A Healthy Buildings Guideline for the COVID-19 Pandemic and Beyond

Federspiel, C.

Public health experts have confirmed that airborne transmission of SARS-CoV-2 (COVID-19) is one of the primary mechanisms of infection (CDC, 2020). In addition to social distancing, mask wearing and hand washing, experts now recommend increasing the ventilation and filtration of indoor air. While there is widespread consensus on this general approach, to date there are no published guidelines for the levels of ventilation, filtration, etc. that are required to control the pandemic. This is an urgent concern because colder weather in the Northern Hemisphere has moved social activity indoors where the risk of infection is higher. We propose a Guideline that provides a Criterion for integrating the effects of engineering and administrative controls with personal protective equipment (PPE) for indoor environments. The Guideline takes into account ventilation, filtration, temperature control, humidity control, masks, occupant density, occupancy category and activity. The design of the Guideline integrates recently published research regarding COVID-19 characteristics (a topic of ongoing scientific investigation) with well-established models for contaminant accumulation and infection risk (Wells-Riley), and is informed by the SIR model of epidemic dynamics. We mathematically determine a minimum threshold for the loss rate (combination of air change rate, removal rate by filtration, inactivation rate, and settling rate) that will keep the expected number of secondary infections from a single infected person less than 1.0 over the sequence of activities performed by the infected person while they are infectious. If the expected number of secondary infections is less than 1.0, then the number of infections at the population level will decrease. We show how the Guideline can be used in conjunction with existing tabulated air quality standards. We also illustrate the importance of masks and occupant density. Though the Guideline has been developed with SARS-CoV-2 in mind, it could also be applied to future epidemics and other pathogens using different pathogen-specific characteristics.