Effects of occupationally low styrene exposure on workers' mitochondrial DNA copy number and micronucleus frequency

  Objective  To investigate the adverse effect of styrene exposure at low concentrations on workers' mitochondrial DNA content and micronucleus frequency, in order to explore these as potential exposure biomarkers.

  Methods  Totally 127 male workers exposed to styrene in a plastic production enterprise in Shanghai were recruited as the study subjects. The concentration of styrene in the air of their workplaces was detected. High performance liquid chromatography was applied to detect urinary metabolites of styrene, and spectrophotometric method was applied to determine urinary creatinine for calibration purposes. The workers were divided into 3 groups, namely low, medium and high exposure, according to the urinary phenylglycolic acid levels, ≤ 0.000 9, 0.000 9 -0.001 4 and > 0.001 4 mg/g creatinine, respectively. Both mitochondrial DNA content and micronucleus frequency of peripheral blood were assessed by real-time fluorescence quantitative PCR assay and cytokinesis -block micronucleus assay, respectively.

  Results  Area sampling at workplaces revealed that ambient styrene concentrations were all below 1.2 mg/m³. The median and interquartile range of workers' urinary phenylglycolic acid levels were 0.001 1 (0.000 8, 0.001 8) mg/g creatinine. Generalized linear model analysis showed that mtDNAcn (after natural log transformation) was elevated by 0.271 (P = 0.042) in workers with medium exposure compared to the workers with low exposure； mtDNAcn (after natural log transformation) decreased by 0.227 (P = 0.002) with an increase in the ratio of neutrophil count to lymphocyte count. Poisson regression model analysis showed that the micronucleus rate increased to 1.043 times the original rate for each year of age (P < 0.001)；urinary phenylglycolic acid level was not an influential factor in the micronucleus rate (P > 0.05).

  Conclusions  Even exposure level was lower than the occupational exposure limit, styrene could still cause changes in mitochondrial copy number levels but not in the genotoxicity index of micronucleus frequency, which suggested that low concentrations of styrene may cause early effects through oxidative stress.