Ann. occup. Hyg., Vol. 46, No. 1, pp. 79-87, 2002
© 2002 British Occupational Hygiene Society
Published by Oxford University Press
Article |
Bitumen, Polycyclic Aromatic Hydrocarbons and Vehicle Exhaust: Exposure Levels and Controls among Norwegian Asphalt Workers
1Division of Environmental and Occupational Health, Institute for Risk Assessment Sciences, Utrecht University, PO Box 80176, 3508 TD Utrecht, The Netherlands; 2Unit of Environmental Cancer Epidemiology, International Agency for Research on Cancer, Lyon, France; 3Center for Occupational and Environmental Medicine, The National Hospital, Oslo, Norway; 4The Public Roads’ Administration, Norway
Received 9 November 2000; in final form 8 May 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Objectives: To characterize exposures of asphalt workers in Norway and to evaluate exposure control measures.
Methods: Representative asphalt paving and mixing operations were monitored in Norway in 1991–92 for exposures to bitumen fume, organic vapor, polycyclic aromatic hydrocarbons (PAHs) and vehicle exhaust (NO2, CO). Linear regression was used to evaluate introduced control measures.
Results: A total of 320 samples of airborne organic matter were gathered (279 from paving). Median personal bitumen fume measurements ranged from 0.03 to 0.15 mg/m3 and were similar in paving and asphalt mixing. According to principal component analysis, there were three independent sets of PAHs: (i) PAHs lighter than 228 g/mol; (ii) 4- to 6-ring PAHs non-detectable in 80–90% of samples; and (iii) naphthalene. Some NO2 (1/49) and CO (12/58) concentrations near paving equipment exceeded 15 min exposure limits, 2 and 25 p.p.m., respectively. Changing sampling methods midway through the study had a significant impact on the measured bitumen fume and organic vapor levels. For pavers, lower application temperatures reduced organic vapor, but not bitumen fume, exposures. Retrofitting a paving machine produced at least a 5-fold reduction in exposure to airborne organic matter. Work in tunnels increased PAH exposures, but general ventilation partially counteracted this effect.
Conclusions: The observed exposure levels indicate that some potentially hazardous exposures may have occurred during paving in Norway. Bitumen fume, organic vapor and PAH exposures can be reduced using appropriate engineering control measures.
Keywords: road paving; asphalt plant; carbon monoxide; determinants of exposure; naphthalene; nitrogen dioxide; volatile organic compounds
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INTRODUCTION |
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Asphalt workers are involved in production and application of various asphalt mixes for road maintenance and construction. [In Europe, asphalt refers to a mixture of organic binder (bitumen in modern operations) and filler (sand and/or gravel). This paper adheres to European nomenclature.] Asphalt mixing is performed at plants that combine a petroleum-derived organic binder (bitumen) with sand, gravel or crushed rock to form hot mix asphalt. This mixture is transported to the road construction site, where it is transferred to a paving machine. It discharges asphalt onto the road surface through a screed. Newly laid asphalt is subsequently smoothed using rakes and shovels, and compacted by rollers. Priming agents, typically bitumen emulsions that may contain aliphatic amines, are often sprayed onto the road surface prior to paving. Both the use of oil gravel (at ambient temperature) and surface dressing (75–150°C) involve application of binder directly onto the road, followed by embedding and compression of gravel into it. In oil gravel paving ‘road oil’, consisting of 20–30% of bitumen mixed with heavy oil (also derived from crude oil originally), was typically used in Norway as binder. Diesel oil was not used for this purpose. In modern operations, the asphalt surface is often recycled and re-mixed with new asphalt. Until about 1995 diesel oil was used to clean tools and as a release agent on truck beds in Norway. A detailed description of such operations can be found elsewhere (Burstyn et al., 2000a).
In Norway, the state is responsible for the planning, construction and maintenance of main highways. Since 1965 the Public Roads’ Administration in each county has been producing and laying down oil gravel. However, since 1979 it began to produce hot mix asphalt that has since replaced oil gravel. The private asphalt industry has been involved in the application of primarily hot asphalt mixes. It currently produces and applies 80% of the total volume of asphalt in Norway.
It is not clear whether bitumen fume is carcinogenic to humans because of the potential confounding of its effect by co-exposure to coal tar in epidemiological studies (Partanen and Boffetta, 1994). However, the latest cohort study of European asphalt workers provides some evidence of bitumen fume carcinogenicity after adjustment for coal tar exposure (Boffetta et al., 2001). Neurological and psychological symptoms were reported among asphalt workers in Norway (Vågsholm et al., 1991). Following that initial report, an exposure survey indicated a potential for exposures to high concentrations of hydrocarbons among asphalt workers (Waage, 1986, 1987). However, a follow-up investigation failed to confirm these findings of high exposure levels (Norseth et al., 1989), but did provide additional evidence of work-related symptoms, such as eye and throat irritation, tiredness and nausea, among asphalt paving workers (Norseth et al., 1991). In 1991–92, to resolve the uncertainty about exposure patterns, the Public Roads’ Administration, in collaboration with the Norwegian asphalt industry, launched an exposure survey (Lien, 1993), the results of which are presented in this paper. The three specific goals we pursued were: (i) to describe exposures of asphalt workers in Norway in 1991–92; (ii) to identify the determinants of their exposure; and (iii) to evaluate control measures for reducing exposures among these individuals.
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MATERIALS AND METHODS |
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Measuring air concentrations
All situations present in the Norwegian paving industry were monitored by applying a stratified measurement strategy, selecting worksites for monitoring from locations across the country. In paving, surface dressing, oil gravel paving and hot mix paving were monitored. In selecting individuals to monitor, preference was given to foremen/screedmen and paver operators, who were expected to incur the highest concentrations of workplace chemicals (henceforth referred to as ‘exposures’). However, all jobs in the above-mentioned types of paving operations were sampled. Some repeated exposure measurements for individuals were collected. Screedmen, standing on the screed, control the discharge of asphalt from the screed of the paving machine. Paver operators drive paving machines that discharge asphalt onto newly paved road surface.
Between 1991 and 1992 sampling methods for gasses and vapors were changed to avoid material loss due to electrostatic properties of XAD-2. Data on the side-by-side comparison of the two sampling methods were not available. In 1991, Millipore 37 mm closed-face filter cassettes with a glass-fiber filter followed by 5 g of XAD-2 were used to collect organic matter and dust. In 1992, Millipore 25 mm closed-face cassettes with glass-fiber filters were used to collect the particulate phase of emissions and silica absorbent tubes collected gaseous emissions. Both cassettes had a 4 mm inlet. The cassettes were positioned in the individuals’ breathing zones and air was drawn through them at a rate of 4 l/min using portable sampling pumps. Air was drawn through samples only when individuals were actively engaged in work. Thus, if work stopped, the sampling pumps were turned off, and they were restarted when work resumed. Rainy weather and waiting for the delivery of the asphalt were the primary reasons for interruptions in sampling. The duration of active work varied between 4 and 6 h. Some of the collected samples were stationary, and were placed in the vicinity of potential sources of asphalt emissions.
After collection, samples were sent by express mail to Chemlab Services A/S (Bergen, Norway) for analysis. Particulate emissions (collected by glass-fiber filters) and gas phase emissions of asphalt (absorbed onto XAD-2 or silica) were analyzed separately. The samples were extracted with 30 ml of either chloroform (in 1991) or n-hexane (in 1992) in a Soxhlet apparatus for 16 h. The solvents were subsequently evaporated in a Rota-Vapor to ~10–15 ml, then concentrated under a gentle flow of dry nitrogen to 1 ml at room temperature. Subsequently, the organic matter content (OMC) of the samples was determined using gas chromatography. The OMC of particulate emissions was used as a measure of bitumen fume levels, and that of gas phase emissions was used as a measure of organic vapor levels (likely to contain organic matter of non-bitumen origin, such as vehicle exhaust and solvent vapor). Esso marine diesel oil (special grade 08644, supplied by Exxon) was used as an external standard in OMC determination. Following the OMC determination, samples were further concentrated under the gentle flow of nitrogen to ~100–200 µl, and their polycyclic aromatic hydrocarbon (PAH) content was determined using a combination of gas chromatography and mass spectroscopy. The PAH standards were obtained from Teknolab A/S (Drøbak, Norway). Calibration curves for each PAH were constructed with external standards using three points: 25, 50 and 100 µg/ml.
Field blanks were not collected systematically; therefore, the limits of detection were estimated using the uncertainty of 15–20% in quantifying airflow through samplers (as reported by the pump manufacturer) and analytical limits of detection. Non-detectable values were used as the values reported by the analytical laboratory.
Stationary monitors were used to measure NO2 and CO levels as proxies of vehicle exhaust from paving machines and rollers (Metrosonic PM 7700-C logger). During a single sampling shift, NO2 and CO monitors were placed either on a paving machine or on a roller.
Collection of information about determinants of exposure
During monitoring, information about potential determinants of exposure was collected using standardized data forms. The following factors were ascertained: date and location, bitumen type, application temperature of asphalt mix, paving type, and type of material used in priming road surface. In addition, meteorological conditions (temperature, precipitation, wind direction and force), job performed, degree of enclosure of the paving site, exposure control measures and cigarette smoking by a subject during sampling were recorded.
Data analysis
Distributions of exposure variables were examined using histograms and normal probability plots to determine if transformations were warranted prior to statistical modeling. Skew to the left in the distributions signaled that logarithmic transformation (base e) was needed to satisfy the assumptions of normality and homoscedasticity of variance. Principal component analysis (Kleinbaum et al., 1988) was used to identify multicolinearity among total PAH levels (fume phase plus vapor phase) and to reduce the number of dependent variables in exposure modeling.
Statistical models were constructed only on the 1991 subset of personal exposure measurements among pavers with the goal of identifying determinants of exposure that can be used to reduce exposure levels. This restriction on the data was imposed because: (i) in preliminary analyses analytical methods had a significant effect on the measured bitumen fume and organic vapor exposures; (ii) the 1992 subset of data for pavers was too small to derive stable regression models; and (iii) a significant proportion of the measurements from the asphalt plants (76%) and the 1992 survey among pavers (24%) were stationary area samples, precluding inferences about personal exposures. Factors that can be potentially altered to control exposures (modifications of paving machine, application temperature, work in tunnels and use of mobile fans in tunnels) were forced into all statistical models. Next, bitumen type, asphalt type, priming agent, wind and ventilation, and rain were added one at a time in order to assess whether they could confound the estimates of the effect of the potential exposure controls. These potential confounders were entered into the final models if they were statistically significant at the 10% level in the initial screen for potential determinants of exposure. In order to correct for within-worker correlation of exposures for workers with repeated exposure measurements, a random worker effect was added to all of the above models utilizing mixed models methodology. Compound symmetry in the covariance matrix was assumed [i.e. (i) independence of between- and within-person variances and (ii) the same within-person variance estimate being valid for all persons]. In these mixed effects models, the worker effect was treated as random, while other variables were treated as fixed effects. The restricted maximum likelihood algorithm was used to estimate variance components.
The data were part of The Asphalt Workers’ Exposure database, developed for the IARC study of lung cancer risk among European asphalt workers (Burstyn et al., 2000b). These data were part of the data set previously analyzed for epidemiological purposes (Burstyn et al., 2000c). This analysis utilized the more detailed supplementary information available only from the Norwegian survey. All statistical analyses were executed in SAS 6.12 (SAS Institute).
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RESULTS |
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Exposure levels
Exposure levels for bitumen fume, organic vapor, CO and NO2 are summarized in Table 1. The same methods were used to measure CO and NO2 in both years of the study, and therefore we pooled the exposure levels. These measures of vehicle exhaust exposure varied over a wide range of values. There was no correlation between CO and NO2 concentrations. It appears that there were substantial differences in bitumen fume and organic vapor exposure levels between the series of measurements collected in 1991 and those from 1992. Data collected in 1992 seemed to have higher concentrations of bitumen fume but lower concentrations of organic vapor than those collected in 1991.
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Levels of exposure to bitumen fume and organic vapor followed skewed distributions that were well approximated by log-normal distributions.
Preliminary analysis demonstrated that there were few systematic differences between exposures to individual PAHs measured by the two different methods in 1991 and 1992 (data not shown). As shown in Table 2, organic vapor generated during asphalt work tended to contain higher average levels of PAHs than fume. Most average PAH exposures were on the order of 0.01–1.00 µg/m3. However, a substantial proportion (up to 97%) of measurements for individual PAHs were below the limit of detection. Hence, caution is advised in interpreting absolute PAH levels, especially in the case of fume-phase PAHs.
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The principal component analysis revealed that three principal components accounted for 76% of multiple correlation among PAH exposure levels. The first principal component consisted primarily of light and medium molecular weight PAHs (Table 2). It explained 54% of multiple correlation, suggesting that most of the PAHs in the study have a common source. The second principal component consisted exclusively of heavier PAHs, which were non-detectable in most samples. The third principal component was associated almost exclusively with naphthalene. On the basis of the above observations, we pursued the identity of the predictors of the score of the first principal component and total naphthalene. Exposure levels measured by the score of the first principal component and naphthalene fitted the log-normal distribution.
Determinants of exposure concentrations in road paving
The results of statistical modeling of exposures among road pavers are presented in Table 3. A paving machine equipped with both the carrying basket and the ventilated tarpaulin covering the screed is depicted in Fig. 1. Using such modified paving machines reduced bitumen and organic vapor exposures, while it did not affect PAH exposure levels.
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all ißi x (determinant of exposure i) + random worker effect + error (1991 data set: 175 personal measurements, 88 workers)a
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Paving in tunnels was associated with a 3- to- 6-fold increase in PAH exposures compared with open-air paving. However, introduction of general ventilation into tunnels by means of fans reduced PAH exposures by a factor of 2–3.
The use of cutback bitumen was associated with elevated organic vapor exposures and reduced naphthalene concentrations. Oil gravel paving was associated with elevated organic vapor exposures, but predicted exposures for this application were still lower than for the use of cutback bitumens because the latter were applied at higher temperatures (another exposure-increasing factor statistically significant in the model). Paving with hotter asphalt tended to increase only organic vapor exposures, but the effect was weak. The effects of bitumen hardness on exposure concentrations, when corrected for application temperature, were negligible. All jobs within a paving crew were associated with similar exposures.
Rain lowered organic matter exposures, probably due to work interruption during the rain. We did not detect statistically significant effects of other meteorological conditions on exposures.
Cigarette smoking by pavers did not influence measured exposure levels, probably because monitoring stopped during breaks in which the majority of the cigarettes were most likely smoked.
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DISCUSSION |
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Sampling and analytical methods
Measured exposure concentrations for bitumen fume and organic vapor depended on the sampling and analytical methods used (Table 1). There were differences between sampling and analytical methods between the 1991 and 1992 portions of the survey in terms of (i) dust samplers, (ii) the type of sorbent used to collect organic vapor and (iii) the solvent used to extract organic matter from dust samples and sorbent tubes. Wall deposition is a well-known problem with these Millipore cassettes (Puskar et al., 1991; Baron and Chen, 1994; Chen and Baron, 1996; Lafontaine et al., 1999). However, this effect may be expected to be similar for both 37 mm and 25 mm cassettes, especially for small particles such as those comprising bitumen fume (G. Lidén, personal communication). Operating both sampler types at twice the ordinary airflow of 2 l/min was likely to have had a negligible effect on sampling efficiency (G. Lidén, personal communication). Large differences in sampler performance can be expected to be due to variations in local air currents (Baron and Chen, 1994; Chen and Baron, 1996) similar to those that occur during outdoor work (e.g. road paving), but we have no reason to believe that this would cause the 37 mm sampler to systematically undersample relative to the 25 mm cassette. Thus, we do not expect the observed differences between the 1991 and 1992 portions of the study to be attributable to different performances of the dust samplers.
A more plausible explanation for the apparent difference in performance of the dust samplers lies in the efficiencies of the solvents used to extract organic matter in the 1991 (n-hexane) and 1992 (chloroform) portions of the study. It is likely that bitumen fume and vapor are not equally soluble in the two solvents used to extract bitumen-attributable organic matter. Being the more polar of the two, chloroform should be more efficient for extraction of substituted aromatics and saturated hydrocarbons found in bitumen (R. Vermeulen, personal communication). Furthermore, according to the Merck Index (Anonymous, 1976), bitumen is soluble in chloroform. However, its solubility in n-hexane was not mentioned (Anonymous, 1976). Lastly, n-hexane is used to precipitate asphaltenes (5–25% by weight) from bitumen (Evans, 1978; IARC, 1985). Therefore, it can be expected that only the oily fraction (maltenes) was extracted with n-hexane from bitumen condensate. This would explain why 37 mm samplers tended to collect more organic fume than 25 mm samplers, since the organic matter collected by the 37 mm samplers was extracted with a more efficient solvent (chloroform) for bitumen-attributed organic matter. In conclusion, chloroform appears to be a promising solvent for extracting all organic matter attributable to bitumen.
We may have expected that the use of a more efficient solvent would also lead to higher reported organic vapor concentrations in the 1992 portion of the study. However, the opposite was observed. The observed differences in organic vapor concentrations can be attributed primarily to the different affinities of XAD-2 and silica gel sorbents for organic vapor generated during asphalt work. This is so because if less organic matter were retained on the sorbent in the first place, a more efficient sorbent in extraction would not compensate for undersampling of organic matter. Silica sorbent has an overall low affinity and absorbs mostly polar compounds, whereas XAD-2 has greater binding efficiency in general, and traps both polar and non-polar compounds (R. Vermeulen, personal communication). This suggests that XAD-2 may be preferable for studies of bitumen vapour exposure. However, since (i) the XAD-2 sorbent tubes in our study were extracted with the less efficient (with respect to bitumen) n-hexane and (ii) the electrostatic properties of XAD-2 may have contributed to material loss (see Materials and methods), our measurements from the 1991 portion of the survey probably underestimate the true concentrations (but to a lesser extent than the 1992 organic vapor measurements collected with silica gel sorbent).
It is evident that standardization of the methods used to measure bitumen emissions is urgently needed (Burstyn et al., 2000a).
Exposure levels and regulatory limits
Our method of treating non-detectable values may have affected both observed exposure levels and effects of determinants of exposure. Many of the non-detectable values for PAHs were reported as zero, and these zero values were used in the analyses. This was likely to have two consequences: (i) reported mean exposures are probably underestimates of actual exposure levels; and (ii) for PAHs, with many non-detectable values, the exposure contrasts that we see in regression modeling are probably overestimated. For the statistical model with the dependent variable constructed from the principal component score of PAHs, none of the principal component scores had zero values. In other statistical models, bias due to treatment of non-detectable values, if any, can be expected to be small due to the low proportion of non-detectable values for bitumen fume (3%), organic vapor (0%) and naphthalene (13%, none reported as zeroes in the data).
Since most of the measurements made during asphalt mixing were stationary, we could not assess whether elevated personal exposures occurred at these workplaces and what measures could be taken to reduce them. However, exposure levels at the asphalt plants indicate that both asphalt plant employees and pavers are potentially exposed to similar levels of organic matter, with the highest potential exposures reaching 1.8 mg/m3 of bitumen fume and 204 mg/m3 of organic vapor.
Even though a large proportion of full-shift time-weighted average concentrations for the exhaust gasses in road paving were not detectable, some short-term measurements exceeded exposure levels permissible in Norway (Direktoratet for arbeidstilsynet, 1996). Thus, the Norwegian ceiling exposure limit for NO2 (2 p.p.m.) was exceeded by one 15 min NO2 sample. The Norwegian short-term exposure limit for CO (25 p.p.m.) was exceeded on 12 occasions (27–172 p.p.m.). It is not clear if these area concentrations can be extrapolated to personal exposures.
Tar use was discontinued in Norway’s road paving industry 20 years before the start of the current survey. This accounts for the low levels of exposure to 4- to 6-ring PAHs such as benzo[a]pyrene, since the PAH content of bitumen can be expected to be much lower than that of coal tar (Brandt et al., 1985). Whether these low PAH exposure levels pose a health hazard is unclear, but in the light of growing evidence of PAH carcinogenicity, their concentrations in workplace atmospheres should be kept as low as possible (Lindstedt and Sollenberg, 1982; Darby et al., 1986; Machado et al., 1993). The highest naphthalene exposure level (0.09 mg/m3) was below the 50 mg/m3 Norwegian 8 h recommended normative occupational exposure limit (Direktoratet for arbeidstilsynet, 1996). However, the carcinogenicity of naphthalene is under review (IARC, 2000), and this may result in a lowering of the allowable naphthalene exposure level.
Our data did not allow us to identify the source of naphthalene exposure. However, all other PAHs seemed to have originated from a source common to all asphalt paving operations, which is probably bitumen.
Because the measured air concentration of bitumen fume depends on the sampling method, we could not compare the levels we observed with other studies. This further emphasizes the urgent need for standardization of sampling and analytical methods for bitumen fumes. However, the observed bitumen fume and PAH levels were of the same order of magnitude as in other published exposure reports in the road construction industry (Burstyn et al., 2000a).
Exposure control measures during paving
Pavers outnumber asphalt plant employees by approximately five to one in the asphalt industry. Therefore, in the light of a likely similarity in exposure levels between the two groups, controlling exposures during road paving may have the more significant impact on safeguarding health.
It is unclear from our results what steps can be taken to reduce vehicle exhaust exposures among road paving workers, or whether they originate from the passing traffic or the paving machines themselves.
Ventilating the screed of the paving machine appears to reduce the bitumen fume and organic vapor exposures of a paving crew. Similar results were obtained in the experimental evaluation of this control measure (Norwegian Road Research Laboratory, 1992). This control measure is analogous to the method recently proposed by the National Institute of Occupational Safety and Health (NIOSH, 1997; Mead et al., 1999). Since all members of a paving crew appear to have similar exposures (Burstyn and Kromhout, 2000), this control measure is likely to benefit all members of a paving crew.
Introducing general ventilation into tunnels seems to have reduced PAH exposures that were otherwise elevated relative to open-air paving. It is not clear why the same effect was not observed for bitumen fume and organic vapor.
Our results support the notion (Norseth et al., 1991; Brandt and De Groot, 1999) that keeping the asphalt temperature as low as possible should provide some respite from bitumen exposure produced during paving. Examining the properties of asphalt and bitumen types that were associated with reductions in exposure levels may lead to the development of ‘low-emission paving materials’. However, preventing overheating of the asphalt mixed may well prove to be a simpler and more practical approach to reducing exposure levels than the development of new asphalt types.
The impact of meteorological conditions on bitumen fume, organic vapor and PAH exposures needs to be studied further. We were not able to detect any statistically significant trends with wind strength and direction, but our data may have lacked the required statistical power.
In general, the introduction of comprehensive exposure control measures in road paving is a challenging task. Even if the issue of appropriate exposure reduction, if any, for each agent is resolved, it is likely to be reached through different strategies for each agent. This is because each agent may originate from a different source. It should also be noted that dermal exposure to bitumen has not been taken into account in our study. It may well prove that different measures are needed to control the deposition of asphalt emissions onto skin rather than to reduce their inhalable concentrations, as was demonstrated in the rubber manufacturing industry (Kromhout et al., 1994).
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CONCLUSION |
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Exposure levels observed in our study indicate that hazardous exposures may have occurred in the Norwegian asphalt industry in 1991–92. However, the lack of standardized methods for measuring bitumen emissions hampered such an evaluation. We lack a clear understanding of the health hazards and risks involved in asphalt work. Nonetheless, ‘action should be taken to protect those working with bitumen/asphalt...by appropriate control measures’ (CROW, 1992). Such measures may include the reduction of asphalt temperatures, but would have to be considered in light of asphalt performance requirements. It is also likely that retrofitting paving machines with engineering controls can effectively reduce exposure to bitumen fume and organic vapor during asphalt paving.
Acknowledgments—I.B. was supported by an IARC Special Training Award. The study was funded from the grants of European Commission, DG-XII, the Biomed-2 program (contract no. BMH4-CT95-1100) and the following European industrial associations: EAPA, Eurobitume and CONCAWE. Seema Singh provided valuable assistance in computerizing the data. Dr Göran Lidén helped with interpretation of the effects of sampling devices. Pamela Cruise edited the manuscript. The authors thank Torbjørn Jørgensen for his insightful comments. Jan Erik Lien, Arne Solhaug, Torbjørn Jørgensen, Olav Ruud, Leif Christensen, Torstein Hansen and Bjørn Langedal managed the original survey. The following individuals were responsible for collecting exposure measurements: Einar Njøs, Arvid Øygård, Gunnar Gjerdingen, Bjørn Langedal, Svein Bjørdal, Bjørn Eriksen, Svein Borgersen, Rolf Gravseth, Albert Skarstad, Egil Bakke, Olav Kleven, Ottar E. Heia, Thorbjørn Mortensen. The authors are deeply indebted to all the road paving crews and asphalt plant employees who participated in the survey.
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FOOTNOTES |
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* Author to whom correspondence should be addressed. Fax: +31-(0)30-253-5077; e-mail: h.kromhout{at}iras.uu.nl
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