Last week’s events at the nation’s Capitol once again raised questions about the risk of Covid-19 transmission in large gatherings not just inside but outside—in this case, where people were packed tightly on and near the steps of the building for several hours. We may never know the outcomes in great detail, but the science is clear: The risk outdoors is lower than indoors, all other things being equal, but it is not zero. Especially when most people are maskless and in close proximity for long durations.
There’s no longer any question that the coronavirus infects people through the air, both by larger respiratory droplets that tend to fall to the ground within a few feet and smaller, lighter aerosols that can stay aloft for minutes or even hours. Research has shown convincingly that the six-foot rule is not adequate to eliminate risk.
Inside, without good ventilation of fresh air or upgraded filtration, the virus-laden aerosols can accumulate dangerously, much as cigarette smoke would. That’s why indoor dining is far riskier than outdoor seating, and why there’s significant concern of a superspreader event potentially resulting from those who infiltrated the Capitol or among lawmakers forced to cloister together for hours—or both.
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Outside, virus particles expelled by an infected person disperse much more quickly. Kimberly Prather, PhD, an atmospheric chemist at the Scripps Institution of Oceanography, offered an analogy when I wrote about the outdoor risk last summer: Imagine a tablespoon of salt water dispersing in a bucket of fresh water versus in a small glass of water.
Dispersion is not instant in any situation. Being near an infected person outside, or downwind from one, can pose a significant risk of infection. Several other factors come into play.
Coronavirus infection depends not just on exposure, but on the number of virus particles a person inhales. That’s why masks can protect both the wearer and others. But masks are only one layer of protection, or conversely, not wearing a mask is just one layer of added risk. (The virus can also spread via contaminated surfaces, so hand-washing remains important).
“Transmission is facilitated by close proximity, prolonged contact, and frequency of contacts,” says Muge Cevik, MD, an infectious diseases researcher at the University of St. Andrews School of Medicine in Scotland and associate editor of Clinical Microbiology and Infection. “So, the longer the time you spend with an infected person and the larger the gathering, the higher the risk is.”
Standing right next to an infected person who shouts in your direction is riskier than standing far away from someone who is just breathing normally. Being near an infected person, or alternately one and then another, for hours of accumulated time is riskier than passing briefly by.
Activity matters, too. If people are breathing heavily due to physical exertion, they can expel (and breathe in) some 20 times more droplets than with normal breathing, says Richard Corsi, PhD, dean of the College of Engineering and Computer Science at Portland State University.
Likewise, talking loudly or shouting spews out far more droplets, large and small, than normal conversation.
It’s impossible to put odds on the myriad scenarios of transmission risk, but overall, researchers are getting a handle on the difference inside versus outside. One review of studies on the topic concludes that 10% or fewer of Covid-19 transmissions occur outdoors.
“Contact tracing studies suggest that transmission risk is 20 times higher in indoor settings compared with outdoor environments,” Cevik explains in a Twitter thread. “The risk is not zero [outdoors] but significantly lower.”
Two big caveats: With the daily number of new cases soaring, the chances of being near someone who is infected—whenever you might be near strangers in any setting—go up. And now an emerging mutated variant of the coronavirus, which has been found in several U.S. states and is expected to become the dominant strain within a couple of months, is thought to be 50% more contagious. That will change all the risk calculus, inside or out.