droplets expelled by a single breath ( a lot, perhaps 100 or more), by a person talking ( 10 times more than breathing), about how far a sneeze can propel those droplets ( much farther than 6 feet), how long those viral droplets linger in the air (around eight to 14 minutes, at least in a controlled indoor lab setting).

But really, what all this means is that the greatest Covid-19 risk is being around breathing, laughing, coughing, sneezing, talking, people.

Amanda Northrup/Vox

It’s still hard to visualize the risk, though, as the respiratory droplets are invisible to our eyes.

Perhaps helpful: Imagine everyone is smoking, as Ed Yong reported in the Atlantic, and you’d like to avoid inhaling as much smoke as possible. In a cramped indoor space, that smoke is going to get dense and heavy fast. If the windows are open, some of that smoke will blow away. If fewer people are in the space, less smoke will accumulate, and it might not waft over to you if you’re standing far enough away. But spend a lot of time in an enclosed space with those people, and the smoke grows denser.

An alternative image to thinking about this risk: “With my kids, I just sort of joke around that if you can smell their farts, you need to move farther apart,” Bromage says. So if not smoking, imagine everyone is farting. Keep this in mind and surely you’ll realize outdoor activities are better than indoor ones. “This tells you the gradient of risk,” Bromage says. “The closer you are, the more it’s gonna smell, the more dangerous it is.“

At a barbecue, you can still imagine being close enough to people to smell their farts. So even in outdoor spaces, we need to limit our contacts.

A crowded indoor place, then, with poor ventilation, filled with people talking, shouting, or singing for hours on end will be the riskiest scenario. A sparsely populated indoor space with open windows is less risky (but not completely safe). Running quickly past another jogger outside is on the other end of the spectrum: minimal risk.

There are many scenarios in between. “In general, outdoors is lower risk,” Muge Cevik, a physician and virology expert at the University of St. Andrews, says. But “if you have a gathering or a barbecue outside, and you spent all day together with your friends, your risk is still higher.”

What recent contact tracing studies can teach us about risk

Scientists pointed out a few recent contact tracing studies that nicely illustrate the dimensions of Covid-19 risk.

In China, 8,437 shoppers and employees of a supermarket were tracked in late January after one of the employees was confirmed positive with Covid-19 while working in the store.

The risk for infection was much higher for the workers than for the shoppers. Around 9 percent of the supermarket employees (11 out of 120 employees) got sick as a result. But just 0.02 percent of the shoppers (2 out of 8,224 shoppers) got sick.

What does this show?

The employees are at a much larger risk due to the time they spent working in the store. Both the employees and shoppers were in the same physical space, but their risk was not the same. (The study did not note whether the shoppers and customers were wearing masks in the store.) The employees may have interacted more with their colleagues, but they also had a greater chance of breathing in the virus.

Caroline Brehman/CQ-Roll Call, Inc via Getty Images An employee fulfills grocery orders at the Takoma Park Silver Spring Food Co-op in Maryland on April 22. Grocery store employees are at much higher risk for contracting the coronavirus than shoppers.

What we should learn from this: If we have to spend time with people indoors, try to make it quick!

Another recent study out of China investigated an outbreak that started at a Buddhist temple event.

Two buses brought people to the function. On one of the buses, there was a person who later tested positive for the coronavirus who had not yet started to feel symptoms. The other bus was free of infected people.

Both buses brought people to the same temple, where they mixed and mingled. But who was most at risk of getting sick? Those who rode the bus with the infected person. Twenty-four out of 67 people on that bus got sick. No one on the other bus did. The event was attended by another 172 people who arrived by other transportation. Only seven of these people got sick.

The lesson? The confines of a bus are a much, much riskier environment for viral spread than a much larger space, like at the temple. The risk in the temple was not zero. But it was much reduced compared to the confines of the bus. So if you have to choose between a big open indoor space and a smaller one, choose the larger one, where people can spread out.

“When you look at public transport, work spaces, restaurants — places where we’re just trying to fit many people in a small confined space — respiratory viruses like those spaces,” Cevik says. It’s “just common sense.”

There’s no set time that’s safe to be in these places. “Generally, for droplet transmission, we say 15 minutes,” Cevik says. “So if you spend 15 minutes face to face with somebody, you’re close contact [and at high risk], but that doesn’t mean if you spend 14 minutes your risk is zero.”

And it’s not just the location or the time spent together: The activity people are engaged in matters, too.

In Washington state, a person with the virus attended a choir practice, and more than half of the other singers subsequently got sick. This was labeled a “superspreading” event, as one infection led to 32 others. Why was this so risky?

“The superspreading event is about the behavior of the person involved,” Cevik says. There are many reasons why a person could become a “superspreader”: Some people shed more of the virus than others, and it appears people shed most of it when they are just starting to feel symptoms. But what made this event so risky was the convergence of many risk factors: the singing activity (during which the infected person released viral particles into the air), the time spent together (the practice was 2.5 hours), and the interaction between the choir members in an enclosed space (not only did they all practice together, they also split up into smaller groups and shared cookies and tea).

What about touching something with infected droplets? Is that still a risk?

In its updated guidelines, the CDC says the coronavirus does not spread easily from people touching surfaces. That is, if someone with Covid-19 touches a hand railing, does that make that hand railing dangerous for other people to touch? The CDC is now saying that such events are not a huge risk for Covid-19 transmission.

But, there’s a catch: It is still the case that surface transmission is possible. Scientists believe the virus can remain viable on a hard, non-porous surface like plastic or steel for around three days, and a rough surface like cardboard for about a day.

Bromage cautions that it’s just really hard to study surface transmission. In contact tracing studies, it’s much easier to ask people who they’ve been in contact with than to have them remember every surface they’ve touched.

“I agree with this [CDC] statement,” Cevik says, agreeing that surfaces aren’t the most significant mode of transmission. “But this does not mean it does not happen.” Cevik points me to a contact tracing study that suggests (with a good deal of uncertainty) that some people caught the infection in a mall via the restroom. “Bottom line,” she says, “it’s still important to maintain personal hygiene and wash hands.”

Also consider how scientists recently found live Covid-19 virus in feces. So good bathroom hygiene is still as important as ever.

There are no magic numbers to eliminate risk

It would be great if there were very specific numbers and guidelines we could follow to minimize coronavirus risk to zero.

But there aren’t. While 6 feet away from another person, it’s not like the virus will immediately decide to drop dead. That’s why we need to think of risk in terms of many dimensions: so we can each think critically and not fall back on rules that are too simplified.

“When I first said restaurants were risks, people interpreted that as ‘every restaurant is a risk,’” Bromage says. “Each restaurant has its own unique environment, its own unique challenges that need to be worked out. If you’ve got a large open-seating area, and you can open up the windows and doors ... the risk there is much lower than a boutique restaurant with five tables that creates that really intimate atmosphere.”

When we venture out into the world, we need to remember we can reduce risk, but never eliminate it.

“Wearing a mask is not going to completely reduce your risk, hand-washing is not going to completely reduce your risk, and staying a distance away from people in an enclosed space is not going to completely reduce your risk,” Haas, the Drexel professor says. “But the concurrent use of all those strategies will hopefully reduce your risk down to a lower level. We can never get to zero. There’s no such thing as zero risk.”

And we still need more data.

Contact tracing studies have taught us a lot so far. But as of now, most of this work has been done in Asian countries, which may have different expectations around mask-wearing, among other differences.

“Contact tracing, testing, isolating — these are the building blocks to understand where the transmission is occurring,” Cevik says. And the more we learn, the more powerful we become to stop the spread of this pandemic.

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