Student Research: Christopher Jacomme
MS, , 2007
Evaluation of a Two-zone Model Used to Predict Workplace Air Contaminant Concentrations
The prediction of airborne contaminant concentration has an important role in exposure assessment and control. While direct measurement gives the best estimate of exposure to airborne contaminants, there are many scenarios in which monitoring is not possible or practical. During the design phase, in process development, for instance, an educated guess of future exposures is invaluable. It may also be necessary to reconstruct an exposure, as in the case of litigation or retrospective epidemiological study. A validated model can be used to make judgments about whether physical monitoring is necessary. The two-zone well-mixed room model has the distinct advantage of a balance of relative simplicity while still taking into account the variability in concentration with distance from the source. The two-zone model, though in widespread use for predicting concentrations, has undergone very little experimental evaluation, and only so in controlled environments with unidirectional airflow. In this experiment, direct measurements made while a contaminant was being released in a laboratory workplace, with the obstructions and non-unidirectional airflow more typical of indoor spaces, were compared to concentration predictions made by the two-zone well-mixed model. Concentration and airspeed measurements were used to estimate non-measurable model parameters. The resulting model was found to have inconsistent predictive capabilities. Ratios of observed to predicted steady state concentration ranged from 0.74 to 10.4 for the inner zone and 0.44 to 7.41 for the outer zone, with only two of six experimental runs showing relatively acceptable ratios of 0.74 and 1.0 for the inner zone and 0.51 and 0.90 for the respective outer zones. Intensity of turbulence and presence of obstructions to free airflow were investigated or their effect on observed concentrations.