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

Abstract

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 (10^th–90^th percentile 34–140 µg m⁻³) and 2.6 µg m⁻³ (10^th–90^th percentile 1.6–7.3 µg m⁻³), respectively. The GM for OC and TC was 28 µg m⁻³ (10^th–90^th percentile 20–42 µg m⁻³) and 31 µg m⁻³ (10^th–90^th percentile 20–50 µg m⁻³), respectively. The GM for RD and particle number was 180 µg m⁻³ (10^th–90^th percentile 20–530 µg m⁻³) and 47 900 cm⁻³ (10^th–90^th 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.

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