important part of this study is to understand existing natural environmental background levels for various microorganisms and their potential effect on the surrounding population. Individuals tolerate certain levels of microbial pathogens in the environment and these levels need to be taken into account in assessing risk. The committee will also determine the cleanup levels associated with a range of infectious doses (e.g., 1:1,000,000 to 1:10,000, or ID10−6 to ID10−4) and describe how these data could be used to assist in establishing acceptable measures of decontamination for selected organisms.

Quantitative risk assessment models: The committee will examine existing quantitative risk assessment models and evaluate whether these models can be adapted for purposes of assessing the safety of decontaminated public transportation facilities or whether new models need to be developed. In either case, the committee will develop the conceptual components of the four risk assessment steps (hazard identification, exposure assessment, dose–response assessment, and risk characterization) for the key organism types considered in the study. Hazard identification identifies aspects of the organisms (e.g., infectivity) and situations (e.g., form of biological hazard, such as fine aerosol) that represent threats to human health. Exposure assessment estimates the dose encountered considering the sources (including existing environmental background levels), spatial distribution of organisms, duration of exposure, and exposure pathways (ingestion, inhalation, and dermal exposure). Dose–response assessment uses available data to relate dose to adverse health response. Risk characterization combines exposure and dose–response assessment to quantify, for a defined population (considering, for example, age, sex, ethnicity, and overall health of population) the risks predicted to result from the exposure. The committee will test the models for relevant representative organisms to assess the potential risk associated with identified options for clean up levels.

Natural and residual contamination: An additional component for the committee to consider is the means of estimating the exposure level that could arise from residual contamination at various locations in a facility (e.g., inside air ducts or from equipment). The committee will comment on the role of the time factor for degradation in various environments (with and without treatment) to help determine decontamination approaches and requirements. The committee will evaluate various approaches (for example, monitoring schemes and performance evaluation targets) and describe how this information could be used to assist in determining safe levels of residual contamination. Relevant information on natural environmental backgrounds that individuals encounter with few or no health effects will also be included and evaluated here.

Past cleanup efforts: The committee will review the various efforts put into the cleanup of B. anthracis following the events of last year in order to more completely understand the implications of exposure and dose to infectivity and immunity. These reviews will look at both federal and private efforts, including the ongoing cleanup in Florida.



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