Is human exposure to styrene a cause of cytogenetic damage? A re-analysis of the available evidence

The possibility that occupational exposure to styrene causes genotoxic effects in humans has been the focus of many biomonitoring studies based on classic cytogenetic biomarkers. Contrasting results have been reported, positive studies being counterbalanced by a number of negative findings. The strength of the conclusions of single studies, either positive or negative, was often weakened by factors such as limited sample size, inadequate exposure assessment, poor epidemiological design, or inappropriate statistical analysis. We have undertaken a meta-analysis of 25 biomonitoring studies of occupational exposure to styrene, in the attempt to discover whether, regardless of the limitations of the individual studies, a general trend could be evinced from a quantitative review of the available evidence on this topic. Essentially, our approach involved a dichotomic classification of all studies according to the median environmental exposure level to styrene, i.e. 125 mg m(-3) (30 ppm), and a quantitative evaluation of the biological effects comparable among the studies considered, i.e. frequency ratio (FR). In order to provide combined estimates of effect across all studies, a weight was attributed to each study depending on its sample variance, and weighted frequency ratios (wFR) were calculated for the endpoints considered, i.e. chromosome aberrations (CA), sister-chromatid exchanges (SCE), and micronuclei (MN). A significant increase of the wFR was found for CA from the studies performed on workers with 'high level' exposure to styrene (wFR = 2.18; 95% Cl = 1.52-3.13), while inconclusive data were obtained for SCE and MN.