Evaluating well-mixed room and near-field-far-field model performance under highly controlled conditions

Exposure judgments made without personal exposure data and based instead on subjective inputs tend to underestimate exposure, with exposure judgment accuracy not significantly more accurate than random chance. Therefore, objective inputs that contribute to more accurate decision making are needed. Models have been shown anecdotally to be useful in accurately predicting exposure but their use in occupational hygiene has been limited. This may be attributable to a general lack of guidance on model selection and use and scant model input data. The lack of systematic evaluation of the models is also an important factor. This research addresses the need to systematically evaluate two widely applicable models, the Well-Mixed Room (WMR) and Near-Field-Far-Field (NF-FF) models. The evaluation, conducted under highly controlled conditions in an exposure chamber, allowed for model inputs to be accurately measured and controlled, generating over 800 pairs of high quality measured and modeled exposure estimates. By varying conditions in the chamber one at a time, model performance across a range of conditions was evaluated using two sets of criteria: the ASTM Standard 5157 and the AIHA Exposure Assessment categorical criteria. Model performance for the WMR model was excellent, with ASTM performance criteria met for 88-97% of the pairs across the three chemicals used in the study, and 96% categorical agreement observed. Model performance for the NF-FF model, impacted somewhat by the size of the chamber was nevertheless good to excellent. NF modeled estimates met modified ASTM criteria for 67-84% of the pairs while 69-91% of FF modeled estimates met these criteria. Categorical agreement was observed for 72% and 96% of NF and FF pairs, respectively. These results support the use of the WMR and NF-FF models in guiding decision making towards improving exposure judgment accuracy.

Models for nearly every occasion: Part I - One box models

The standard "well mixed room," "one box" model cannot be used to predict occupational exposures whenever the scenario involves the use of local controls. New "constant emission" one box models are proposed that permit either local exhaust or local exhaust with filtered return, coupled with general room ventilation or the recirculation of a portion of the general room exhaust. New "two box" models are presented in Part II of this series. Both steady state and transient models were developed. The steady state equation for each model, including the standard one box steady state model, is augmented with an additional factor reflecting the fraction of time the substance was generated during each task. This addition allows the easy calculation of the average exposure for cyclic and irregular emission patterns, provided the starting and ending concentrations are zero or near zero, or the cumulative time across all tasks is long (e.g., several tasks to a full shift). The new models introduce additional variables, such as the efficiency of the local exhaust to immediately capture freshly generated contaminant and the filtration efficiency whenever filtered exhaust is returned to the workspace. Many of the model variables are knowable (e.g., room volume and ventilation rate). A structured procedure for calibrating a model to a work scenario is introduced that can be applied to both continuous and cyclic processes. The "calibration" procedure generates estimates of the generation rate and all of remaining unknown model variables.

Models for nearly every occasion: Part II - Two box models

The "well-mixed room" two box models are often used to predict near and far field concentrations for a specific task, but are limited to scenarios where local exhaust controls are not used. In Part II of this series, new two box models are presented that permit local controls that either exhaust to the outside or return filtered air to the workspace. Additional models are presented that also allow for the recirculation of a filtered portion of the general ventilation flowrate. During the concentration increase phase the emission rate is assumed to be relatively constant. Both steady state and transient equations were developed for each scenario. An additional factor, representing the fraction of time that the substance is being emitted during a task or set of tasks, was added to the standard and new steady state models. This simple modification permits the easy calculation of the average near and far field concentrations for cyclic and irregular emission patterns, provided the starting and ending concentrations are identical (e.g., zero or near zero) or the cumulative task time is long (e.g., two or more task cycles to a full shift, depending upon the number of room air changes per task). Additional variables are introduced with the new models, such as the efficiency of a local control to immediately capture freshly generated contaminant and the filtration efficiency whenever filtered exhaust is returned to the workspace. Many of the model variables are knowable (e.g., room volume and ventilation rate). Others can be approximated using manufacturer specifications or published values (e.g., filtration efficiency). A structured procedure for calibrating a model to a work scenario is presented that can be applied to both continuous and cyclic processes. The "calibration" procedure generates estimates of all of the unknown model variables, including the generation rate and the effective near field flowrate (which takes into account potentially complex near field air currents as well as any thermal plumes created by a hot process).