Effectiveness of urban shelter-in-place—I: Idealized conditions

For a large-scale, unanticipated release of a toxic chemical into the atmosphere, it is recommended for nearby populations to shelter indoors. Two new metrics to quantify the community-scale effectiveness of shelter-in-place (SIP) are introduced. The casualty reduction factor (CRF) quantifies the expected reduction in casualties if SIP is performed. The safety-factor multiplier (SFM) quantifies the extent of toxic-load reduction for individuals in each exposed building. In this paper, idealized models are combined to explore the relationships among important input parameters and the SIP-effectiveness metrics. A Gaussian plume model predicts ambient concentrations for a hypothetical release event. A box model predicts indoor concentrations in buildings. A toxic-load model links exposure to health consequences. SIP effectiveness varies significantly with the toxic-load exponent, m, which characterizes the dose–response relationship. Another influential variable is a dimensionless time scale, ξ, equal to the release duration multiplied by the building air-exchange rate. Other factors that influence SIP effectiveness include the magnitude of the release relative to the toxicity of the pollutant, atmospheric transport and dispersion rates, and punctual termination of SIP once the toxic cloud has passed. SIP can be effective for short-duration releases (ξ<1), especially for chemicals with m of 2–3 or higher. If m=1, punctual termination at the end of the event can be important to ensure SIP effectiveness.