The protection of building occupants from hazardous outdoor releases can involve many strategies of varying cost and complexity. One method is known as "shelter-in-place," in which a space within the building is isolated to a practical degree from ambient and the remaining building air. The design of such a space involves decisions about size and level of permeability. An obvious issue is the comfort and health of occupants during the event. Because a design cannot satisfy all needs entirely, engineering the space becomes an optimization problem. This research provides an analytical framework for considering the effects of safe-room volume; ambient, building, and safe-room concentrations; ambient/building and building/safe-room air exchange rates; contaminant generation rate within the safe-room; contaminant toxicity; building volume and time. Intuition suggests that the room should be as large as possible to keep the balance of oxygen and carbon dioxide at safe levels. However, the current work quantifies the optimal balance, using a systems analysis of a three-compartment building model, consisting of ambient, building, and safe-room zones. In an example calculation involving an outdoor release of chlorine gas and a safe- room release of carbon dioxide from occupant respiration, safe-room contaminant concentration was plotted vs the safe-room air exchange rate, β. It was found that the intersection of the decreasing carbon dioxide curve and the increasing chlorine curve occurred at a β of 0.70 h^-1. This permeability was interpreted as optimal, since it resulted in the lowest total exposure, relative to hazardous levels of these toxicologically independent agents. The analysis can be used to rank the importance of the variables affecting safe-room concentration, so that control efforts can be efficiently applied. This information would be helpful in choosing among existing rooms to use for shelter, for making room modifications or designing a new space, and for making decisions as an incident unfolds.

Disclaimer: The findings and conclusions in this article have not been formally disseminated by the National Institute for Occupational Safety and Health and should not be construed to represent any agency determination or policy.