“We should all stand six feet apart to prevent the spread of COVID-19.” If you haven’t had that hammered into your head since March, let me be the hammer that takes the last lick at the nail.

Where did that number come from?

Six feet seems arbitrary. Why not five feet? Why not ten feet? In the August 25 issue of the British Medical Journal, “BMJ” for short, a few authors take us on a journey six feet long (or two meters, if you’re in one of the all-but-three countries that has switched from imperial to metric measurement; or two metres if you’re a true pedant).

The two-meter rule has a long history. Scientists in the 19th century, shortly after the very invention of germ theory, placed culture plates around patients who were then asked to cough or sneeze. Then the scientists could culture out whatever bugs landed on the plates. Most of the plates more than 1-2 meters from the test subject failed to grow anything scary. So, two meters became the default distance.

As cameras got better in the 1940s, visual representation of sneezes became possible:

One experiment showed that only 10% of droplets traveled as far as 5 ½ feet. So even though 10% of sneezers were able to eject harmful bacteria up to 9 ½ feet away, the two-meter rule was cemented. Unfortunately for hard-core two-meter adherents, recent studies have failed to fully support these original conclusions. Eight out of the ten studies included in a recent systematic review, for example, showed large numbers of respiratory droplets landing beyond 2 meters for particles up to 60 micrometers, a particle size large enough to contain thousands of copies of SARS CoV-2, although not small enough to deposit those viruses into your lungs.

What do we do with this information?

Not to pat my family on the back, but you do the same thing we did this weekend. You pretend that everyone in the world is a smoker, and you protect yourself from the imaginary PM2.5-rich smoke by wearing a mask and keeping your distance. On a shopping trip this past weekend, my wife and I ran into an acquaintance and talked with her, indoors, for several minutes. In our minds, she was actively smoking: we did not shake hands, hug, or get closer than a few feet, and everyone wore a mask.

Two days later, that person tested positive for COVID-19.

I’m going to be careful and self-isolate myself for a couple of weeks just in case; I’m fortunate that my work allows it fairly easily. But for the most part I feel safe because we were all masked, including the infected person, and we stayed relatively physically distant. Because as the authors of this review noted, the risk of transmission, even with prolonged exposure, is relatively low if everyone is masked and the environment is reasonably well-ventilated:

As we mentioned in a previous blog post, for example, two Missouri hairstylists who unknowingly exposed hundreds of people to the virus appear to have infected zero clients because of their fastidious facemask use.

So six feet isn’t magical. But it’s still a reasonable number, especially combined with good ventilation and good adherence to masking. The next time you see me, if I keep my distance and don’t shake your hand, it isn’t personal. It’s epidemiology.

As the Medical Director of the Kansas Business Group on Health I’m sometimes asked to weigh in on topics that might affect employers or employees. This was a reprint of a blog post from KBGH.