When and how to end shelter-in-place protection from a release of airborne hazardous material : report on a decision-making concept and methodology.

Shelter-in-place (SIP) is considered a credible alternative to immediate evacuation to protect the population on and around Army chemical warfare agent stockpile storage sites from accidental agent releases of short duration. To be effective, this strategy requires immediate SIP to minimize initial exposure to agent vapor, followed by timely and appropriate termination of SIP to minimize additional exposure to agent vapor accumulations in the shelter when the air outside becomes less hazardous. However, a major challenge facing emergency managers has been how to decide the best time and way to end SIP to obtain this ideal. This report describes a concept to make this decision, and suggests a methodology to apply the concept as a site-specific response tool. The major conditions that influence the exposure of a population are the source term values of the agent that is released, meteorological conditions, shelter air change rates, the distance of the shelter from the source, and th e dose-response relationship of the hazardous material. The circumstances that contribute to overall exposure associated with a SIP strategy involve exposure during the time before taking shelter, exposure while sheltered due to vapor infiltration, and additional exposure (if any) following the termination of SIP. Options to end SIP are to resume normal activities with no restrictions, to ventilate the shelter but remain indoors, to exit from the shelter and remain nearby, or to relocate to a designated facility. The optimal time and way to end SIP involves examining the relationships among the conditions and circumstances listed above to find the combination of these variables that gives the smallest area where a sheltered population might receive a certain level of toxic effect. For example, find the combination of times, conditions, and circumstances that produce the smallest area where fatalities are possible. In this case, the best time and action to end SIP to minimize fatalities is that combination of variables which produces the smallest area where this level of effect is expected. The methodology to apply the concept is to use a computer model to examine the relationships among these conditions and circumstances (many of which are pre-planned default inputs), and display the best time and action to end SIP quickly, in a user-friendly format. A computer model that was developed to prove the concept and demonstrate the methodology (called the TSIP Model) is described in the report, and the use of the TSIP Model is illustrated in a case study in an appendix to the report. The report also discusses public education and emergency instructions essential for implementing this concept, and makes recommendations for agreements, plans, and exercises relevant to deciding when and how to end SIP. This concept and methodology is independent of the atmospheric dispersion model used, and is not limited to chemical warfare agent vapor hazards. Thus it can help make decisions on when and how to end SIP following the accidental release of many other non-flammable non-reactive hazardous vapors if sufficient information is available about the characteristics of the material and the circumstances of the release