The following excerpt is adapted from our scientific paper that explains and characterizes the science of respiratory cross-infection with practical applicability: "Preventing Respiratory Infections."
While ACH is useful to characterize airborne pathogen mitigation strategies, it is not linear with the time to infection, because of varying source/sink significance to the overall pathogen balance. To better inform the public and guide facility managers about the respiratory infection health risk in buildings, a more practical and actionable metric is correlated with time. In addition to site-specific indoor air conditions and occupant behavior, the time to infection is highly dependent on the infectivity of a specific pathogen of interest.
Hence it is useful to define a linear, non-dimensional parameter, proposed herein as the Airborne Pathogen Mitigation Index (APMI) to characterize the relative airborne infection risk in a particular indoor setting. Various indoor air quality indices have been proposed by others including Breeze Technologies and Berkeley Lab – these approaches focus on indoor carbon dioxide, contaminants of concern (chemicals or non-pathogen particles), and comfort factors such as odors and humidity. Currently, there is no established index that evaluates the potential airborne infection risk for an indoor space.
The normalized APMI ranges from 0 to 10, with 0 representing the least protection (e.g., no mitigation with ACHe = 2.8) and 10 representing the highest protection from cross-transmission risk of respiratory infection. Because infection risk is determined by both the dose delivery rate (e.g., PPQ intake per breath) and exposure time, the APMI is calibrated from the base case with short-term exposure ~ 15 min (CDC 15-min close contact standard) to full day exposure (8+ hours).
To allow for a 13% model departure from the CDC guideline (Section 2.9), APMI is calculated as follows:
APMI = [1.13 x (time to infection in minutes/60)] – 0.283 (Eq. 33)
Hence APMI is correlated with time to infection for the base case and non-variant SARS-CoV-2. Figure 10 depicts APMI for the various scenarios.