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Annals of Occupational Hygiene Advance Access published online on August 26, 2005
Annals of Occupational Hygiene, doi:10.1093/annhyg/mei041
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© 2005 British Occupational Hygiene Society
Received May 27, 2004
Accepted July 5, 2005

Article

Are Variance Components of Exposure Heterogeneous Between Time Periods and Factories in the European Carbon Black Industry?

Martie van Tongeren 1*, Igor Burstyn 2, Hans Kromhout 3, and Kerry Gardiner 4

1 Centre for Occupational and Environmental Health, University of Manchester, UK; Institute of Occupational Health, University of Birmingham, UK
2 Department of Public Health Sciences, University of Alberta, Edmonton, Canada; Institute for Risk Assessment Sciences, Utrecht University, The Netherlands
3 Institute for Risk Assessment Sciences, Utrecht University, The Netherlands
4 Institute of Occupational Health, University of Birmingham, UK; The Medical School, The University of the Witwatersrand, Johannesburg, South Africa

* To whom correspondence should be addressed.
Martie van Tongeren, E-mail: martie.van-tongeren{at}manchester.ac.uk


  Abstract

Occupational exposure to chemical agents can vary enormously within- and between-workers, even when carrying out the same jobs. When repeated measurements are available, the variance components can be estimated using random- or mixed-effects models. Pooling the variance components across the fixed effects, in mixed-effects models, reduces the complexity of the models; especially, when there are a large number of fixed effects. The analyses presented in this paper tested the assumptions of homogeneity in the variance components between factories and surveys for inhalable dust exposure in the European carbon black manufacturing industry. In total, 5296 measurements from 1771 workers were available collected during two surveys carried out between 1991 and 1995. Workers were grouped into eight job categories, and for each of these separate mixed-effects models were developed, including factory, survey and in some cases the interaction term as the fixed effects. The likelihood ratio test was used to test the assumptions of homogeneity of the variance components. Statistically significant heterogeneity of the variance components was observed for two of the eight job categories, ‘Fitter/Welder’ and ‘Warehouseman’. The heterogeneity was due mainly to differences in variance between the factories. When estimating the probability of overexposure for all the factories combined, there was little difference between the models with and without heterogeneous variance components for ‘Fitters/Welders’. For the ‘Warehousemen’ the probability of overexposure in the last survey changed marginally from 4% in the pooled model to 6% in the heterogeneous model. Larger differences between the models were observed when estimating the probability of overexposure for individual factories, which was due to over- or under-estimation of the variance components in the pooled models. In conclusion, for most job categories pooling of the variance components appears to be justified in this database. In addition, no large differences were found when determining the industry-wide probability of ‘overexposure’ when comparing the pooled with the heterogeneous models. However, when evaluating the factory-specific probability of ‘overexposure’ or when using the models to provide exposure estimates for epidemiological studies heterogeneity in the variance components should be investigated.


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