Assessment of an irritant gas plume model for epidemiologic study.

Previously, we reported the development of a Hazard Prediction and Assessment Capability plume dispersion model of the 2005 Graniteville, South Carolina, USA accidental release of chlorine. Here, we assess this model by spatial and statistical comparison with post-incident observed environmental indicators of exposure and other types of observations. Spatial agreement was found when the model was compared to phytotoxic bleaching and corrosion events observed in 2 km radius around the release site. When spatially compared to locations of injured or killed animals, model predictions of the plume footprint were in relatively good agreement. Model-predicted human casualties differed from observed casualty counts primarily due to the shielding effect of buildings. A statistical comparison of observed dog health outcome-derived exposure vs. model predicted exposure showed relatively good agreement, particularly when a subcohort of indoor dogs was excluded. Evaluation and assessment of the building infiltration effect would further improve the model prior to application in epidemiologic study.

Modeling an irritant gas plume for epidemiologic study.

Plume dispersion modeling systems are often used in assessing human exposures to chemical hazards for epidemiologic study. We modeled the 2005 Graniteville, South Carolina, 54,915 kg railcar chlorine release using both the Areal Locations of Hazardous Atmospheres and Hazard Prediction and Assessment Capability (HPAC) plume modeling systems. We estimated the release rate by an engineering analysis combining semi-quantitative observations and fundamental physical principles. The use of regional meteorological conditions was validated by comparing concentration estimates generated by two source-location weather data-sets. The HPAC model estimated a chlorine plume with 20 ppm outdoor concentrations up to 7 km downwind and 0.25 km upwind/downgrade. A comparative analysis of our two models showed that HPAC was the best candidate for use as a model system on which epidemiologic studies could be based after further model validation. Further validation studies are needed before individual exposure estimates can be reliable and the chlorine plume more definitively modeled.