Is it safe to go outside?

Even if coronavirus particles can move through the air, they would still diffuse over distance. “People envision these clouds of viruses roaming through the streets coming after them, but the risk of [infection] is higher if you’re closer to the source,” says Linsey Marr, who studies airborne disease transmission at Virginia Tech. “The outside is great as long as you’re not in a crowded park.”

In February, scientists in Wuhan, China—where the coronavirus outbreak originated—sampled the air in various public areas, and showed that the virus was either undetectable or found in extremely low concentrations. The only exceptions were two crowded sites, one in front of a department store and another next to a hospital. Even then, each cubic meter of air contained fewer than a dozen virus particles. (No one knows the infectious dose of SARS-CoV-2—that is, the number of particles needed to start an infection—but for the original SARS virus of 2003, one study provided estimates many times higher than the levels detected in the Wuhan spaces.)*

These particles might not even have been infectious. “I think we’ll find that like many other viruses, [SARS-CoV-2] isn’t especially stable under outdoor conditions like sunlight or warm temperatures,” Santarpia said. “Don’t congregate in groups outside, but going for a walk, or sitting on your porch on a sunny day, are still great ideas.”

You could tie yourself in knots gaming out the various scenarios that might pose a risk outdoors, but Marr recommends a simple technique. “When I go out now, I imagine that everyone is smoking, and I pick my path to get the least exposure to that smoke,” she told me. If that’s the case, I asked her, is it irrational to hold your breath when another person walks past you and you don’t have enough space to move away? “It’s not irrational; I do that myself,” she said. “I don’t know if it makes a difference, but in theory it could. It’s like when you walk through a cigarette plume.”

Indoors, experts’ opinions start to diverge. Consider, for example, the grocery store—one of the last vestiges of public life. There, Santarpia is far more concerned about touching shared surfaces than breathing shared air, and he makes sure to sanitize his hands before he leaves. Marr said that she tries to go when it’s less crowded, although that’s obviously harder in a big city. Bourouiba’s best advice is to always keep as much distance from other people as possible, and she adds that the onus is on stores to improve their ventilation or limit the number of concurrent customers. Stores must also devise ways of protecting the people at greatest risk: the cashiers and the workers stocking shelves.

Then there are shared spaces like hallways, stairwells, and elevators in apartment buildings. Elevators pose the highest risk, Bourouiba told me, since they’re enclosed boxes with limited airflow. For stairwells and hallways, she advocated a commonsense approach: “If you hear neighbors going out, and there are 10 people in the corridor right now, maybe wait and go later.”