6 feet (180 cm) individual distance is the commonly accepted implementation of individual social distancing. This post is going to present scientific stipulation data behind this notion. There will be repetitions of data mentioned in the previous posts, because each post addresses specific application of the commonly available data, but at a different angle.
6 feet (approximately 2 meters) origin
In 1934, William F. Walls, the Instructor for Sanitary Science at Harvard School of Public Health, published his studies of water droplets expelled from the human mouth in coughing, sneezing or loud talking. He calculated time taken by droplets of various size to fall two meters – the ‘ height of a tall man. He concluded that the falling velocity of a small droplet is proportional to the square of its diameter. The two meters distance which was determined by the instrument for bacterial examination in the air, developed at the Harvard School of Public Health in 1931, became the starting point of most studies, and eventually was transformed in 6 feet physical distance for social distancing.
Walls’ work is assumed as classic. There is the Wells Curve, which presents the effects of gravity and evaporation.
The Wells’ evaporation–falling curve of droplets helped in understanding the transmission by large and small droplets of infectious material. Wells’ study also presented the transformation of large droplets into ‘droplet nuclei’ by evaporation. Small droplets evaporate fast. The residual particulates are referred as aerosols (a suspension of particles in the air).
According to Wells (1955), the vehicle for airborne respiratory disease transmission is the droplet nuclei, which are the dried-out residual of droplets possibly containing infectious pathogens. Droplet nuclei is the main object of modern understanding of infectious material in viral diseases. Moreover, the Well’s inclination of using Newton’s gravitation notion led to the application of basic Stokes’s laws to connects velocity and movement of small spherical particles for better understanding of droplets.
In his articles, Walls warned that his experiments are intended to stimulate wider and more thorough studies of air-born infections. (Wells, W. F. (-11-01). “On Air-Borne Infection”. American Journal of Epidemiology. 1934; 20 (3): 611–618; Wells, W. F. On Air-borne Infection. Study II. Droplets and Droplet Nuclei. American Journal of Hygiene; 1934; Vol.20 pp.611-18).
Although later studies demonstrated that the droplet size at which evaporation outpaces falling is smaller than that described by Wells, and the settling time is longer, his work remains important for understanding the physics of respiratory droplets.
Current droplets studies
The concepts of large droplet transmission and airborne transmission have been extended and investigated over the last 70 years (Fennelly et al., 2004; O’Grady and Riley, 1963; Riley, 1974; Riley and O’Grady, 1961; Riley et al., 1962; Wells, 1955; Yassi and Bryce, 2004).
Two meters, or 6 feet, were conditionally chosen as the commonsense distance for experimental studies at the time when nobody expected that 6 feet notion would be the buzz word in the fight with COVID-19. Outbreaks of ‘Asian” epidemics in XXI century urged to revisit 2 meters previous studies at the more advanced experimental level. These studies naturally were done predominately in China. They were performed for the infectious disease transmission airborne routes developing engineering control projects.
The effects of droplet size, exhaled air velocity, and relative humidity on droplet evaporation and dispersion were examined following Walls methodology. When the relative humidity of the ambient air was taken into account in the indoor air environment , expelled free‐falling large droplets were carried away more than 6 m by sneezing, more than 2 m by coughing, and less than 1m through breathing. Horizontally expelled large droplets can also penetrate a longer distance. At a low relative humidity, more droplets and droplet nuclei could suspend in air, increasing the probability of aerolization. (How far droplets can move in indoor environments – revisiting the Wells evaporation–falling curve; 2007 Department of Mechanical Engineering; The University of Hong Kong).
The velocity parameters can be used to calculate the droplet spread distance and the safe distance to control the disease spread. Apparently, the breathing droplet velocity is lower than coughing and sneezing. In the review of more recent studies, the ranges of breathing droplet velocity are 0.1 to 1m/s, the transmission distance is about 1 m; the speaking droplet velocity is 2-10 m/s (average 3m/c). The patients’ coughed droplet concentrations change with the size into a peak rule. The velocity of the cough droplets is the biggest, the range of 10 to 25m/s, the transmission distance is more than 2m. (Documentary Research of Human Respiratory Droplet Characteristics Procedia Engineering; Volume 2015, Pages 1365-1374; Chongqing University, China).
However, there are also different data. For example, a direct quote from an article: “In order for droplet transmission to occur infected and susceptible persons have to be in close contact (several tens of cm apart), of comparable height and the sneeze or cough has to be directed in the “right” direction. The stopping distances of expelled particles provide another telling illustration of the complexities involved in droplet transmission: particles smaller than 488 μm (cough) or 232 μm (sneeze) will not travel further than 60 cm. (Quantifying the routes of transmission for pandemic influenza. Bulletin of Mathematical Biology. 2008; 70:820–867). Although these data are related to influenza viruses, the physical properties of a droplets are the same. In the latest World Health Organization recommendations for COVID-19, health care personnel and other staff are advised to maintain a 3-foot (1-m) distance away from a person showing symptoms of disease, such as coughing and sneezing.
A researcher in MIT from Fluid Dynamics of Disease Transmission Laboratory, however, found the distance should be 27 feet (810 cm) or even more due to turbulent gas cloud dynamics during sneezing by infected person (Bourouiba L, Dehandshoewoercker E, Bush JWM. Violent respiratory events: on coughing and sneezing. J Fluid Mech. 2014;745:537-563.Google ScholarCrossref; Bourouiba L. Images in clinical medicine: a sneeze. N Engl J Med. 2016;375(8):e15.PubMedGoogle ScholarLydia Bourouiba PhD,Turbulent Gas Clouds and Respiratory Pathogen Emissions. Potential Implications for Reducing Transmission of COVID-19. JAMA online, March, 26, 2020).
At a briefing by the White House’s coronavirus task force, a reporter asked Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases (NIAID), about the potential for the coronavirus to travel 27 feet. Fauci went on to say he was “disturbed” by headlines about the virus traveling such distances “because that’s misleading. That means that, all of a sudden, the 6-foot thing doesn’t work.” The virus traveling distances that might be achieved after a vigorous sneeze is “not what we’re talking about” when it comes to social distancing, Fauci said, defending the 6-foot guideline.
Where are CDC’s and Dr. Fauci’s NIAID studies corroborated by other independent institutions about basics in SARS-CoV-2 spread, viability and other parameters crucial for everyday life and governmental policies?They had at least half a year to conduct these studies, never mind a decade since the Asian respiratory tract epidemic’s outbreaks. What are people doing in the huge building in Atlanta?
Presented material allows to come to some conclusions:
Center for Disease Control and Prevention (CDC) had not conducted any study for social distancing scientific support unless these data are under nondisclosure policy.
6 feet (2 meters) distance is elective commonsense measurement for social distancing which has not been supported by scientific data.