Exposure modelling : estimating dermal and inhalation exposures for epidemiological research
This study reviews available methods of exposure assessment, examines the effects of exposure misclassification on the ability to identify an association, and focuses on recent progress in subjective exposure modelling. Subjective exposure modelling utilises a framework of parameters identified as likely to control personal exposure levels together with detailed guidance and expert judgement in order to estimate exposure concentrations. As part of a case-control study, the Neurotoxicity of Paint Solvents (NPS) study, the process of evaluating subjects' lifetime exposure to solvents was reviewed. A training program and detailed guidance material was produced and the ability of assessors to reconstruct inhalation exposures from textual data was tested. In the training study, assessors were shown to estimate exposures that were well correlated with measured levels (correlation coefficients for log estimate compared to log measured values ranging from 0.73 to 0.85). The assessors tended to overestimate levels with the estimates ranging from 1.6 to 3.5 times the measured results. A series of painting simulation exercises was carried out to validate the use of certain guidance values for the model parameters. Using different paint application methods and different paint types, the influence of these variables on exposure levels was assessed. The results agreed closely with the guidance produced for the inhalation exposure model. Analysis of the solvent exposure histories of the NPS study group suggested that dermal exposure and uptake of solvents was important. To this end a novel dermal exposure model was developed for spray painting tasks. Using a conceptual model of the process, a method to describe both the likely dermal solvent exposure and solvent uptake through the skin was created. Mechanisms for combining exposures from the dermal and inhalation exposure pathways are described. Using occupational history information together with workplace monitoring records and data from paint manufacturers, both inhalation and dermal exposure to solvents was estimated for the one-hundred and twenty NPS study subjects. Two solvent exposure metrics were calculated. Cumulative exposure was the product of exposure level and time, while average annual intensity was the cumulative exposure figure divided by the number of years in solvent using employment.