How much physical distancing is safe?

As the pandemic drags on and countries try to grapple with how to achieve some semblance of normalcy in the fact of restrictions, they seem to have arrived at three general recommendations to help slow the spread of the virus. Wash hands with soap and water or use sanitizers, wear masks, and keep one’s distance from other people. While those seem straightforward enough, there are a lot of uncertainties within them. For example, when it comes to masks, while there has emerged a broad scientific consensus that wearing them is a good thing, what types of masks are better and are they meant to prevent the wearer from spreading infections to others or from getting infected by others or both?

The question of what distance is appropriate is even more complicated and this article goes into some of the complicating factors that have emerged from recent studies.

Since early January, the World Health Organization has been calling for separation of at least one meter (a little more than 3 feet). The Centers for Disease Control and Prevention, meanwhile, has been recommending nearly twice that distance — 6 feet or more.

And it turns out that questions about distancing don’t end at 6 feet. In the past few months, environmental scientists, physicists, engineers, epidemiologists, and others have become increasingly vocal with concerns that the virus might be transmitted through smaller droplets that can reach as far as 26 feet on violent exhalations like coughs and sneezes. The droplets can slow down, dry out, and hang in the air for hours. (No matter what distance they support, pretty much everyone weighing in on distance recommends that people wear face masks when out in public and close to others who might be potentially infected with SARS-CoV-2.)

In the 1930s, American scientist William F. Wells wrote that pathogens could ride out on people’s breath in two forms. The larger form, today often called “droplets,” are subject to gravity. They fall to the ground within a few feet. The tinier aerosols can float in the air, riding air currents. (There’s some debate within the scientific community over the vocabulary of particles in the air, but we’ll use droplets and aerosols here.)

So if large droplets are a distinct category from small aerosols, and if a bacterium or a virus spreads mostly on the large droplets, and if the large droplets fall from the air within just a few feet, then it makes sense to keep just a few feet apart from other people, with about 3 feet versus 6 feet being a key question.

Distance should be looked at in combination with other factors, not in isolation, said Bourouiba. Air changes, occupancy, length of exposure, indoor vs. outdoor, whether it’s a health care setting with infected patients or not, all are important. “An indoor space usage for an extended amount of time with poor ventilation, even sitting more than 6 feet apart would not be sufficient,” she said. “If you are in a big park with air flow and winds and everybody wearing a mask, then 6 feet could be OK.”

It is hard for people to know what exactly they should do. But certain principles hold good, even if the precise quantified requirements are in flux. Minimizing indoor contact, wearing masks, keeping as far away from others as is feasible, and washing hands and sanitizing seem like reasonable practices to follow.


  1. aquietvoice says

    Eh, to be honest I think it’s much more about compliance than anything else -- my state (~8 million people) has gotten to the point where individual new cases are a news item, just because we’ve done the ‘reasonable measures’ you’re talking about, but we’ve got just about everyone doing them every day.

    It’s gotten to the point where life is back to normal for many people. It’s good.

    In short: high compliance beats high quality by a wide margin.

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