Why 6 feet?

Coronavirus Apr 08, 2020

I don't think I have to say that it's been recommended we keep 6 feet apart. But, here's the CDC's (Archive) guidelines:  "Stay at least 6 feet (2 meters) from other people".

I've talked a bit about the science or lack thereof behind the response to Wuhan, China Coronavirus. However, this made me wonder, why 6 feet? Why not 5 feet? Why not 7? Surely 6 wasn't just a made up number. There must be some level of evidence for such a strong and wholly implemented guideline.

6 feet in the media

But, why 6 feet? From LiveScience (Archive):

The 6 feet of distance is designed to put up a roadblock to  the aerosolized and droplet methods of transmission. But that standard is best understood as a reference point — not a hard line beyond which you are absolutely protected, said Krys Johnson, an epidemiologist at Temple University.
When moving around outdoors, she said, 6 feet is a good minimum distance at which to pass other people, if you can't give them a wider berth. In indoor settings (think the grocery store), she said, it's more of an "absolute minimum."

If not 6 then how many?

Okay, so, 6 feet is a minimum? Then what's enough? From USA Today (Archive):

Lydia Bourouiba, an associate professor at MIT, has researched the dynamics of exhalations (coughs and sneezes, for instance) for years at The Fluid Dynamics of Disease Transmission Laboratory and found exhalations cause gaseous clouds that can travel up to 27 feet.
Dr. Paul Pottinger, an infectious disease professor at the University of Washington School of Medicine, said questions remain about the distances at which the virus is effective.
“For me, the question is not how far the germs can travel, but how far can they travel before they’re no longer a threat. The smaller the germ particles, the lower the risk that they might infect somebody who would breathe them in or get them stuck in their nose or their mouth,” Pottinger told USA TODAY.

What's the basis though?

That makes sense. So, where's the evidence? Back to the LiveScience article:

"Six feet is probably not safe enough," Raina MacIntyre, a professor of global security and the head of the Biosecurity Program at the Kirby Institute in Australia, told Live Science in an email. "The 3-6 foot rule is based on a few studies from the 1930s and 1940s, which have since been shown to be wrong — droplets can travel further than 6 feet. Yet hospital infection control experts continue to believe this rule. It's like the flat-Earth theory — anyone who tries to discuss the actual evidence is shouted down by a chorus of believers."

So, the data is wrong and from the 30s and 40s? I'm not happy with that answer... to Google Scholar!

Off to the journals

The screenshot of Google scholar results is at the bottom of the page. But, the first article has no relevance to 6 feet.

From the second article (Extensive Viable Middle East Respiratory Syndrome (MERS) Coronavirus Contamination in Air and Surrounding Environment in MERS Isolation Wards):

US Centers for Disease Control and Prevention guidelines define close contact as being within 6 feet of an infected patient or within the room or care area of such a patient for a long time[.]

6 feet is a CDC guideline? Next from the same article:

The Korea Centers for Disease Control and Prevention (KCDC) initially quarantined and followed up, by personal interview or closed-circuit television review, only those who had been in close contact (6 feet) with the index patient or who had shared the same room. However, many patients and guardians became infected and were later recognized to have been >6 feet away from the index patient, though in the same ward[.]

6 feet was ineffectual in the referenced study. Moving on.

From the third article (The 2019 Novel Coronavirus Outbreak – A Global Threat):

Spread from person-to-person happens among close contacts (about 6 feet) mainly via respiratory droplets through contact with mucous membrane of mouth, nose and possibly eyes.

They cite the CDC on that claim. Nothing new about 6 feet. Although, notably, they do question asymptomatic transmission:

The human to human transmission is mostly from symptomatic infected patients. But a report of transmission of 2019-nCoV infection from an asymptomatic contact in Germany had created widespread panic questioning our understanding of current transmission dynamics of this virus. Infection appeared to have been transmitted during the incubation period of the index patient. But this has been disputed in other reports suggesting that patient had nonspecific complaint like myalgia and fatigue during transmission and was not totally asymptomatic

But, I'm looking for 6 feet... Onto article number 4!

From the fourth article (Coronavirus (COVID-19) outbreak: what the department of endoscopy should know):

Endoscopy procedures demand short physical distance from patients to personnel, and according to studies performed during the global SARS outbreak of 2003, droplets from infected patients could reach persons located 6 feet (approximately 1.83 meters) or more from the source.

The cited article (Cluster of SARS among Medical Students Exposed to Single Patient, Hong Kong) doesn't mention 6 feet. The only measurement I can find is 1 meter. However, from the images one could infer that people who hadn't been within 6 feet of the patient got sick. From the citation:

Proximity to the index case was associated with transmission, and all three students who examined the patient in bed 12 (within 1 m of the index patient) contracted SARS. As the index patient was bedridden during this period, this observation is compatible with transmission by droplets. However, that a few ill students were never within 1 m of the index patient raises the possibility of transmission by other mechanisms.

Given that it was a number of people in a hospital ward who were confined/examined (for 40+ minutes) a SARS patient. I'm not sure how that's relevant to my question, though.

The fifth article (Coronavirus: Why You Must Act Now):

The current scientific consensus is that this virus can be spread within 2 meters (6 feet)
if somebody coughs. Otherwise, the droplets fall to the ground and don’t infect you.

What the... What scientific consensus? Seriously, is this guy just making stuff up? At this point, the words "scientific consensus" are synonymous with "bullshit". Moving on.

The next 3 articles just cite the CDC.

Perhaps Raina MacIntyre was correct calling it like the flat earth theory. The CDC said 6 feet seems to be the extent of the evidence. Nothing more. No scientific backing whatsoever.

If science isn't based in the scientific method, is it actually science? Are the experts making policy based on science? No, no they aren't.