Environ. Sci. Technol. 2006, 40, 5661-5667
Status of Worker Exposure to Asphalt Paving Fumes with the Use of Engineering Controls R . L E R O Y M I C K E L S E N , †,‡ STANLEY A. SHULMAN,‡ ANTHONY J. KRIECH,§ L I N D A V . O S B O R N , * ,§ A N D ADAM P. REDMAN§ National Institute for Occupational Safety & Health, Cincinnati, Ohio, Heritage Research Group, 7901 West Morris Street, Indianapolis, Indiana 46231
Since 1996, industry, labor, and government have partnered to minimize workers’ exposure to asphalt fumes using engineering controls. The objective of this study was to determine the use after some years of experience and to benchmark the effectiveness of the engineering controls as compared to the current exposure limits. To accomplish this objective, the current highway class pavers equipped with controls to reduce asphalt fumes, occupational exposure levels, and ventilation flow rates were monitored, and a user acceptance survey was conducted. Personal breathing-zone sampling was administered to determine concentrations of total particulate matter (TPM) and benzene soluble matter (BSM). Personal monitoring of workers yielded a BSM arithmetic mean of 0.13 mg/m3 (95% confidence limits (0.07, 0.43) mg/m3). All site average worker BSM values are below the American Conference of Governmental Industrial Hygienists (ACGIH) adopted threshold limit value (TLV) time weighted average (TWA) of 0.5 mg/m3 as benzene soluble inhalable particulate, although five sites contained 95% confidence limits slightly above the ACGIH TLV. The TPM arithmetic mean was 0.35 mg/m3 (95% confidence limits (0.27, 0.69) mg/m3). All sites showed average worker and area TPM values below NIOSH’s recommended exposure limit for asphalt fumes (5 mg/m3, 15 min). One screed area sample and one operator area sample were also taken each day. Area samples followed a similar pattern to the worker breathing zone samples, but were generally slightly higher in TPM and BSM concentration. The effect of work practices and application temperatures appears to have an impact on the ability of the engineering controls to keep exposure below the TLV for BSM. To gain a better understanding of the aerodynamic properties of asphalt fumes, particle size and airborne concentrations were also monitored using a TSI model 3320 aerodynamic particle sizer spectrometer. The geometric mean particle size was between 0.64 and 0.98 micrometers for the worker breathing zone samples, with a geometric mean of 0.73 micrometers for all sites. Total airborne concentrations were typically higher for the * Corresponding author phone: 317-390-3188; fax: 317-486-2985; e-mail:
[email protected]. † Currently affiliated with the Environmental Protection Agency. ‡ National Institute for Occupational Safety & Health. § Heritage Research Group. 10.1021/es060547z CCC: $33.50 Published on Web 08/12/2006
2006 American Chemical Society
asphalt fume exposed groups than for the background samples. During high fume events, four 15-minute samples were taken each day. Only one 15-minute sample was above the limit of quantification. Stack flow rates were measured, and results are discussed and compared to the manufacturers’ nominal values. Survey results were generally positive, with recommendations discussed for continuous improvement.
Introduction The asphalt paving industry places 550 million tons of asphalt yearly and directly employs 300 000 people. Although the health risks from asphalt fume exposures during paving are not yet fully defined, review of the health effects data show that the principal adverse health effects were irritation of the mucous membranes and upper respiratory tract. With this in mind, all partners (paving industry, unions, and government) agreed that prudent action was needed to reduce worker exposures. Consequently, this engineering controls partnership secured industry-wide participation and voluntary agreement to incorporate fume emission controls on all new highway class pavers in hopes of minimizing exposures for the 300 000 asphalt paving workers. Highway class pavers (pavers g16 000 lbs) account for approximately 90% of the 550 million tons of hot mix asphalt placed annually. These pavers are the largest in the industry and typically pave in a continuous or semi-continuous mode which will result in higher potential for worker exposure as compared to smaller pavers that place asphalt at a lower rate and with more frequent breaks during a typical paving day. For greatest worker protection impact, addressing worker exposures on the highway class pavers is the logical place to start for this industry sector. By inference, the significant majority of paving workers were affected by this development. Initiated by the National Asphalt Pavement Association (NAPA) in 1993, the Phase I effort of this project involved six asphalt paver manufacturers who represented the significant majority of the highway-class paver market and who independently designed engineering controls for their respective pavers. At the request of NAPA, through an agreement with the United States Department of Transportation (USDOT), the National Institute for Occupational Safety and Health (NIOSH) assisted the manufacturers with technical review and recommendations for their prototype designs and independently assessed the performance of each prototype (1). NIOSH researchers developed and published a test protocol for evaluating the asphalt pavers under controlled conditions (2) using the information gathered through the application of this protocol, and NIOSH researchers proposed design modifications to optimize the performance of engineering controls during actual paving operations. As a result, the asphalt paving industry determined that each paver manufacturer should develop and install fume exhaust ventilation systems. The engineering control systems for highway class pavers manufactured after July 1, 1997 all incorporated exhaust systems, hoods, and enclosures, although each manufacturer developed its own unique design. These components capture the asphalt fumes within the paver’s auger area and exhaust them before they enter the workers’ environment. Control systems were designed to capture, in an indoor environment, at least 80% of surrogate fume emissions coming from the auger area, which is the greatest source of fume emissions from paving machines VOL. 40, NO. 18, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(1). Asphalt fumes are transported through a duct system and exhausted through a stack whose discharge is up and away from the worker breathing zones. NIOSH recommended that each paver manufacturer test the exhaust ventilation system for each model of paver and certify that each model meets the minimum indoor capture efficiency of 80% when tested using the NIOSH-developed protocol. Exposure data taken at outdoor work environments have a high degree of variability due to the weather and the construction tasks. Because of this variability, the sample size necessary to show a statistically significant reduction associated with use of controls, when compared to past studies of uncontrolled paving emission, would make such a study untenable. To overcome this problem of variability, the phase I study, using prototype paver engineering controls, measured the effectiveness of the controls to capture surrogates that were released in the auger area of the paver and used a study design where controls were turned off and on randomly within the same day at each site. This phase I study did not measure BSM or TPM for the worker, but did show the effectiveness of the controls to capture surrogates released in the auger area ranging from 64 to 99% capture (impressive for an outdoor environment) (3). During the study protocol development for the current evaluation, it was determined that showing the effectiveness of controls versus no controls would not be achievable with the available budget, even pooling resources from all the partners. The focus of this current study was to benchmark BSM and TPM worker exposures when using engineering controls. This study (phase II) was conducted during 2003 and 2004 to establish a baseline of asphalt fume exposures in the U.S. while utilizing these controls and to determine their effectiveness as related to the ACGIH TLV. Only highway class pavers that have engineering controls and have been operating in the field for more than 1 year were included in this study. In addition to obtaining worker exposure levels with the engineering controls in place, a survey was conducted to obtain input from the end users. This engineering controls partnership included the Asphalt Institute, Association of Equipment Manufacturers, Center to Protect Workers’ Rights (CPWR), Federal Highway Administration, Heritage Research Group, International Union of Operating Engineers, Laborers’ International Union of North America, NAPA, NIOSH, and paver manufacturers (Caterpillar, Dynapac, IR Blaw-Know, Roadtec, TerexCedarapids, and Vogele America). Non-highway-class pavers are not addressed in either phase of this study. Current research applies only to new equipment that has integrated the engineering control system into the basic paver design.
Study Design Primary analytical methods used in this study to assess the effectiveness of engineering controls include TPM and BSM using NIOSH Method 5042 (4). This method was used for these studies since it was the most recently approved test specific to asphalt fume exposure quantification. These gravimetric analyses were performed through a NIOSH contract laboratory. In this method, drawing a known volume of air through a tared PTFE filter collects the TPM and BSM sample. After the tared PTFE filter is gravimetrically analyzed to determine the concentration of TPM, the filter is extracted with benzene and the organic residue is gravimetrically determined and reported as BSM. Although both TPM and BSM are nonspecific, TPM could include dust and other particulates not related to asphalt fume exposures. The BSM represents the fraction of the total particulates that could be caused by asphalt fume exposure, although it is possible for other contaminants to be benzene soluble but not a function of asphalt fume exposure. 5662
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Time weighted averages (TWA’s) are an average value of exposure over the course of an 8-hour work shift. The computed concentrations were converted to TWAs by site for comparison to the American Conference of Governmental Industrial Hygienists (ACGIH) adopted threshold limit value (TLV) for asphalt fumes: 0.5 mg/m3 as “benzene extractable inhalable particulate” (5). In addition, geometric means were computed, averaging over all sites. ACGIH has prepared this TLV as a guideline to assist decision-making processes by industrial hygienists regarding safe levels of exposure found in the workplace. These guidelines reflect an estimate of exposure level, below which should be safe for the workers, above which there may be irritation in some workers. Traditionally, a 37-mm closed-faced cassette (NIOSH Method 5042) has been used for collection of asphalt fumes in the workplace to evaluate potential exposures with reference to the prior TLV and the current NIOSH recommended exposure limit (REL). This closed-faced total sampler was utilized during the current phase II study. This sampler’s measurements have been shown to correlate well with the IOM sampler’s measurements (benzene extractable inhalable particulate) for asphalt fume exposure. For sampling asphalt fumes without high levels of non-asphalt fume particulate confounders, a comparison study showed a 1:1 ratio between these two samplers (6, 7). It was determined by the engineering controls partnership that the engineering controls would always be “on” in this study. There were six different paver manufacturers involved; each manufacturer selected two sites to be monitored. Every site was evaluated for two consecutive days, scheduling and weather permitting. For one site, a second day was cancelled due to lack of asphalt on the job and was rescheduled for a different site. This second site evaluation was subsequently cancelled due to an engineering control malfunction. Thus, there were a total of 23 sampling days for phase II. Each day, total samplers were mounted on three workers and in two areas. Workers monitored included the raker (responsible for raking the hot mix asphalt to the correct thickness to reduce high and low areas in the pavement), the screedman (duties consist of raising and lowering the paver screed to the proper depth and width for the application), and paver operator (responsible for maneuvering the paver while laying down hot mix asphalt). Each wore two samplers, one on his left side and one on his right side, since exposure may vary by side. In addition, a total of two area samplers were affixed to the paver, one at the operator area and the other to the back corner of the paver at a height representative of the workers’ breathing zone. Figure 1 is a photograph showing the general location of workers during a normal U.S. paving operation. At four time-periods of potentially high exposure each day, one of the industrial hygienists stationed alongside one of the workers also wore a sampler collection system for measuring 15-minute short-term exposures. When the operator first started the paver, the first of these four samples was always collected. The timing of the other three samples was based on visual appearance of asphalt fumes. These samples were collected and analyzed to determine the efficacy of utilizing 15-minute monitoring. Additionally, two stationary points taken upwind of the site were monitored for background levels using one sampler at each point on each day. Five “trip” blanks per day were also prepared, as required by the method. Six worker breathing zone samples, two area samples, two background samples, four 15-minute samples, and five trip blanks resulted in 19 samples per day. With 2 days monitored at twelve sites (except for site 4, which had just 1 day) the total number of TPM/BSM samples was 437. Site locations, in alphabetical order by city, included Alton, NH; Ashland, WI; Aurora, CO; Burnsville, MN; Cincinnati,
FIGURE 1. Worker location during a normal paving operation. OH; Edinburg, TX; Grand Rapids, MI; Indianapolis, IN; Jacksonville, FL; Joplin, MO; Manassas, VA; and Rome, NY. Real time exposure assessments using a TSI model 3320 aerodynamic particle sizer spectrometer and stack velocity measurements using a Velocicalc Plus air velocity meter were additional tools used to evaluate the engineering control systems. Aerodynamic particle size measurements were made to determine the particle size distribution and light-scattering intensity in real time using low particle accelerations. This time-of flight particle sizing technology involves measuring the acceleration of aerosol particles in response to the accelerated flow of the sample aerosol through a nozzle. Instrument setup conditions included a 20 s sampling time in summed mode (continuous) with a baud rate of 9600 and an inlet pressure of ∼980 mbar. With a total flow rate of 5 liters per minute (Lpm), the sheath flow rate was 4-Lpm and the aerosol flow rate was 1-Lpm. Size distributions from 0.5 to 20 µm were counted and light-scattering intensity for particles from 0.3 to 20 µm detected. Special carbon impregnated conductive silicone tubing (8 ohms/cm), 7.62 m in length and 11.2 mm internal diameter, was used for all particle size samplings. Number, surface, and mass particle size information was generated for each sample along with total concentrations for these parameters. With a Velocicalc Plus (TSI, Inc. Minneapolis, MN) airvelocity hotwire anemometer and digital manometer, two velocity determinations through the engineering control stacks were conducted each day of sampling. Two inch diameter holes were drilled in each duct at right angles to each other. Since the velocity of an air stream in a duct is not uniform throughout the cross-section, a series of ten duct velocity pressure readings, commonly referred to as a traverse, were taken using the digital manometer and Pitot tube at points of equal area across the duct. One exception was a certain brand of paver where the hood face velocity had to be measured with the hotwire anemometer at the hood inlet in the screed area due to design differences that prevented adequate duct access for a traverse. For this design, nine measurements were taken across the hood face. Documentation regarding the stack included the area of the duct, traverse location, velocity pressure measures at each point, the temperature of the air stream, elevation, and humidity. Velocity pressure readings were converted to standard velocities using eq 1. The only significant deviation from standard conditions was elevated temperatures of the air stream caused by hot asphalt heating the air as it passed by. The density factor was used to adjust to standard conditions in eq 1. Multiplying average velocity and the cross-
TABLE 1. Averages over 11 Sites Where Engineering Controls Were Used (TWAs) total particulate (mg/m3)
operator raker screedman
benzene soluble (mg/m3)
left
right
left
right
0.34 0.31 0.37
0.34 0.33 0.36
0.16 0.081 0.16
0.17 0.075 0.14
sectional area yields the volumetric flow rate. In the case of the face velocity measurements, these measurements were multiplied by the open area of the inlet to obtain the volumetric flow rate. Equation 1 shows conversion of velocity pressure readings to standard velocities:
V(std) ) 4005 × square root (VP/df)
(1)
where V(std) is the velocity at standard conditions in feet per minute, VP is the velocity pressure measured in the duct in inches of water gage, df is the density ratio of actual to standard conditions, for temperature the ratio is T(std)/ T(measured in the duct)
Statistical Information An important observation was that the variance of the worker and area concentrations increased with their magnitude, which suggests that for either geometric or arithmetic means, the natural log transformation is appropriate. Since there were some negative values, this required some procedures that would allow these values to be used, for instance, by averaging the simultaneous measurements taken on each worker. A justification for this procedure is that there is no statistically significant difference (at even the 10% level) between the right and left side samples. Average results are shown for both total particulate and benzene soluble fraction in Table 1. Some of these negative values resulted after blank correction, especially for sample determinations close to the LOD. Blank corrections are required within method 5042 using five field blanks, due to artifacts and limitations of this gravimetric analysis (4). Among the samples taken on the workers or in their work area, there were three TPM and twenty BSM values less than the LOD. Almost all of these occurred at sites 1 and 2. Negative concentrations estimates arise because of blank correction. Since the estimated blank means used in correction are themselves subject to variation, it seems best to use the negative values, which is done here by averaging these values of the right and left-side samples, which usually produced positive values. By substituting zero VOL. 40, NO. 18, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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for these values or by removing the values altogether, both bias and underestimation of variability may result. For the 15-minute samples, there were negative values and relatively few samples. Results are summarized by site medians. Since the median of a lognormal distribution equals the geometric mean, there is still some consistency in using the median. (The Supporting Information contains more details about the statistical procedures used to handle these data).
TABLE 2. BSM and TPM Average TWA Exposures (mg/m3) for all Sites (Personal and Area Sampling) and 95% Confidence Limits BSM-worker TPM-worker breathing zone breathing zone site samples samples 1 2
Results Primary emphasis in this study was placed on exposures of the workers as measured by historically accepted methods of asphalt fume quantification, i.e., the TPM and BSM. All geometric mean data are reported as mg/m3 and are based on the actual sampling times. Generally, this time period ranged between 6 and 8 h, except for the 15-minute samples. For calculation of the TWA, the assumption is made that the exposure during the unsampled portion of 8 h was zero. This corresponds to multiplying the measured concentrations by the factor (actual minutes sampled divided by 480). This assumption seems reasonable, since for each shift, samples were taken for the entire time that workers were paving. Any non-paving portion of the 8-h shift should have led to minimal exposure.
TPM/BSM Results Background samples were relatively low in concentration showing a range of nondetected to 0.38 mg/m3 for TPM and nondetected to 0.032 mg/m3 for BSM. (See the Supporting Information for a graph of the average TPM and BSM TWAs of the background samples by site as well as averages over all sites.) One problem observed was negative BSM data indicative of the challenges of operating near the limit of detection of the method (LOD) and limit of quantitation (LOQ). Negative values are related to the NIOSH method requirement that sample mass values be corrected for the average of five field blanks. Accuracy of the gravimetric measurements is greatly affected by changes in temperature or humidity during pre and post-collection weighing. Personal monitoring of workers yielded a benzene soluble matter (BSM) arithmetic mean of 0.13 mg/m3 with 95% confidence limits (0.07, 0.43) mg/m3. The corresponding geometric mean, based solely on measured concentration during paving activity, was 0.089 mg/m3. Site 12 is not included in the statistical calculations of the overall arithmetic mean since controls were not functioning at this site. Categorized by site, the TPM and BSM TWA results are shown in Table 2, along with the 95% confidence limits. The average TWA BSM concentrations were all below the ACGIH TLV. Ninety seven percent of the 138 individual worker measurements were below the TLV using engineering controls. Only three workers showed concentrations that were slightly above 0.5 mg/m3. (At site 6, day 2, both operator samples (left and right) were slightly above 0.5 mg/m3. At site 9, day 2, the operator had a right-side sample of 0.516 mg/m3, compared to 0.229 mg/m3 on the left side. Finally site 10’s operator on day 1 had one exposure concentration slightly above 0.5 mg/m3.) Two of these workers were involved with jobs where the asphalt was at an elevated temperature as compared to the other sites (up to 325 °F), one due to a significant amount of handwork required (site 6) and one stone matrix asphalt (SMA) job (site 10). The average temperature taken behind the screed area after laying the pavement was 280 °F excluding sites 6 and 10. (In comparison, the average temperature taken at the back of the screed was 303 °F for site 6 and 315 °F for site 10.) Site 9 also had one worker result that exceeded the TLV, but it did not appear to be related to elevated temperature of the asphalt. The few 5664
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3 4 5 6 7 8 9 10 11 12
0.050 (0.018,0.147) 0.032 (0.013,0.111) 0.060 (0.021,0.172) 0.019 (0.006,0.109) 0.188 (0.068,0.558) 0.292 (0.120,0.988) 0.066 (0.029,0.242) 0.029 (0.012,0.103) 0.223 (0.088,0.729) 0.338 (0.148,1.218) 0.161 (0.065,0.536) 0.084 (0.026,0.211)
0.233 (0.110,0.728) 0.151 (0.072,0.456) 0.188 (0.098,0.644) 0.283 (0.093,1.251) 0.359 (0.157,0.999) 0.444 (0.205,1.302) 0.323 (0.139,0.883) 0.277 (0.098,0.621) 0.491 (0.228,1.452) 0.523 (0.248,1.575) 0.471 (0.185,1.178) 0.199 (0.087,0.552)
BSM-area
TPM-area
0.036 (0.012,0.108) 0.204 (0.053,0.499) 0.060 (0.016,0.152) 0.026 (0.007,0.167) 0.140 (0.043, 0.408) 0.791 (0.306,2.875) 0.123 (0.048,0.563) 0.054 (0.022,0.207) 0.358 (0.160,1.507) 1.187 (0.401,3.762) 0.515 (0.187,1.760) 0.149 (0.045,0.426)
0.168 (0.079,0.521) 0.292 (0.109,0.727) 0.116 (0.042,0.278) 0.054 (0.017,0,246) 0.268 (0.105,0.695) 0.872 (0.373,2.473) 0. 272 (0.096,0.687) 0.334 (0.100,0.664) 0.586 (0.263,1.745) 1.259 (0.489,3.244) 0.929 (0.312,2.073) 0.228 (0.093,0.619)
exposures that slightly exceeded the TLV-TWA were found at the operator location. All site average worker BSM values are well below the ACGIH TLV, although for five of the 12 sites surveyed, the 95% upper confidence limit on the BSM personal exposures exceeds the TLV(see Table 2). The TPM arithmetic mean was 0.35 mg/m3 with 95% confidence limits (0.27, 0.69) mg/m3. The corresponding geometric mean, based solely on measured concentration during paving activity, was 0.33 mg/m3. Table 2 indicates that all sites’ TWA worker and area TPM average TWAs are well below NIOSH’s REL for asphalt fumes of 5 mg/m3. Site 10 has the highest worker and area TPM results, which is most likely related to materials within the SMA formulation and high application temperatures. For worker breathing zone samples, a breakdown of the arithmetic means of TWAs by worker function for each site showed all TPM TWA averages below 0.66 mg/m3 and all BSM TWA averages below 0.49 mg/m3 (Table 3). Over all sites, the range of individual TWA exposures was from -0.0135 to 1.114 mg/m3 for TPM. BSM values ranged from -0.155 to 0.594 mg/m3. Examination of this gravimetric data categorized by worker functions reveals approximately equal TPM exposures for the operator, screedman, and raker. However, based on the BSM results, the raker is generally exposed to less asphalt fumes than the operator or screedman, whose average concentrations were similar. Area samples follow a similar pattern to the worker breathing zone samples, although concentrations are generally higher (Table 2). The median value for the 15-minute samples was calculated as the median of the sites 1-11 site medians. The resulting estimates are 0.40 for TPM (95% confidence limits: (0.0067, 0.59)), and 0.080 for BSM (95% confidence limits: (-0.018, 0.31)). These values are well below the official limits of detection (∼1 mg/m3) for these short collection times. For the BSM fraction, only one 15-minute sample was above the LOQ concentration. Eighty-six percent (76/92) of the 15-minute samples and the median value for all 15-minute samples were below the LOD concentrations. Twelve of these
TABLE 3. Arithmetic Means by Worker Function and 95% Confidence Limits for AveragessPhase II (TWA) TPM (mg/m3) site
1
2
3
4
5
6
7
8
9
10
11
12
operator, n ) 4
0.28
0.16
0.19
0.32
0.22
0.62
0.38
0.18
0.54
0.61
0.24
0.27
raker, n ) 4
0.15
0.17
0.15
0.3
0.33
0.31
0.23
0.34
0.45
0.41
0.66
0.12
screedman, n ) 4
0.27
0.13
0.23
0.23
0.53
0.41
0.35
0.31
0.48
0.54
0.51
0.2
average
0.23
0.15
0.19
0.28
0.36
0.44
0.32
0.28
0.49
0.52
0.47
0.2
averagea (2.5%CL,97.5% CL)b 0.34 (0.26,0.69) 0.32 (0.26,0.70) 0.36 (0.28,0.75) 0.35 (0.27,0.69)
BSM (mg/m3) site
1
2
3
4
5
6
7
8
9
10
11
12
operator, n ) 4
0.08
0.02
0.08
0.03
0.11
0.49
0.08
0.04
0.32
0.47
0.1
0.15
raker, n ) 4
0.02
0.05
0.04
0.01
0.11
0.13
0.03
0.02
0.10
0.17
0.19
0.038
screedman, n ) 4
0.05
0.02
0.06
0.01
0.35
0.26
0.08
0.03
0.25
0.37
0.19
0.065
average
0.05
0.03
0.06
0.018
0.19
0.29
0.07
0.03
0.22
0.34
0.16
0.084
a
0.16 (0.083,0.56) 0.08 (0.041,0.28) 0.15 (0.081,0.55) 0.13 (0.067,0.43)
Site 12 is excluded from the average because controls were not working there. b Sites treated as random in computation of confidence limits.
samples were just above the LOD concentrations but below the LOQ concentrations. It should be emphasized here that the LODs and LOQs are much higher for the 15-minute samples than for the 8-hour samples at 1 mg/m3 versus ∼0.04 mg/m3, due to the difference in volume of air sampled. However, the LOQ is one-fifth of the NIOSH REL for asphalt fumes of 5 mg/m3. The required method blanks performed by the contract laboratory show high variations that indicate the limitations of this gravimetric procedure. Sites 1 and 2 were particularly problematic in terms of the blank data. To better understand the aerodynamic properties of asphalt fumes, the particle size distribution in paving operations was measured. The geometric count mean diameter was between 0.64 and 0.98 micrometers for the worker breathing zone samples, with an overall geometric count mean diameter of 0.73 micrometers. Similarly, the overall geometric count mean diameter for area samples was 0.74 micrometers. This study illustrated that the particle size of the fume is clearly within the respirable range. (See the Supporting Information for more detailed results.) Site 2 showed the highest geometric mean particle size (0.98 micrometers), but the data are suspect due to baseline problems resulting possibly from high atmospheric moisture on these 2 days. Instrument software did not allow for background subtraction, which would have minimized the inaccuracy of site 2 results. Mass weighted concentration per channel equals the particle density times the volume weighted concentration per channel. The default value of 1.00 g/cm3 was used for the density, although the density of asphalt fumes is ∼0.88 g/cm3. In general, these concentration values were slightly higher for the asphalt-fume exposed groups than for the background samples. Specifically, mass weighted concentrations taken by the background samples averaged 0.021 mg/m3 as compared to an average of 0.033 mg/m3 for measurements taken in the worker-breathing zone. Mass weighted concentration measurements taken during the 15-minute sampling averaged 0.06 mg/m3 and the measurements taken in the same location as the area samples averaged 0.07 mg/m3.
FIGURE 2. Stack flows (percent of nominal). Site 6 had the highest total concentration mass particle size data for both worker and area samples.
Engineering Control Flow Rates Stack flow rates were measured toward the beginning and end of each day of sampling. Results were averaged and reported as a percentage of the actual flow divided by the nominal flow as shown in Figure 2. This method of reporting is consistent with phase I, designed to protect the identity of specific manufacturers. One out of 12 pavers had flow rates within 15% of the nominal flow rates as listed in the equipment specifications. Specifically, the flow rate of the engineering control system used at site 3 was 86% of what the manufacturer specified. Three other pavers had engineering controls with flows within 30% of nominal. The remaining eight pavers were either above 130% or below 70%, which means that their flow rates were either too high or too low when compared to the manufacturer’s recommended value. At two sites, the engineering control systems had to be repaired prior to sampling. Site 12’s paver had controls, but there was an electrical short in the system that caused the fuses to blow immediately; thus the flows were zero for this site. Workers at a couple of sites reported that particulates spewed out from the engineering controls. This appears to VOL. 40, NO. 18, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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be more of an issue during granular base applications. Granular base applications involve jobs where the hot mix asphalt is applied over a layer of crushed stone or gravel that is compacted using water as a lubricant. It is not bound by asphalt or pozzolanic materials. If not protected by a seal coat or some other sprayed treatment the exposed surface of granular base will dry out and dust (particles abraded from the surface) will be generated by vehicles or equipment traveling on the granular base. This is evidenced by the increased TPM concentrations relative to the BSM concentrations. One of the above-nominal measurements was associated with the paver requiring face measurements instead of traverse. Turbulence at slot entries often leads to artificially high readings when using a hot-wire anemometer, which may have contributed to this higher reading.
Confounders on the Job Field documentation records included potential confounders, which predominantly involved diesel fuel use, diesel exhaust, and cigarette smoke. If present, these airborne contaminants can be captured on the sampling media potentially influencing the quantitative results of asphalt fume exposure as determined by TPM and BSM concentrations. Since these methods are not specific for asphalt emissions, verification of these parameters as confounders was not possible in this study. Thirty-six percent of the workers classified themselves as smokers. Diesel fuel was used for cleaning equipment and was the most common confounder. Diesel exhaust from other equipment, especially the dump trucks, was also noted during these site investigations.
Survey Results Crewmember survey results were generally positive. Pavers used in this study were an average of approximately 2 years old, all originally built with the engineering control exhaust systems. All individuals interviewed had regularly operated or worked on pavers equipped with engineering controls for an average time of 1.8 years. Eight out of 12 of those surveyed regarded the engineering controls as reliable. Ten out of 12 systems come on when the engine starts. The two systems that do not come on when the engine starts have manual on/off switches that engage the engineering controls. Operators and foremen were generally the most knowledgeable about these control systems, with a few screedmen trained. Eleven workers responded that they had no maintenance concerns and that no other controls were needed. One person did not respond to this question. Seven out of nine paver operators stated that they use the controls 100% of the time. Two others estimated the percent time that they use the controls: estimates of 70% and 80%. (The remaining three paver-operators did not respond to this question.) Reasons for not using the controls 100% of the time included ineffectiveness, noise, increased particulates, smoke, and improper operation. Other reasons included lack of training, ambivalence about exposure to asphalt fumes, and lack of visibility of the screed augers blocked by the collection hood. Being able to see the screed augers was important to the workers, and some designs block this view. In some cases, workers complained about the stack height being too tall, which caused logistical problems in tree-lined streets. Conversely, other workers complained about stacks being too short, causing fumes to be too close to the paver operators. Nine of those interviewed had no suggestions for improving the operations of the engineering controls. Recommendations from the foremen include noise reduction, lessening of particulates from the unit onto the crew, 5666
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providing more information on troubleshooting the control system, improving the hydraulic system performance, and improving the electrical system to keep it from failing due to vibration and heat. Since the engine-cooling fan for one model pushes fumes (fumes that were discharging from the stack or from another source not associated with the EC) back at the crew, it was recommended that new designs vent engine-cooling air to the side instead of the back. Some workers expressed the desire for continued checking and testing in the field.
Discussion Both TPM and BSM results are consistently below U.S. government recommended values with the use of engineering controls. When the asphalt temperature was elevated (up to 325 °F), operators from site 6 and site 10 (three samples out of 138 worker measurements) had levels of exposure to asphalt fumes that slightly exceeded the current ACGIH recommended TLV of 0.5 mg/m3 as benzene extractable inhalable particulate (BEIP) (4). Overall, the individual BSM levels were less than the recommended ACGIH TLV and all average TWA BSM concentrations were below this level. The impact of work practices and HMA material design relating to the product application temperature appears significant. All 15-minute worker samples were below the NIOSH REL: 5 mg/m3 ceiling (15 min) total particulates. Since only one 15-minute sampler out of 96 was above the limit of quantitation, it seems impractical to use this short-term sampling procedure for asphalt fume field exposure assessments. Survey results indicate that the majority of engineering controls were successful in terms of their use, general reliability, and overall effectiveness in keeping exposures below the recommended exposure limits. Recommended best practices by the manufacturers based on the results of this study include regular inspections of the engineering controls system to ensure unobstructed flow, to verify that it is free of leaks or damage to components, and to confirm that the system functions per manufacturer’s specifications. Finally, adequate training in accordance with the manufacturer’s manuals prior to operating or maintaining the system is paramount to providing the most effective protection against asphalt fume exposure for the asphaltpaving workers.
Acknowledgments The Engineering Controls Partnership thanks all of the manufacturers, contractors, and their crews for the tremendous cooperation and assistance during these field studies.
Supporting Information Available More detailed statistical information including data-handling statistical models, confidence intervals for TWAs, confidence intervals for 15-minute sample medians, and confidence intervals for geometric count mean diameters. This material is available free of charge via the Internet at http:// pubs.acs.org.
Literature Cited (1) National Institute for Occupational Safety and Health (NIOSH)s Reducing Exposure to Asphalt Fumes; http://www.cdc.gov/ niosh/innovatn.html. (2) Engineering Control Guidelines for Hot Mix Asphalt Paverss National Institute for Occupational Safety and Health; U.S. Department of Health and Human Services: Washington, DC, 1997. (3) Mickelsen, R. L.; Mead, K. R.; Shulman, S. A.; Brumagin, T. E. Evaluating Engineering Controls During Asphalt Paving Using a Portable Tracer Gas Method. Am. J. Ind. Med. Suppl. 1999, 1, 77-79.
(4) National Institute for Occupational Safety and Health (NIOSH). Benzene solubles and total particulate (asphalt fume). (Analytical Method 5042). In NIOSH Manual of Analytical Methods, 4th ed.; Eller, P. M., Ed.; NIOSH: Cincinnati, OH, 1998. (5) TLVs and BEIs: threshold limit values for chemical substances and physical agents; and biological exposure indices, American Conference of Governmental Industrial Hygienists. 2002, ACGIH: Cincinnati, OH, 2002. (6) Ekstrom, L. G.; Kriech, A. J.; Bowen, C.; Johnson, S.; Breuer, D. International Sampler Study for Bitumen Fumes. J. Environ. Monit. 2001, 3 (5), 439-445.
(7) Kriech, A. J.; Osborn, L. V.; Wissel, H. L.; Kurek, J. T.; Sweeney, B. J.; Peregrine C. J. G. Total Versus Inhalable Sampler Comparison Study for the Determination of Asphalt Fume Exposures Within the Road Paving Industry. J. Environ. Monit. 2004, 6, 827-833.
Received for review March 8, 2006. Revised manuscript received July 14, 2006. Accepted July 14, 2006. ES060547Z
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