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.