MIT Finds CDC COVID Six-Foot Rule Is Wrong

The social distancing, specifically the Six-Foot Rule, a guideline that offers little protection from pathogen-bearing aerosol droplets sufficiently small to be continuously mixed through an indoor space. The importance of airborne transmission of COVID-19 is now widely recognized. While tools for risk assessment have recently been developed, no safety guideline has been proposed to protect against it.

Six Foot Rule Only Applies to Big Drops but We Expel Small Drops Too

The Six-Foot Rule is a social distancing recommendation by the US Centers for Disease Control and Prevention, based on the assumption that the primary vector of pathogen transmission is the large drops ejected from the most vigorous exhalation events, coughing and sneezing. Indeed, high-speed visualization of such events reveals that 6 ft corresponds roughly to the maximum range of the largest, millimeter-scale drops. Compliance to the Six-Foot Rule will thus substantially reduce the risk of such large-drop transmission. However, the liquid drops expelled by respiratory events are known to span a considerable range of scales, with radii varying from fractions of a micron to millimeters.

There is now overwhelming evidence that indoor airborne transmission associated with relatively small, micron-scale aerosol droplets plays a dominant role in the spread of COVID-19 especially for so-called “superspreading events” which invariably occur indoors. For example, at the 2.5-h-long Skagit Valley Chorale choir practice that took place in Washington State on March 10, some 53 of 61 attendees were infected, presumably not all of them within 6 ft of the initially infected individual. Similarly, when 23 of 68 passengers were infected on a 2-h bus journey in Ningbo, China, their seated locations were uncorrelated with distance to the index case.

The authors found that all clusters of three or more cases occurred indoors, 80% arising inside apartment homes and 34% potentially involving public transportation; only a single transmission was recorded outdoors. Finally, the fact that face mask directives have been more effective than either lockdowns or social distancing in controlling the spread of COVID-19 is consistent with indoor airborne transmission as the primary driver of the global pandemic.

When people cough, sneeze, sing, speak, or breathe, they expel an array of liquid droplets formed by the shear-induced or capillary destabilization of the mucosal linings of the lungs and respiratory tract (8, 34, 35) and saliva in the mouth. When the person is infectious, these droplets of sputum are potentially pathogen bearing, and represent the principle vector of disease transmission. The range of the exhaled pathogens is determined by the radii of the carrier droplets, which typically lie in the range of 0.1 μm to 1 mm. While the majority are submicron in scale, the drop size distribution depends on the form of exhalation event (11). For normal breathing, the drop radii vary between 0.1 and 5.0 μm, with a peak around 0.5 μm. Relatively large drops are more prevalent in the case of more violent expiratory events such as coughing and sneezing (20, 40). The ultimate fate of the droplets is determined by their size and the airflows they encounter.

In such well-mixed spaces, one is no safer from airborne pathogens at 60 ft than 6 ft. The Wells–Riley model highlights the role of the room’s ventilation outflow rate Q in the rate of infection, showing that the transmission rate is inversely proportional to Q, a trend supported by data on the spreading of airborne respiratory diseases on college campuses.

MIT Research Recommendations

To minimize risk of infection, one should avoid spending extended periods in highly populated areas. One is safer in rooms with large volume and high ventilation rates. One is at greater risk in rooms where people are exerting themselves in such a way as to increase their respiration rate and pathogen output, for example, by exercising, singing, or shouting. Since the rate of inhalation of contagion depends on the volume flux of both the exhalation of the infected individual and the inhalation of the susceptible person, the risk of infection increases as Q2b. Likewise, masks worn by both infected and susceptible persons will reduce the risk of transmission by a factor p2m, a dramatic effect given that pm≤0.1 for moderately high-quality masks.

Box Fan Filters and HEPA Filters Increase Air Filtration Indoors

Nine months ago, Nextbigfuture featured the work of Evan Wang on making inexpensive box fan filters to improve air changes indoors.

You want to add more ventilation. How do you determine how much? Cubic feet per minute (CFM) is used to measure the airflow in a room. You can estimate the required CFM based on how big your room is.

You can add CFM by increasing the ventilation but that is problematic because the ventilation unit can only vent so much. Just increasing the intensity on standard AC will just push the viruses around the building because it is not hospital-grade filtering and there is little outside air. To filter viruses, you can add a MERV13 filter. However, unless you have hospital-grade ventilation, you will burn out the AC. In my experiments, a MERV13 filter reduced the air flow by 4x. Weaker fans treated it like a wall.

I have a video that shows you how to add 600 CFM of hospital-grade filtration to a room for only $30. HEPA fans work great too but they are much more expensive. My Dad said that he would have two on his desk if he was going back to the office. I also have a spreadsheet of a number of options that I have tested.

If you can’t get to 12 ACH, you can vent the room every hour or so to prevent the COVID-19 particles from building up in the room due to insufficient ventilation. If you have access to an outside window then venting the room using fans is ideal. It takes 20 minutes to cycle out 98% of the air in a room with 12 ACH. It would take an entire day to effectively kill COVID-19 in a room with humidity. However, between classes you could take the filters off the fans and vent at 60 ACH which would cycle out all the air between classes and make sure there is no cross-contamination between groups.

MIT Model

MIT researchers build on models of airborne disease transmission in order to derive an indoor safety guideline that would impose an upper bound on the “cumulative exposure time,” the product of the number of occupants and their time in an enclosed space. How long you are safe depends on the rates of ventilation and air filtration, dimensions of the room, breathing rate, respiratory activity and face mask use of its occupants, and infectiousness of the respiratory aerosols. By synthesizing available data from the best-characterized indoor spreading events with respiratory drop size distributions, they estimate an infectious dose on the order of 10 aerosol-borne virions. The new virus (severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]) is thus inferred to be an order of magnitude more infectious than its forerunner (SARS-CoV), consistent with the pandemic status achieved by COVID-19. Case studies are presented for classrooms and nursing homes, and a spreadsheet and online app are provided to facilitate use of our guideline. Implications for contact tracing and quarantining are considered, and appropriate caveats enumerated. Particular consideration is given to respiratory jets, which may substantially elevate risk when face masks are not worn.

Written By Brian Wang with material from Evan Wang,