Issues When Modeling Benzene, Toluene, and Xylene Exposures Using a Literature Database

Abstract

A database of benzene, toluene, and xylene measurements was compiled from an extensive literature review that contained information on several exposure determinants, including job type, operation, mechanism of release, process type, ventilation, temperature, distance from the source, quantity, and location. The database was used to develop statistical models for benzene, toluene, and xylene exposure as a function of operation and other workplace determinants. These models can be used to predict exposure levels for subjects enrolled in community-based case-control studies. This article presents the derived parameter estimates for specific operations and additional workplace exposure determinants and describes a number of statistical and data limitation issues that are inherent in determinants modeling of historical published data.

[Supplementary materials are available for this article. Go to the publisher's online edition of Journal of Occupational and Environmental Hygiene for the following free supplemental resource(s): a PDF file of QQ plots and a Word file with references used in the benzene/toluene/xylene exposure database.]

Keywords:

• case control studies
• exposure assessment
• exposure determinants
• occupational exposure

ACKNOWLEDGMENTS

This research was supported, in part, by the Intramural Research Program of the National Institutes of Health, National Cancer Institute, Division of Cancer Epidemiology and Genetics.

The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health.

Notes

^A The term “reported level” refers to both individual measurements and summary measures.

^B The mixed sample type denotes summary measures comprised of both personal and area samples.

^C Levels reported as non-detect or below the limit of detection (LOD) where the LOD was not provided were assigned a level of 0.05/√2, 0.14/√2 and 0.07/√2 mg/m³ for benzene, toluene and xylene, respectively.

^A Measures of fit: R² is the multiple coefficient of determination reported by the GLM procedure, and MSE_w is a unitless quantity defined as the mean of the squared differences between the observed and predicted log transformed exposure levels weighted by sample size.

^B In addition to operation, duration, and type, the full model included process-ventilation, temperature, distance, quantity, location, and publication year.

^C In addition to operation, duration, and type, the reduced model for benzene included distance, quantity, and publication year; the reduced model for xylene included process-ventilation and location.

^D Model cross-validation limited to operations with 20 or more exposure levels. The data set was randomly split into a model data set (80%) and a validation data set (20%); the former was used to estimate model coefficients which were applied to the data in the latter in order to estimate predicted values. The process was repeated 1000 times. Spearman and Pearson denote the mean correlation between the observed and predicted exposure levels in the validation data sets. R² denotes the mean of the square of the Pearson correlations. Numbers in parentheses denote 95% confidence intervals estimated using the 2.5 and 97.5 percentiles.

^A All exposure determinant terms were statistically significant, so a reduced model was not necessary.

^B In the models for benzene, the rolling operation was combined with coating and the mixing operation was combined with dipping.

^C REF denotes the reference category.

^D In the reduced model for benzene, the low and medium categories combined served as the reference category for quantity.

^E Mixed sample type denotes summary measures reportedly comprised of personal and area samples.