A recent study has reported a safety guideline to limit airborne transmission of COVID-19 that goes beyond the six-foot social distancing guideline.

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The safety guideline imposes an upper bound on the time spent in an enclosed space that is developed by considering disease infectivity inferred from case studies of indoor spreading events, rates of ventilation and air filtration, room dimensions, number of occupants, breathing rates, respiratory activity, and occupants' face mask use, according to the authors.

By assuming that the respiratory droplets are mixed uniformly through an indoor space, the study derives a simple safety guideline for mitigating airborne transmission that would impose an upper bound on the product of the number of occupants and their time spent in a room. The findings of the research were published in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS).

"Our theoretical model quantifies the extent to which transmission risk is reduced in large rooms with high air exchange rates, increased for more vigorous respiratory activities, and dramatically reduced by the use of face masks. Consideration of a number of outbreaks yields self-consistent estimates for the infectiousness of the new coronavirus", the study said.

The current revival of the American economy is being predicated on 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.

"We here 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", the study said. By synthesizing available data from the best-characterized indoor spreading events with respiratory drop size distributions, the study estimate an infectious dose on the order of 10 aerosol-borne virions.

The new virus 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. 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.

The study subsequently refers to these three modes of transmission as, respectively, "large-drop", "contact", and "airborne" transmission, while noting that the distinction between large-drop and airborne transmission is somewhat nebulous given the continuum of sizes of emitted droplets. "We here build upon the existing theoretical framework for describing airborne disease transmission in order to characterize the evolution of the concentration of pathogen-laden droplets in a well-mixed room, and the associated risk of infection to its occupants", the study said.

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, millimetre-scale drops. Compliance with 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 millimetres.

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, the study said. It further said the theoretical model developed by them informs the risk of airborne transmission resulting from the inhalation of small, aerosol droplets that remain suspended for extended periods within closed, well-mixed indoor spaces.