An Improved Model of Human Response to Bioaerosol Exposure
This chapter describes the Deposition and Response in the Respiratory Tract (DARRT) model, an improved medical model of particle size effects for bioaerosol inhalation hazards. The DARRT model accounts for variations in human response caused by differences in particle size, in particular for coarse particles that may be present near an aerosol dissemination source and that may remain suspended long enough in an urban environment to expose large numbers of people. Plume dispersion calculations with the US Defense Threat Reduction Agency’s Hazard Prediction and Assessment Capability (HPAC) show that, contrary to common beliefs, particles with aerodynamic diameters as large as 20–40 µm can be a significant hazard tens of kilometers downwind of a release. DARRT predicts the probability of infection or injury from inhalation exposure to bioaerosols with diameters between 0.01 and 100 µm and describes the resulting medical impact. Many current models assume that only 1–5 µm “respirable” particles capable of reaching the pulmonary region of the respiratory system are of concern; however, we find that inclusion of coarse (>5 µm diameter) particle deposition is also important. Coarse particle deposition in the nose, mouth and throat may pose a substantial health risk.