Beyond Six Feet: A Guideline to Limit Indoor Airborne Transmission of COVID-19

Abstract

The revival of the global economy is being predicated on the Six-Foot Rule, a guideline that offers little protection from pathogen-bearing droplets sufficiently small to be continuously mixed through an indoor space. The importance of indoor, airborne transmission of COVID-19 is now widely recognized; nevertheless, no quantitative measures have been proposed to protect against it. In this article, we build upon models of airborne disease transmission in order to derive a safety guideline that would impose a precise upper bound on the cumulative exposure time, the product of the number of occupants and their time in an enclosed space. We demonstrate the manner in which this bound depends on the ventilation rate and dimensions of the room; the breathing rate, respiratory activity and face-mask use of its occupants; and the infectiousness of the respiratory aerosols, a disease-specific parameter that we estimate from available data. Case studies are presented, implications for contact tracing considered, and appropriate caveats enumerated.

Superspreading Events Without Superspreaders: Using High Attack Rate Events to Estimate Nº for Airborne Transmission of COVID-19

“…Because we associate these quanta emission rates with typical peak viral loads, we find that these quanta are a reasonable representation of population-wide risk of aerosols in various scenarios. It should be noted, however, that if, as has been suggested in other analysis [20,21] , the case studies such as the Skagit choir correspond to individuals with a viral load of 10 9 -10 11 copies/mL (as opposed to the ~10 7 copies/mL in this analysis), the population-wide risk will be much lower than that implied by the quanta calculations (since such high viral loads are in the 95+ percentile of measured loads). Finally, we present a calculation indicating that typical fomite exposures might be on the order 1-10 virions, which may motivate the observation that only a small percentage of total transmission appears to be through fomites [37] .…”

“…Support for superspreaders has been provided by measurements of viral loads across all stages of disease progression which indicate that viral loads span ~8-9 orders of magnitude centered on ~ 10 6 -10 7 copies/mL [15][16][17][18][19] . Support for the role of superspreaders was also offered by studies of the Skagit choir that inferred an index patient viral load ranging from 10 9 -10 11 copies/mL [20,21] , which is at the very high end of measured viral loads. Support for "enhanced transmission" is provided by measurements and modeling from Goyal et al, which indicate that the period of peak infectivity for COVID-19 is ~0.5-1.0 days, coinciding with a relatively narrow range of peak viral loads (measured by nasopharyngeal swab) of order 10 7 copies/mL [1,2] .…”

“…For example, Miller et al and Buannono et al have analyzed the Skagit Choir case in detail, [20,22] while Jimenez has published an online tool using the findings to evaluate aerosol infection probabilities in different settings [28] . More recently, Bazant has published estimates of source strengths due to various activities and introduced a framework for interpreting and calculating the subsequent risks [21] .…”

“…While the absolute values of N 0 will change depending on the aerosolized volume assumption, this consistency would hold for any single value of the viral density chosen. Independent of the estimate of N 0 , and as Miller, Jimenez, Buannano, and Bazant have separately done, the case studies can be used to compute the number of aerosolized units of N 0 (quanta) released per unit time, which can in turn be used to estimate the quanta released by talking or breathing [20][21][22]28] . This calculation finds that talking releases ~460 N 0 per hour of aerosolized virus, whereas normal breathing releases ~10 N 0 per hour.…”

Retail store customer flow and COVID-19 transmission

“…As Bazant and Bush write in ref. 40, p. 1, transmission risk "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." To be conservative we use c = 0.001, and the overall transmission rate for our model is linear with respect to c so that the following results can be linearly adjusted up or down for different estimates of c. We also conduct sensitivity analysis, below, to examine how c interacts with another highly uncertain parameter, πi .…”