Diesel Exhaust Exposure Assessment Among Tunnel Construction Workers—Correlations Between Nitrogen Dioxide, Respirable Elemental Carbon, and Particle Number

Objectives Occupational exposure to diesel exhaust is common due the widespread use of diesel-powered combustion engines. Diesel exhaust is chemically complex and consists of thousands of compounds present as gases and particulate matter. Both nitrogen dioxide (NO2) and elemental carbon (EC) have been used as markers for diesel exhaust exposure. Currently EC is regarded as the best surrogate of diesel exhaust. The objective was to quantify the occupational exposure to diesel exhaust in underground tunnel construction work using a multi-metric approach, and to investigate the correlations between NO2, respirable EC, respirable organic carbon (OC), respirable total carbon (TC), respirable dust (RD), and particle number. Also, the use of NO2 as a proxy for diesel exhaust was evaluated, how much of the variability in the diesel exhaust exposure was attributed to within and between individual factors and if there was a difference between expert and self-administered measurements of NO2. Methods The personal exposure to diesel exhaust was assessed by expert supervised measurements of NO2, EC, OC, TC, RD and particle number in the breathing zones of underground tunnel workers. Stationary sampling of NO2, EC, OC, TC, RD, size-fractioned mass concentration, and particle number were conducted. The personal and stationary measurements were conducted on three occasions simultaneously. The workers measured their exposure by repeated self-administered measurements of NO2. The self-administered measurements were performed twice for each worker with at least one month lag between the samplings. Results In the simultaneous sampling of diesel exhaust, the geometric mean (GM) concentration of NO2 and respirable EC were 72 µg m-3 (10th-90th percentile 34-140 µg m-3) and 2.6 µg m-3 (10th-90th percentile 1.6-7.3 µg m-3), respectively. The GM for OC and TC was 28 µg m-3 (10th-90th percentile 20-42 µg m-3) and 31 µg m-3 (10th-90th percentile 20-50 µg m-3), respectively. The GM for RD and particle number was 180 µg m-3 (10th-90th percentile 20-530 µg m-3) and 47 900 cm-3 (10th-90th percentile 27500-94100 cm-3), respectively. A significant correlation was found between NO2 and respirable EC [Spearman's correlation r = 0.53 (P = 0.05)]. The within-worker variability of NO2 was 45.5% and the between-worker variability was 54.5%. The self-administered measured concentrations of NO2 (GM 70 µg m-3) did not statistically differ from the NO2 concentrations measured by an expert (P > 0.35). Conclusion The diesel exhaust exposure in tunnel construction work was low. A significant correlation between NO2 and EC was observed. This indicates that NO2 could be used as a proxy for diesel exhaust in tunnel work if diesel exhaust is the only source of NO2 and if the ratio between EC and NO2 is known and constant. Passive sampling of NO2 is much easier and cheaper to perform compared with active sampling of EC. It is possible to utilize self-administered NO2 measurements in extreme and inaccessible work environments. This study adds support to continued use of NO2 as an exposure marker in combination with EC for diesel exhaust exposure. In tunnel construction work, the variability in the diesel exhaust exposure was high both between- and within-workers.