Occupational Exposure to Diesel Exhaust Fumes

ABSTRACT

There is currently no OEL for diesel fumes in the UK. This study reports parallel measurements of airborne levels of diesel fume pollutants in nine distribution depots where diesel powered fork-lift trucks (FLTs) were in use. Correlations between individual pollutants are assessed as well as their spatial distribution. Samples were collected on board FLTs and at background positions at nine distribution depots. Substances measured and the range of exposures by site were: respirable dust (n = 76) GM ≤ 80-179 µg/m3; elemental carbon (n = 79) GM = 7-55 µg/m3; organic carbon (n = 79) GM = 11-69 µg/m3; ultrafine particles (n = 17) range = 58-231 × 103 particles/cm3; selected particulate phase polycyclic aromatic hydrocarbons (PAHs) (n = 14) range = 6-37 ng/m3. In addition, a tracer method based on ultrafine particle measurements was used to estimate the spatial distribution of total carbon and PAHs at the sites monitored. The spatial distribution was found to be reasonably uniform. Major diesel fume aerosol components were, in general, well correlated (r = 0.62-0.97). CO2 measurements were also made and found to be below the HSE guideline of 1000 p.p.m., with most levels below 600 p.p.m.

MATERIALS AND METHODS

Work activities

Exposure to diesel fumes was measured in nine distribution depots engaged in the supply of drinks to the licensed trade. Diesel fork lift trucks (FLTs) were used to receive incoming stock and assemble orders. All FLTs were Lansing Linde model H300D-03, rated at 3000 kg, and entered service during 1997– 1998. FLTs used red derv, a duty exempt diesel fuel with a colourant additive. Larger depots had an inhouse engineer while smaller depots had access to a contract engineer as required. The number of workers present and the intensity of activity varied widely through a shift. Neither local exhaust ventilation (LEV) nor mechanically assisted dilution ventilation was present at any of the depots investigated.

Sampling was conducted between June and September, periods of relatively high activity. Smoking was not permitted at any depot and no sources of heating were in use, hence confounding sources of pollutants of interest were excluded as far as possible.

Respirable dust

Respirable dust was monitored according to MDHS14/3. Filters (25 mm quartz fibre) (Whatman, Maidstone, UK) were pre-weighed on a Cahn 25 microbalance and loaded in Higgins and Dewell cyclones. These were connected to personal sampling pumps calibrated to a flow rate of 2.2 l/min before and after sampling using a rotameter. The rotameter was calibrated against a primary standard (Gilibrator 2; Gilian Instruments, Clearwater, FL). Filters were removed to clean cassettes after sampling and allowed to equilibriate at the laboratory prior to gravimetric analysis. Control filters, calculated as the mean plus two standard deviations, were used for baseline correction and to establish the limit of detection.

Elemental and organic carbon

After gravimetric determination of respirable dust, EC and OC were determined by two-stage thermal analysis with coulometric detection. This method is described in detail elsewhere .

Polycyclic aromatic hydrocarbons

Particulate phase PAHs were collected onto 70 mm diameter Teflon-coated glassfibre filters (Pallflex T60A20; Pallflex Corp., Putnam, CT) pre-extracted in HPLC grade dichloromethane. These were loaded into borosilicate glass filter holders with a fritted support. Total suspended particulate (TSP) was sampled at a flow rate of 20–25 l/min using a medium flow pump with dry gas meter (Universal Stack Sampler; Andersen Instruments, Atlanta, GA). Sample volumes ranged from 10 to 13 m3.

Filters were stored in a freezer prior to analysis. Desorption was performed by sonication in 2 cm3 dichloromethane for 2 h. The extracts were filtered and internal standards added (perylened12 and chrysened12). Extracts were then analysed by gas chromatography–mass spectrometry. The following PAHs were quantified: phenanthrene, fluoranthene, pyrene, benz[a]anthracene, chrysene, benzofluoranthenes (as the sum of the isomers), benzo[a]pyrene, indeno[123cd]pyrene and benzo[ghi]perylene.

Ultrafine particles

Ultrafine particles were measured using a P Trak condensation particle counter model 8525 (TSI, Shoreview, MN). An integral pump draws a sample through the instrument probe at a flow rate of 100 ml/min. Larger particles are removed by filtration and ultrafine particles grow by condensation in a saturated alcohol vapour. Particles in the range 0.02–1 µm are counted. Counts were integrated over a period of 1 min.

Carbon dioxide and carbon monoxide


CO2 was measured using long-term detector tubes (Drägar, Lübeck, Germany). These were operated using low flow sampling pumps calibrated using a bubble flow meter to 15–20 ml/min. Parallel measurements of CO2, together with carbon monoxide (CO), temperature and relative humidity, were taken using a Q Trak detector (TSI). This instrument uses a non-dispersive infrared cell (NDIR) to measure CO2 and CO.

Monitoring strategy

Samples were collected continuously over periods of 7–10 h as follows. At most depots parallel samples of CO2, EC, OC, PAHs and ultrafines were collected at a position referred to henceforth as the reference position. Depending on the layout of the depot, the reference position was close to the loading bay area where overall FLT activity was highest and where maximum levels of pollutants might be expected. In addition, parallel samples of EC, OC and CO2 were collected at a second position within the depot, mostly in the keg storage area, where FLTs were also in frequent use.

Samples of EC, OC and CO2 were also collected on board FLTs by attaching cyclones to the overhead crossbeam close (∼30 cm) to the driver’s breathing zone. CO2 detector tubes were fixed close to the cyclones but outside the windscreen and were therefore not directly impacted by the driver’s exhaled breath. of detection (LOD) were assigned a value equal to half the LOD.

Levels of ultrafine particles around each depot were mapped using an additional P Trak detector.
Spot measurements were taken repeatedly over 2 min periods at 6–10 monitoring positions covering the
area of the depot. The ratio of each spot measurement to the corresponding measurement at the reference position within the same time frame (∼30 min) was
calculated in each case. This is denoted as Rspot/ref. TWA estimates at individual monitoring positions were calculated as the product of the mean value for
Rspot/ref and the TWA ultrafine level at the reference position: estimated TWA concentration at the spot measurement position = Rspot/ref × TWAref .

The standard error was calculated as an estimate of the uncertainty associated with these estimates. The same equation was used to predict TC concentrations at the second sampling position by assuming that the ratio of TC at the second and reference positions was the same as the ratio of ultrafine particles. Here, Rspot/ref refers specifically to the second and the reference positions, respectively: predicted TWA TC at the second sampling position = Rspot/ref × TC ref. The predicted values were then plotted against the measured value at the second position.

Statistical analysis was performed using Microsoft Excel 2000 and Minitab 13 (Minitab Inc.). In regression analysis, the distribution of residuals was checked by graphical means. Values below the limit of detection (LOD) were assigned a value equal to half the LOD.

RESULTS

Results for respirable dust, EC, OC and CO2, expressed as geometric means (GMs), are shown in Table 1. All TWAs reported are for the sampling period of 7–10 h. EC was approximately lognormally distributed and the distribution of respirable dust, OC and TC was also slightly skewed. A few respirable dust results were clearly anomalous in that the blank corrected concentrations were less than the corresponding result for TC. A similar finding was reported by and attributed to handling losses of fibres from the quartz fibre filters.

The highest levels of EC and OC were recorded at depots 1, 2 and 7. These depots suffered from a combination of intensive FLT usage and architecture which impeded natural ventilation. In addition, lorries were loaded internally at depot 1. The majority of FLT samples (88%) at these depots were >50% of the corresponding German limit, although only one exceeded the limit (depot 1, EC = 117.9 µg/m3). GMs for EC in FLT samples were ∼10–70% higher than the background levels at most locations except depots 5 and 6, where FLT and background levels were similar. GMs for respirable dust ranged from <80 to 179 μg/m3 . Several samples were below the limit of detection of 80 μg/m 3 .

GM CO2 levels ranged from 436 to 535 p.p.m. The highest level measured was 828 p.p.m. (depot 2), with all but two results <600 p.p.m., considerably below the HSE guideline of 1000 p.p.m. CO2 measurements using long-term detector tubes were marginally, but consistently, higher than parallel NDIR results. CO levels ranged from mostly not detected to 1–2 p.p.m., consistent with previous reports.

Results for PAHs and ultrafines at the reference positions are shown in Table 2. Concentrations of the particulate phase PAHs measured ranged from 6.2 to 34.7 ng/m3. The lower end of this range is approaching the expected urban summertime background for the UK and was found in a depot with little FLT activity. The mean ratio of PAHs to TC was 0.035% (range 0.016–0.044%).Despite differences in experimental methods, these values are comparable to similar ratios derived from source sampling of a current technology heavy goods vehicle (HGV) of 0.056% (range 0.036–0.074%) at 1600 r.p.m. and 0.039% (range 0.034–0.049%) at 2600 r.p.m. . A slightly more extensive suite of PAHs was measured in this study, accounting for the marginally higher ratios reported.

Ultrafine levels also varied widely, with a maximum of 2.3 × 105 particles/cm3 (depot 7). Wide temporal variation in ultrafine levels was apparent, with peak concentrations of 4–5 × 105 particles/cm3 over the integration period of 1 min. It is also noted that offscale (>5 × 105 particles/cm3) transient exposure peaks sometimes occurred when the monitoring position was directly impacted by the plume from a passing FLT. These peaks were generally of less than 1 min duration.

The highest value for respirable dust (278 µg/m) had an anomalously low corresponding EC value.
Eliminating this point and restricting the data set to a respirable dust limit of 250 µg/m3, correlation coefficients for the substances measured were calculated. These are shown in Table 3. Most substances were reasonably well correlated (r = 0.62–0.97, P < 0.05 for all particulate phase components). EC and OC were highly correlated in FLT and static samples (Fig. 1, r = 0.88–0.91). The slopes are similar, although there is a suggestion that OC is slightly enriched in FLT samples. PAHs were unexpectedly highly correlated with both respirable dust (r = 0.90) and carbon fractions (r = 0.93).

The spatial distribution of ultrafines was measured and the ultrafine tracer measurements were used to predict TC at the second sampling position as described above. A reasonable correlation between measured and predicted values was found. This is shown in Fig. 2 (r = 0.80). This method therefore provides, within limits, a useful estimate of the spatial distribution of TC and an indication of the extent to which values measured at a selected position(s) are representative of exposures across the workplace.

Given the strong correlation between ultrafines and PAHs, the use of this method for estimation of PAHs should be of comparable accuracy to its use for TC.
Predicted mean TWA levels of TC and PAHs, together with the corresponding range of TWAs expected within each depot, are shown in Table 4.
Mean concentrations of ultrafines at four depots were in the range 180–192 × 103 particles/cm3. The range of predicted daily TWA levels suggests that ultrafine TWA values at points within five depots were in excess of 200 × 103 particles/cm3. It should be noted that these estimates do not represent actual TWA personal exposures but predicted concentrations at defined locations.
