Abstract:
Background Many people throughout the world are exposed to diesel exhaust (DE) in both occupational and non-occupational environments. High DE exposures are associated with acute and chronic adverse health outcomes including cardiovascular and respiratory disease, asthma and lung cancer. In recent years, The International Agency for Research on Cancer (IARC) has categorized DE as a known Group 1 human carcinogen, reaffirming the risk that DE poses to human health and the necessity for continuing research on the topic and enhanced control measures to further reduce risk. Thus, urinary hydroxylated PAHs were explored as biomarkers of exposure to diesel exhaust in underground miners. Methods In this research project, specific job task information was collected, exposure questionnaires were administered and air and biological samples were collected from a cohort of 20 underground miners. Air samples were previously analyzed for elemental carbon (EC) and 1-nitropyrene (1-NP). A subset of the urine samples (n=170) were previously analyzed for metabolites of 1-NP - 8-OHNP, 6-OHNP and 1-hydroxypyrene (1-OHP). The remaining urine samples (n=354) were analyzed using high performance liquid chromatography with fluorescence detection for six different hydroxylated PAH metabolites associated with DE exposure. All urine samples were specific gravity adjusted prior to data analysis. In this project, we estimated associations between (1) hydroxylated PAH urinary metabolites and air exposures of EC and 1-NP and (2) hydroxylated PAH urinary metabolites and 1-NP-8-OHNP and 6-OHNP urinary metabolites. Lastly, the temporal variation was examined for the hydroxylated PAH urinary metabolites to determine if they accumulate in the body over continued exposures. This was done by (1) comparing post-shift urine voids to pre-shift urine voids and (2) comparing post-shift urine voids throughout the week. Results Of the six metabolites examined, the urinary metabolite 1-OHP is positively associated with EC in air. For every doubling of elemental carbon air exposure, 1-OHP in urine increases by 37% (p = 0.001 and 95% CI: 14%,64%). Similarly, 1-OHP is also positively associated with 1-NP in air. For every doubling of 1-NP in air exposure, 1-OHP in urine increases by 15% (p = 0.02 and 95% CI: 2%,28%). An association is observed between 1-OHP and 6-OHNP and 8-OHNP. For every doubling of 6-OHNP and 8-OHNP, 1-OHP increases by 46% (95% CI: 24%, 74%) and 51% (95% CI: 26%, 79%) respectively (p < 0.001 for both associations). When examining temporal variation throughout the week, there were no statistically significant increases in hydroxylated PAH urinary metabolite concentrations. This means that these metabolites are cleared from the body within 24 hours and do not accumulate over weekly exposures. Unlike weekly variability, when examining daily cross shift increases in hydroxylated PAH urinary metabolites, for the metabolite 1-OHP, daily post shift samples were on average, 21% greater than pre-shift samples collected on the same day (p=0.02). No statistically significant daily increases were observed in the five other metabolites. Conclusions Of the six hydroxylated PAH urinary metabolites examined in this study, it appears that 1-OHP has the greatest potential to be established as a reliable surrogate for DE exposure. 1-OHP is associated with EC and EC is frequently used as a surrogate for DE and in combination with total carbon (TC), is used for the determination of compliance with the Mine Safety and Health Administration PEL. 1-OHP is also positively associated with 1-NP in air and OHNPs in urine, which are diesel specific and less prone to interferences to other sources. Thus, in the absence of interfering combustion sources, 1-OHP may be reliable for estimating DE exposure. It can be concluded that occupational DE exposures contribute to elevated hydroxylated PAH levels because of the elevated 1-OHP concentrations seen in miner’s post shift urine samples compared to their pre-shift samples collected on the same day. The ability to estimate DE exposures from hydroxylated PAH urinary metabolites reduces the time and cost burden of collecting air samples or the economic burden of sending samples to a lab to be analyzed. Continuing research to establish hydroxylated PAH urinary metabolites as reliable biomarkers for DE exposure could have many benefits, such as workers not having to don cumbersome sampling equipment, thus increasing their work output overall. Unexplained variability in associations is observed and continued research needs to be completed to determine the source of that variability.
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