J. Spencer, M. Plisko, Environmental Profiles Inc., Baltimore, MD.
When prospectively assessing the need for exposure controls, mathematical modeling may be more useful than traditional air monitoring in instances where operations associated with certain exposures have been discontinued, or historical air monitoring data either are not sufficient or are lacking entirely. In the context of industrial hygiene and retrospective epidemiology as well as legal matters, the need often arises to estimate occupational exposures to airborne chemicals. If sufficient information on chemical use plus the physical and environmental factors is available, mathematical models can be used as a means of estimating exposures. Validation of a model is an important step to reduce the uncertainty associated with a particular model’s outcome and to refine the exposure assessment process. A model is validated by comparing measured air concentrations generated under well-controlled laboratory conditions, with modeled concentrations that use parameters that are the same as the laboratory conditions. The objective of this research was to compare the airborne solvent concentrations measured during three simulated parts disassembly processes with concentrations estimated by a mathematical model. The study involved three test simulations where cyclohexane, used as a penetrating solvent, was squirted onto a gate valve during disassembly of the valve. For statistical considerations, six replicate solvent application trials were conducted for each simulation. The near field-far field (NF-FF) model with a constant generation rate was selected to estimate the solvent vapor concentrations, and Monte Carlo analysis was used to quantify uncertainty in the input parameters of the model. Mean concentration estimates obtained from the modeling process were within a multiplicative factor of 2 of the arithmetic mean of the actual air sample results for all three NF conditions in each simulation. Validation/calibration of the NF-FF model under the conditions described suggests there is a reasonable degree of reliability in forecasting airborne contaminant levels in the workplace environment.
M. Plisko, J. Spencer, Environmental Profiles Inc., Baltimore, MD.
The predictive value of models can be enhanced through validation and refinement of input variables. A study was performed in which solvent air concentration predictions derived using the near field-far field model were compared to actual solvent air concentrations measured during the use of a solvent for the disassembly of metal parts. Input parameters for the model were selected based on the actual workroom volume, air exchange rate, random air speed through the workers’ breathing zone, and solvent generation rate. Comparison of the predicted versus measured solvent air concentrations revealed that model predictions differed from measured results by nearly 200%. A review of the data indicated that the discrepancy was due to differences between the actual and input values for air speed. Entering the actual air speed into the model, the error rate of the predictions was greatly improved from nearly 200% to 16%. Given the limited resources faced today by many industrial hygienists, exposure modeling may serve as a valuable tool for generating the information needed to make informed risk management decisions. However, care must be taken when selecting input parameters, to develop a modeling construct most representative of the process or situation being evaluated. Overprediction of employee exposure may result in the unnecessary expenditure of resources for exposure controls, while underprediction may result in increased potential for employee overexposures to workplace hazards.
S. Zemba, Cambridge Environmental Inc., Cambridge, MA; E. Bullister, Cambridge Technology Development, Inc., Weston, MA; I. Linkov, Intertox, Seattle, WA; S. DiNardi, University of Massachusetts, Amerst, MA.
A health and environmental simulation (HES) tool was developed under a grant from the U.S. Air Force. The tool was applied to evaluate potential health risks to personnel engaged in the servicing and maintenance of aircraft. The HES tool modeled the dispersion of pollutants generated by sanding, coating, refueling, and other typical industrial operations, both within the immediate area of the release (~2 ft) and at other locations in the vicinity of the aircraft (up to 25 ft and more). The computational fluid dynamics (CFD) model was demonstrated to be a capable and powerful tool for estimating the dispersion of contaminant emissions. Software modules were designed to surround the key CFD elements to automate the creation of model input files from user specifications and to process output data in a manner consistent with occupational health and safety data. In conjunction with traditional industrial hygiene (IH) surveys, the HES model can be useful in determining the level of protection needed to protect various workers, and hence to assist in the development of standard operating procedures for repairs and servicing. Application of HES to specific work environments can provide additional health risk information to assist personnel in making informed decisions regarding worker safety and other factors. HES modeling cannot replace the role of traditional IH monitoring, but it was demonstrated to be valuable in (1) designing sampling strategies and (2) helping to identify areas in which bystander workers might need to be protected against inhalation hazards in complex indoor settings characterized by complex air movement/ventilation and gradients in concentrations, as contaminants disperse from release points. Examples of HES application to aircraft maintenance will be presented. HES techniques apply easily to a wide range of applications and can be combined with other CFD software tools to offer versatile, cost-effective design aids to the IH field.
J. Rasmuson, D. Hall, Chemistry and Industrial Hygiene Inc., Wheat Ridge, CO; L. Birkner, (Deceased), AZ; C. Connell, Chemistry and Industrial Hygiene Inc. (former employee), Wheat Ridge, CO; J. Martyny, National Jewish Medical and Research Center, Denver, CO.
Tracer gas (sulfur hexafluoride) measurements were made in an office space under varying ventilation conditions to determine potential bystander exposures. This was done by using simulated contaminant concentrations within a worker breathing zone near a contamination source and by determining the spatial distribution of the contaminant in various zones within the same space. Near field-far field (NF-FF) mass balance models have been used in the past to estimate bystander exposures by determining the ratio of contaminant concentrations in, and surrounding, the primary worker breathing zone. Refinements to NF-FF models to assess contaminant fate and transport can be made by increasing the number of fields or zones in the same area from two zones to thousands of zones utilizing computational fluid dynamic (CFD) methodologies. In the example to be presented, comparisons will be made between CFD modeled contaminant distributions and tracer gas measurements made during a simulated exposure event. Generalized conclusions will be discussed as they relate to bystander exposure factors as a function of ventilation rate, room size, and other factors, from the perspective of the CFD model. CFD methodologies can be applied to room and ventilation design as well as exposure and retrospective exposure assessment methodologies.
D. Hall, J. Rasmuson, Chemistry and Industrial Hygiene Inc., Wheat Ridge, CO.
The distribution of inhalable and respirable size fibrous or nonfibrous particles and their removal from the air onto walls and other surfaces via attractive forces within a building space can be modeled with computational fluid dynamics (CFD). An important parameter in the fate and transport of airborne particulate matter is the percentage of respirable particles that contact and are lost to walls and other indoor surfaces. These losses, and the probability of re-entrainment under various conditions (room size, physical configuration, ventilation rate, etc.), will be compared and discussed from the perspective of a CFD model, the general literature, and particle physics. Situations will be described in which wall loss is an important sink and where particulate re-entrainment is an important exposure pathway. Application of CFD methodologies to exposure and retrospective assessment techniques will also be described.
M. Jayjock, The LifeLine Group, Langhorne, PA; P. Price, The LifeLine Group, Cape Elizabeth, ME; C. Chaisson, The LifeLine Group, Annandale, VA; C. Franklin, The LifeLine Group, Ottawa, ON, Canada; S. Arnold, The LifeLine Group, Roswell, GA.
A number of studies have shown that indoor air levels of most volatile organic compounds are higher in indoor air than outside air, indicating the dominance of residential emission sources. Indeed, for many compounds, residential sources are the dominant cause of individuals’ airborne exposures. The specific sources of the emissions are often unclear. This presentation offers a quantitative modeling method for estimating the magnitude of the sources, based on the measured airborne concentrations and the residences’ ventilation rates. It also presents approaches for using the magnitude of the source to identify potential causes of exposure in residences. We develop this concept and provide a complete source analysis with example data for a commonly found indoor air contaminant, quinoline.