and practicing emergency plans and communication can lead to increased vigilance, security, and confidence among building occupants and can minimize anxieties during an event. We note that it is more likely that a building will have a flood, chemical spill, fire, or be subject to malodorous conditions, “sick building” problems, or disease clusters than that an attack with biological weapons will occur. The practical guidance offered here will have benefit if it is followed and applied to other untoward events that might occur.

Often, public health decisions (that result in regulatory or other actions) with respect to environmental exposures are made without complete knowledge. Some important factors often are not clear; others could be highly variable. A risk analysis methodology has been applied to environmental decision making in the United States over the past 20 years and by that approach, the estimated decreases in damage attributable to increasing regulation or control of exposure are balanced against the quantifiable, nonquantifiable, and noneconomic costs of an action to identify the most desirable decision. Although the metrics used to balance benefits and costs differ with the context, the overall process of assessing risks works the same way for many situations. The committee believes that the risk analysis framework is appropriate in the context of the question “How clean is safe?” for decontamination subsequent to the release of a biological agent.

A risk assessment that involves a microbiological pathogen—also known as quantitative microbial risk assessment (QMRA)—can follow the framework developed for assessing the risk attributable to exposure to harmful chemical agents as outlined by the National Research Council (NRC, 1983), and it broadly includes the following steps:

  • Hazard assessment involves identifying pathogens, determining how exposure occurred, and assessing the potential outcomes of infection (course of disease). The vectors and vehicles for secondary transmission also are identified for transmissible agents.

  • Exposure assessment evaluates the number of people who have ingested, inhaled, or otherwise been in contact with particular amounts (doses) of the infectious agents, and with what frequency.

  • Dose–response analysis examines the relationship between the dose of biological agent to an individual person and the probability of that person’s becoming ill. For populations, dose-response analysis characterizes the relationship between the dose of the agent in a given environment and the number of people who will become ill as a result of exposure to that agent in that place. Given a particular scenario for the distribution of doses and the associated uncertainties, the dose–response relationship provides an estimate of the expected number of adverse outcomes (disease cases) and their distribution and uncertainty.

  • Risk characterization is a “synthesis and summary of information about a hazard that addresses the needs and interests of decision makers and of interested

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