How Sensitive Is Safe? Risk-Based Targets for Ambient Monitoring of Pathogens

Biological agents present hazards at concentrations far below those of concern for most chemical agents. Detecting such low concentrations poses a great challenge to environmental monitoring systems. This study proposed a framework to address the questions of: 1) what level of risk would trigger the use of countermeasures and 2) what environmental sampling rates would be needed to detect the target risk level. In the first part, a decision model is developed to assess the costs and benefits of prophylactic antibiotic treatment for
Yersinia pestis
as a function of the risk of illness. A sensitivity analysis is then conducted to identify a threshold level of risk at which medical treatment of exposed individuals is justified. Risk levels of 1.5 × 10
^{- 4}
are estimated to be sufficient to justify treatment. A dose-response function is developed to map these risk levels to delivered doses to individuals. Estimates are that an average dose of 1.8 organisms is sufficient to trigger medical treatment for an exposed population. A range of human breathing rates are used to estimate the environmental sampling rates required to detect air-phase concentrations corresponding to this dose over an 8-h exposure period. The environmental sampling rates for
Lassa virus
are two orders of magnitude greater than for
Yersinia pestis
, while those for
Bacillus anthracis
are an order of magnitude lower. These sampling rates represent idealized goals for the sensitivity of detector systems. By linking environmental sampling rates to risk-based goals for detection, this paper provides a framework for optimizing sensing systems.