COVID-19 Indoor Safety Guideline

Martin Z. Bazant

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Originally posted September 4, 2020

A growing chorus of scientists is sounding the alarm that COVID-19 is mainly spreading in homes or other enclosed spaces whenever people spend extended periods breathing tiny aerosol droplets suspended in air infected by the virus. Public health advice has been slow to catch up with the rapidly advancing science, and official guidelines still only set a minimum social distance (6 feet in the U.S.) or maximum occupancy (25 persons in Massachusetts) for indoor spaces. Although the need for building engineering to control indoor air quality (IAQ) has been emphasized, no quantitative guideline has been proposed, specific to COVID-19.

To protect against airborne transmission, it is common sense that the exposure time, room size, ventilation and human activity must also be considered:

Using mathematical models from chemical engineering and epidemiology, I have derived a safety guideline for well-mixed indoor spaces, in collaboration with John Bush, which combines all the key variables above in a bound on "cumulative exposure time". The guideline is intuitive and quantitative, calibrated against the latest data for COVID-19 indoor spreading and respiratory aerosol emissions, and easy to apply using a spreadsheet or app (linked above).

Airborne transmission risk, as quantified by the guideline, is always present indoors and critical to consider when devising policies, such as:

The guideline thus provides specific recommendations on how to limit COVID-19 transmission through well-mixed indoor air, but one should also consider various caveats emphasized in the paper and other literature, including the possibility of short-range aerosol transmission in respiratory jets. Such effects, which can lead to fluctuations in droplet concentrations around their mean values, are only partially addressed by choosing a sufficiently small tolerance in the well-mixed guideline and will depend on the details of airflow and human behavior in a specific indoor space. Especially when masks are worn, however, rising thermal plumes around each person disrupt short-range transmission and promote mixing of indoor air.


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