Employee Exposure to Diesel Exhaust in the Electric Utility Industry

ABSTRACT

The purpose of this study was to assess diesel exhaust exposures in the electric utility industry and to compare these findings with worker exposures reported in other industries and to proposed and established occupational exposure limits. Two sampling approaches were used: particulates were analyzed for elemental (EC) and organic (OC) carbon via the thermal-optical method; and gaseous components (NO2, SO2, NO, and CO) were determined using a direct reading instrument, the Metrosonics pm-7400.

Concentrations of the gases were all well within established occupational exposure levels. The EC percentage of the total carbon was generally lower than results reported from other studies resulting in OC levels representing a higher percentage of the total carbon concentrations. Smokers had higher average OC exposure (79 ixg/m3) than nonsmokers (57 pg/m3), but cigarette smoke did not contribute to EC levels in this study (smokers and nonsmokers = 3 pg/m3). 

Two of 120 individual personal exposure levels were found to exceed the proposed threshold limit value of 150 pg/m3 tor total particulate, but geometric mean levels were found to be significantly less than the proposed value. Questions are raised concerning the use of EC as the sole surrogate for estimating diesel content for comparison with an exposure standard.

MATERIALS AND METHODS 

Sampling Sites and Experimental Design

Operating headquarters serve as a base from which utility company employees receive their job assignments for each day, load supplies for assignments, and house fleet equipment and trucks that are used in daily work activities. Air sampling for diesel exhaust concentrations was performed at eight operating headquarters of a major southeastern electric utility company during two separate seasons.

At each facility’ seven to eight employees were monitored over a 2-day period in both the summer of 1996 and the winter of 1997 for exposure to elemental (EC) and organic carbon (OC). Two of these employees were simultaneously evaluated for expo­ sure to nitrogen dioxide (NO,), sulfur dioxide (SO,), nitric oxide (NO), and carbon monoxide (CO). All personal samples were taken in the employee’s breathing zone and the sample duration was between 6 and 7.5 hours of the 8-hour shift. Workers were selected for monitoring from two job categories commonly used in the electric utility industry: lineman and winch truck operator (WTO). Except for a 45-minute to 1 -hour period of time in the morning when the workers are loading the trucks within the par­tially enclosed truck bays, all job activities were performed in an outdoor setting.

Area monitoring, for both particulates and gases, was also con­ ducted during each visit. At each facility several area samples were collected in the truck bay for about 1 hour at the beginning of the shift while the trucks were being “warmed-up” in the morn­ ing. Background sampling was also performed in the areas where the crews were working. This sampling was accomplished by re­ turning to a job site the day following employee sampling (i.e., at a time when no work activity was being performed ) and samples were collected for 3 to 4 hours during normal working hours.

Each employee who participated in the study completed an activity questionnaire at the end of the work shift. The question­naire was used to determine smoking habits; job activity; length of time spent near the trucks; complaints about vehicle exhaust; and configuration of the truck exhaust (e.g., up the cab of the truck, angling to the right/left, etc.) that the employee worked near for that day. At each facility, an individual completed a ques­tionnaire regarding the type of truck bay ventilation system (if any) available at the site; whether the system was used; whether the line crew or office employees had specific complaints about equipment exhaust; and whether these complaints occur in a par­ticular season of the year.

For each day that monitoring was conducted the daily high and low temperatures and the average relative humidity was ob­tained from the nearest airport. Precipitation data were not col­lected because air sampling was not performed during periods of any significant precipitation.

Instrumentation and Analytical Procedures

Particulates

Commonly used air sampling pumps (MSA Escort, Mine Safety Appliances Co., Pittsburgh, Pa., and A1RCHEK, SKC, Inc., Eight}’ Four, Pa.) w'ere employed to collect personal and area par­ticulate samples at a flow' rate between 2.5-3.0 L/min. Air mon­itors were calibrated before use and checked follow-’ing use to en­ sure calibration using a BIOS International DC-1 DryCal (Pompton Plains, N.J.). Personal samples were collected using 37­ mm prefired quartz-fiber filters and analyzed for concentrations of EC and OC using the thermal-optical method (NIOSH Method
5040)54 by Sunset Laboratory Inc., Forest Grove, Oregon. The detection limit (DL) of this method, for both EC and OC, is <2.8 pg per filter.’5’

The thermal-optical method uses various temperatures and fur­nace atmospheres to determine EC and OC in three stages. First, the OC is volatized from the filter and oxidized to carbon dioxide. Second, the carbon dioxide is reduced to methane and subse­quently quantified with a flame-ionization detector. Third, EC re­ mains on the filter and is quantified by filter transmittance.’5 The
total carbon (TC), which is the OC plus EC, can be used to es­timate the total particulate present on the sample. 5’

Gases

Nitrogen dioxide, SO,, NO, and CO concentrations w'ere deter­ mined simultaneously using the Metrosonics pm-7400 Miniature Four-Gas Monitor, and recorded by an internal data logger at a rate of one reading per minute, which provides time-weighted av­erages and short-term exposure concentrations.16’ A battery op­erated pump was used with the monitor, which allowed for the sample to be collected in the employee’s breathing zone. The DL for NO,, SO,, and NO is 0.1 ppm, and 1 ppm for CO. The instrument was calibrated using the appropriate gas standards be­ fore each sampling period and checked following each use to en­ sure continuing calibration.

Statistical Analysis

Descriptive statistics were generated by analyte, including the number of samples greater than their respective DLs. The data w’ere tested for normality using graphical techniques’7’ and found to be approximately lognormallv distributed. Thus, natural log- transformed values were used in subsequent statistical analyses. Nondetects were set to the limit of detection.

A one-way t test was used to test the hypothesis that geometric mean (GM) occupational exposure levels did not exceed the pro­ posed threshold limit value (TLV®) (150 pg/m5)'5’ for diesel ex­haust and the established TLVs for the gases monitored. Analysis of variance and noriparametric (Kruskall Wallis and Wilcoxon rank sums) methods were used to test for differences in occupational exposure levels between seasons (winter and summer), smokers and nonsmokers, job categories (WTO and lineman), and among facilities. All statistical analyses were performed using the statistical software SAS® version 6.08.16


RESULTS

Meteorological Data

The temperature range (daily high and low) during the summer of 1996 was between 16 and 35°C; overall average was 25.7°C. The temperature range during the winter of 1997 was between 0.6 and 27°C and the average temperature was 10.9°C. The daily average relative humidity in the summer ranged from 68 to 77% and average winter relative humidity levels ranged from 49 to 75%.


TC

Descriptive statistics for the 207 samples collected at the eight facilities are presented in Table I. These samples included 18 area background samples, 69 area truck bay samples, and 120 personal samples (90 from linemen and 30 from WTOs).

Mean area background and truck bay TC concentrations were 18 and 121 pg/m5, respectively. Mean personal TC air concen­trations for linemen and WTOs were 62 and 69 pg/m5, respec­tively. TC levels in truck bay samples (GM = 78 pg/m5) were significantly greater (p = 0.001) than those in personal samples (GM = 59 pg/m5). TC concentrations in WTO personal samples (GM = 65 pg/m: were significantly greater (p = 0.038) than those collected for linemen (GM = 57 pg/m5). Background (GM = 16 pg/m5) versus personal samples (GM = 59 pg/m5) and truck ba}’ versus background samples were also found to be sig­nificantly different (p = 0.0001) when evaluating TC. When com­ paring GM concentrations between seasons, samples collected in the truck bay area were found to be significantly higher in the winter than in the summer (GM = 91 |xg/m3 versus GM = 59 p-g/m1). 

No significant differences between seasons were observed for background, linemen, and WTO samples. Linemen and WTOs who smoked (GM = 74 pg/m3 and GM = 83 pg/m3) were found to have significantly higher personal exposure levels than nonsmokers (GM = 54 pg/nv and GM = 57 pg/m3). GM TC levels in truck bay and linemen samples were found to be signifi­cantly (p = 0.0006 and p = 0.0489) different among facilities. However, no significant differences among facilities were observed when background and WTO samples were evaluated.


OC and EC

Summary statistics for OC, EC, TC data are provided in Table II. Mean OC levels ranged from 16 pg/m3 for the background sam­ples to 109 pg/m3 for truck bay samples. EC levels were much lower and generally were less than 10 pg/m3 for all locations and sample types. The ratio of average EC to average TC (EC/TC) was determined for the categories of sample location. This ratio was highest (11%) tor the background samples but all the other categories ranged from 4 to 7%. Elevated OC levels were observed for employees who were smokers (mean = 79 pg/m3) compared with nonsmokers (mean = 57 pg/m3) or those who worked near other employees who smoked (mean = 71 pg/m3). EC levels were approximately equal for all employees.

Gases

Approximately 50 samples each were collected for NO,, CO, and SO, short-term exposure limits (STELs) and time-weighted av­erages (TWAs). Descriptive statistics for these exposure measures are presented in Table I. The majority of the gas concentrations were reported to be less than their respective method DLs. For example, only three out of the 21 NO TWA samples collected for linemen exceeded the DL of 0.1 ppm. No significant differences were observed in NO,, CO, and SO, STELs, and SO TWAs when 
bay samples were compared with background and personal sam­ples. 

However, when the background gas concentrations were compared with personal samples, personal STEL CO concentra­tions were found to be significantly higher (GM = 4.4 ppm versus GM = 2.0 ppm). In addition, SO, TWA concentrations were found to be significantly higher (p = 0.0063) in the winter (GM = 1.1 ppm) than in the summer (GM = 0.2 ppm) for truck bay samples. Likewise, truck bay7 concentrations collected for NO, and SO, STEL value comparisons were found to be significantly higher in the winter (GM = 1.6 ppm and GM =1.1 ppm) than in the summer (GM = 0.4 ppm and GM = 0.3 ppm). Linemen personal exposure concentrations for NO, STEL and SO, TWA were found to be significantly higher in the winter than in the summer also. No significant differences were observed for NO,, CO, and SO, STELs, and SO, TWAs between smokers and nonsmokers, or among facilities.


