Occupational Exposure to Diesel Engine Exhaust in Municipal Household Waste Workers

Abstract

Objectives: The purposes of this study were as follows: 1) to assess the occupational exposure of municipal household waste (MHW) workers to diesel engine exhaust (DE) using a range of surrogates including elemental carbon (EC), organic carbon (OC), total carbon (TC), black carbon (BC), fine particulate matter (PM2.5), and nitrogen dioxide (NO2)

2) to determine appropriate surrogates for DE for MHW workers

and 3) to identify the main exposure determinants that influence personal exposure to DE at the task level, as well as using Time Weighted Average (TWA).

Methods: A total of 72 workers from five MHW collection companies were assessed over seven days in total between June 26 and September 18, 2014. During the field sampling period, 72 respirable EC/OC/TC, 17 BC, 21 PM2.5 and 70 NO2 samples were collected. EC/OC/TC samples were quantified using the thermal optical transmittance method. BC and PM2.5 were measured using real-time monitors, an aethalometer, and a laser photometer. NO2 samples were taken using passive filter badges and were analyzed with a spectrophotometer. For task-based exposure assessment, 1,969 BC and 1,983 PM2.5 measurements (1-min data) collected from nine workers were categorized into six specific tasks based on time activity information. Resultantly, 259 BC task samples and 261 PM2.5 task samples were obtained for assessment. All results were statistically analyzed for occupational and environmental variables in order to identify exposure determinants for DE.

Results: The geometric means (GM) of EC, OC, TC, BC, PM2.5 and NO2 were 4.8, 39.6, 44.8, 9.1, 62.0 µg/m3, and 105.3 µg/m3, respectively. In comparison with other occupations, the EC levels found for the MHW collectors (GM=5.6 µg/m3) were similar or slightly higher than those for truck drivers (GM=1.1‒4.0 µg/m3), railroad crews (GM=1.4‒5.6 µg/m3), mechanics in truck repair garages (GM=3.2–5.9 µg/m3) and in locomotive workshops (GM=2.6–3.2 µg/m3), and surface workers at mining facilities (GM=1–4 µg/m3). The EC levels for MHW truck drivers (GM=3.8 µg/m3) were comparable to those of local truck drivers (GM=1.2‒4.0 µg/m3) and long-haul truck drivers (GM=1.1‒3.8 µg/m3). On the other hand, all of their exposures proved much lower compared to those of mining workers (GM=62‒85 µg/m3). NO2 exposure levels for MHW workers showed similar comparison results with those for EC levels. Among the five surrogates for diesel particulate matter (DPM: EC, OC. TC, BC and PM2.5), EC measurements showed consistent and relevant exposure patterns against various exposure factors, such as job title (collector > driver), European engine emission standards (Euro 3 truck > Euro 4 truck), distance from the rear of the truck to the engine tailpipe (longer > shorter), age of truck (older > younger), average driving speed (slow > fast), number of containers collected (more > less), and others. More importantly, EC was not affected by worker smoking, ambient dust, weather, and malodors from food waste, as opposed to OC, TC, BC and PM2.5. This indicates that EC is the most appropriate surrogate for DPM exposure among MHW workers. NO2 measurements were also consistent and predictable according to various occupational and environmental factors such as job title (collector > driver), Euro engine standard (Euro 3 truck > Euro 4 truck), engine size (bigger > smaller), driving distance (longer > shorter), location (urban area > suburban area), weight of collected waste (more > less). Like EC, NO2 was unaffected by worker smoking habit, ambient dust, and malodors from waste. Furthermore, NO2 was significantly correlated with EC levels, indicating a consistent association between both surrogates (r=0.339, p=0.002). This suggests that NO2 can be used as an alternative surrogate for EC to assess MHW workers using Euro 3‒4 standards trucks. Task-based exposure assessment identified the task of collection < 2 m (GM of BC=9.4 µg/m3) as the highest-exposure task in MHW collection work. For collectors, the task of collection < 2 m was the task that contributed most to TWA exposure with a contribution rate of 56.6%. The task of riding in the cabin (driving) was the greatest contributor (76.4%) for drivers. Between BC and PM2.5, BC was the better surrogate for DPM for real-time measurements. BC was less affected by smoking or weather than was PM2.5. We also found how well task-based exposure assessment refined exposure characterization at the task level with a small number of subjects. In addition, the task-based method was more effective for assessing repetitive work composed of multiple tasks with significantly different exposure levels. Based on the multiple regression model, workers job title, European engine emission standard of the truck and average driving speed were the most influential factors in determining EC exposures. For NO2 exposure levels, workers job title, engine size, and driving distance were the main exposure determinants. The BC levels of the highest exposure task, collection < 2 m, were mainly affected by job title and Euro engine emission standard of the truck. In summary, job title and the Euro engine emission standard of the truck were the most important factors to predict EC and BC exposure levels.

Conclusion: This study assessed the exposure of MHW workers to DE using parallel samples of six surrogates: EC, OC, TC, BC, PM2.5 and NO2. The levels of exposure to DE were slightly higher than those found among mechanics in both truck repair garages and locomotive workshops, truck drivers, railroad crews and surface workers at mining facilities. In particular, the MHW workers were exposed to higher levels of DE when they remained near the tailpipe of truck (the task of collection < 2 m) indicating that the primary source of DE for MHW workers is the trash truck. A workers job title and the European engine emission standard of the truck were the most influential factors for exposure to DE. Among the six surrogates, EC was the most appropriate surrogate for DPM exposure because it showed less interference compared to the other surrogates and demonstrated the most relevant exposure pattern for occupational and environmental factors. NO2 levels also can be used as an alternative surrogate for EC as well as DE among MHW workers using Euro 3‒4 standard trucks, since it was significantly correlated with EC levels and showed consistent and predictable exposure patterns for occupational and environmental factors.