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Annals of Occupational Hygiene Advance Access originally published online on September 23, 2006
Annals of Occupational Hygiene 2006 50(8):813-819; doi:10.1093/annhyg/mel055
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© The Author 2006. Published by Oxford University Press on behalf of the British Occupational Hygiene Society

Collection, Validation and Generation of Bitumen Fumes for Inhalation Studies in RatsPart 3: Regeneration of Bitumen Fumes, Inhalation Setup, Validation

G. POHLMANN*, A. PREISS, W. KOCH, H. KOCK, M. ELEND and M. RAABE

Fraunhofer Institute of Toxicology und Experimental Medicine, Nikolai-Fuchs-Strasse 1 30625 Hannover, Germany

*Author to whom correspondence should be addressed. Tel: +49 511 5350 116; fax: +49 511 5350 155; e-mail: pohlmann{at}item.fhg.de


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION AND SUMMARY
 REFERENCES
 
Undertaking a chronic inhalation study on bitumen fume presents a challenge in terms of generating sufficient amounts of representative fume. The objective of the study described in this and in previous publications was to collect sufficient fume and use this to develop a laboratory-generated exposure atmosphere, for use in chronic inhalation toxicity studies in rats that resembles, as closely as possible, personal exposures seen in workers during road paving operation. To achieve this goal, atmospheric workplace samples were collected at road paving work sites and compared with bitumen fume condensate samples collected from the headspace of hot bitumen storage tanks. In Parts 1 and 2, we described the collection and analysis of workplace samples, the strategy for in-line extraction of a suitable fraction of bitumen fume collected from the headspace of a bitumen storage tank and the comparison of the collected condensate to the workplace samples. This paper (part 3) describes the regeneration of bitumen fume for inhalation and the exposure setup used for inhalation studies.

Keywords: asphalt • bitumen • chemical analysis • fume • polycyclic aromatic hydrocarbons • workplace


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 DISCUSSION AND SUMMARY
 REFERENCES
 
A complex mixture of hydrocarbon vapors and aerosols is normally found at asphalt road paving sites. This mixture consists of hydrocarbon species present in the environmental atmosphere, supplemented with bitumen emissions derived from the road paving activities. The hydrocarbon contributions originate not only from hot bitumen but also from resuspended dust, cigarette smoke, roadwork vehicles and other nearby sources. Looking closer at the process of bitumen fume formation, it is obvious that aerosol particles already present in the atmosphere have a strong influence on the gas-to-particle ratio and on the particle size, in which bitumen fume compounds are most likely to be found. Using a simple model, bitumen fumes are released from the hot asphalt mixture as gaseous compounds, and depending on their vapor pressure, these compounds will either tend to persist in the gaseous state or to condense as aerosol droplets. Since the workplace atmosphere already contains an amount of particles, vapors with low vapor pressure will predominantly condense onto these particles. The size of particles carrying bitumen fume components will therefore depend on the pre-existing environmental aerosols and furthermore will vary from workplace to workplace, depending on prevailing weather conditions.

The objective of the inhalation studies conducted was to investigate the effects of bitumen fume only. Combination effects caused by the above mentioned confounders were intended to be excluded. Since the confounders are responsible for the aerosol size distribution found at the workplace, a new rationale had to be applied for generating an appropriate size distribution for the inhalation experiments. In this context, constraints arising from two aspects had to be taken into consideration. On the one hand, a fume generation method had to be used that would allow for production of a consistent inhalation atmosphere for more than 2 years. On the other hand, the lung deposition probability had to be considered to ensure a reasonable particle deposition in rat lung.

Koch et al. (1993) developed a method that allows the generation of stable, fine aerosols under well-defined conditions. When used with bitumen fume condensate, this method is able to regenerate a fume with particles in the size range of 140 nm (mass median diameter), which was considered suitable for rat inhalation studies.


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 DISCUSSION AND SUMMARY
 REFERENCES
 
Bitumen fume generation
The method developed by Koch et al. (1993) for regeneration of bitumen fume basically uses a free jet to recondense hot vapor in a stream of cool air. Since in this system heat and mass transfer are mainly determined by the free jet, the particle generation process is very robust, is not sensitive to external factors and yields a stable fume in respect to its physical and chemical composition.

The bitumen fume test atmosphere is generated using the free jet principle by means of a laboratory setup developed at the Fraunhofer ITEM. In this apparatus (Fig. 1), liquid bitumen fume condensate is evaporated and recondenses on a large number of condensation nuclei also generated from the condensate by the apparatus. This leads to a highly dispersed aerosol phase in equilibrium with the corresponding vapor phase.


Figure 1
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  		Fig. 1 Setup of the fume generator.

 
In the free jet evaporation–condensation generator, a well-defined mass flux of vaporized material is issued at high velocity, through a nozzle, together with a carrier (nitrogen), into a stream of slowly flowing, cool air. An expanding turbulent jet forms, due to mixing of the surrounding air with the vapor stream. The bitumen vapor is cooled and diluted downstream of the nozzle as the jet develops. The initial vapor phase contains very fine seed particles, originating from the vapor generation process. The vapor is generated by first nebulizing the condensate, using a pneumatic nebulization nozzle, and subsequently evaporating the droplets in a heated tube. The seed particles result from lower volatility constituents in the condensate. Due to the nonlinear temperature behavior of the saturation concentration, the saturation ratio, S, goes through a maximum as a function of distance, x, from the nozzle. Depending on the temperature of the surrounding air in the generator and the mass flux of the vaporized material, supersaturation (S > 1) of the vapor is eventually achieved, leading to formation and growth of liquid aerosol droplets by condensation of the vapor phase on the seed particles.

The setup of the fume generator is shown in Fig. 1. A peristaltic pump, driven by a stepping motor, maintains the pump feed rate of the bitumen fume condensate. For the nose-only system used in the inhalation studies and the concentrations envisaged, the feed rate has to be of the order of 3 ml h–1, or even smaller. The condensate is pumped via a stainless steel tube directly into the pneumatic dispersion nozzle. This nozzle is operated with nitrogen at a flow rate of 5 l min–1 and generates droplets with a mean diameter of ~6 µm. The droplets are fed directly into a tube heated at ~220°C, where they evaporate. The vapor is then issued through the nozzle and is recondensed in the condensation section as described, by mixing with cool air.

Inhalation system and dilution
From the generator, the fume is directed through stainless steel tubes to the different inhalation units (Fig. 2). Flow resistors control the flow of bitumen fume to each inhalation unit. The flow rate through these resistors is maintained by keeping a constant pressure difference between the inhalation units and the generator by controlling the flow rate of the cooling air in the generator. The final concentrations are achieved by mixing the bitumen fume with dilution air, regulated by mass flow controllers. The whole inhalation system is microprocessor-controlled and supervised by a central computer.


Figure 2
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  		Fig. 2 Scheme of the inhalation setup. Flow resistors control the flow rate of the bitumen fume from the generator into the inhalation units.

 
The exposure to bitumen fume takes place in direct flow nose-only inhalation exposure systems as shown in Fig. 3. These inhalation exposure units are placed in closed laboratory hoods that are maintained at a constant pressure. Within each unit, fumes are supplied to each animal individually, and the exhaled air is exhausted immediately. Animals are placed around the exposure cylinder in tapered acrylic glass tubes with adjustable backstops. Nominal target concentrations at the different inhalation units are 4 (Unit 2), 20 (Unit 3) and 100 mg m–3 (Units 1 and 4). Concentrations are given in the so-called BIA equivalents and have to be multiplied by a factor of 1.66 to give the true concentrations; this factor was calculated by division of the gravimetrically determined concentration by the concentration determined using the BIA method (BIA Method 6305).


Figure 3
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  		Fig. 3 Partial view of the direct flow nose-only inhalation exposure system. The picture on the left shows the upper part of the unit with the fume inlet installed, the inhalation ports and, as an example, one acrylic glass tube installed. The picture on the right shows a top view of the inhalation unit with the fume inlet removed, providing a view of the inner inlet cylinder with the stainless steel tubes directed to the inhalation ports as well as the outer cylinder that serves as exhaust drain.

 
Monitoring of exposure atmosphere
Relative humidity (RH), temperature (T) and Total Organic Matter (TOM) of the inhalation atmosphere are monitored continuously for every inhalation unit, using commercially available RH-T sensors and flame ionization detectors (FID's). TOM and polyaromatic hydrocarbon (PAH) content are frequently determined by atmosphere sampling and subsequent chemical analysis. The atmosphere samples are generally collected and analyzed by means of an unused inhalation port of the inhalation exposure unit. For on-line TOM measurements, each inhalation unit is connected to an FID, via a heated Teflon tube.

TOM sampling is carried out following the method used by the German employers' liability insurance association (BIA Method 6305). The respective sampler system is used to collect substances occurring both as particles and vapor. The sampler is a closed face sampler, where the inlet air passes through an inlet cone that is directly attached to the sample port of the inhalation exposure unit. The sampling flow rate is set to 2 l min–1. The particle phase is collected on a 37 mm glass fiber filter and the gas phase is adsorbed in a cartridge containing 3 g XAD2, a macro-porous polystyrene-divinylbenzene copolymer. Before use, the XAD2 has to be purified using the following procedure. About 100 g XAD2 is dried at 50°C, rinsed overnight, decanted and dried twice using ethanol. This procedure is then repeated using trichloroethene. Afterwards, XAD is treated for 15 min in an ultrasonic cleaning bath and dried. Drying is accomplished using a rotary evaporator. Finally, the purified XAD2 is filled into the cassettes. The analysis following sample collection has been described in a previous publication (Preiss et al., 2006).

Sampling of PAH's is accomplished using a 250 ml dropping funnel filled with XAD2. The funnel is filled with a 2 cm layer of quartz wool followed by ~200 ml of XAD2 and a final 2 cm layer of quartz wool. The flow rate through the funnel is set to 5 l min–1. Sampling duration depends on the concentration and ranges from 1 (nominal concentration 100 mg m–3) to 6 h (nominal concentration 4 mg m–3). The PAH analysis again has been described in a previous publication (Preiss et al., 2006).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
DISCUSSION AND SUMMARY
 REFERENCES
 
Validation of chemical composition of regenerated fumes
To ensure that the relative chemical compositions in the different inhalation units are comparable and correspond to workplace measurements, prior to commencing the animal studies, the inhalation atmospheres were analyzed. Table 1 gives an overview of the results of PAH analysis of the inhalation atmospheres in the different inhalation units. Figure 4 shows the relative PAH content of the regenerated bitumen fume compared with the PAH content of the bitumen fume condensate.


Figure 4
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  		Fig. 4 Comparison of the PAH profiles of sampled bitumen fume condensate and fume sampled from inhalation atmosphere at the different inhalation units. (for analytical reasons, acenaphthylene and fluorene have been omitted).

 


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  		Table 1 Overview of the average PAH concentrations (ng/m3) found in the inhalation atmospheres of the different inhalation units

 
In Fig. 5, by way of example, the boiling point distributions for samples taken from inhalation Units 1 and 4 are compared with the boiling point distribution of the pooled bitumen fume condensate. The curves are in good agreement.


Figure 5
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  		Fig. 5 Comparison of the boiling point distributions of bitumen fume condensate sampled from bitumen tanks and fume sampled from inhalation units 1 and 4.

 
The TOM in the inhalation atmospheres was monitored before starting the study and during the exposure period. Mean concentrations (aerosol + vapor phase) over the 2 year study, determined by IR spectroscopy according to the BIA guideline, during the 6 h exposure periods were 4.1 ± 0.3 mg m–3 TOM for the low-dose, 20.6 ± 1.6 mg m–3 TOM for the medium-dose, and 104 ± 9.6 mg m–3 respective 98.7 ± 7.1 mg m–3 TOM for the high-dose groups. Taking into account the factor (1.66) between the absolute bitumen fume concentration and the bitumen fume concentration determined using the BIA method (BIA Method 6305), concentrations were 6.8 mg m–3 THC for the low-dose, 34.4 mg m–3 THC for the medium-dose and 172.5 mg m–3 (158 mg m–3) TOM for the high-dose groups. The total amount of bitumen fume condensate used for generation of the exposure atmosphere was 7768.5 g.

The aerosol generated is semivolatile in nature and in a dynamic equilibrium with the surrounding vapor phase. The partitioning between aerosol and vapor phase at the different inhalation units therefore depends on temperature at the units (in this case 22°C) and total fume concentration. The aerosol fraction generally decreases with increasing temperature and as is the case with the different dilutions at the inhalation units it decreases with decreasing total fume concentration. This is what actually is found for the different inhalation units shown by the results of the chemical analysis given in Table 2 and Fig. 6.


Figure 6
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  		Fig. 6 TOM as function of TOM in aerosol phase.

 


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  		Table 2 TOM concentrationsa and partitioning between aerosol and vapor phase

 
Physical characteristics of the regenerated fume
Size distributions in the individual units are measured using a scanning mobility particle sizer (SMPS, TSI Inc.). This type of instrument was selected for two reasons. First, it is capable of covering the size range expected for the aerosol to be measured. In addition, it has to be taken into account that the aerosol to be measured is in a dynamic equilibrium with the surrounding gas phase and, therefore, the influence of factors that might cause alterations to the gas phase during measurements needs to be minimized. For this reason, particle size analyzers that rely on impaction stages for smaller particles at very low pressure must be avoided.

Figure 7 shows the average of the number size distributions of the aerosol phase measured at the ports of the inhalation units during the 2 year chronic study [n = 11 (Unit 1), n = 22]. As can be seen from Fig. 8, the number concentration decreases linearly with mass concentration when diluted. From this graph it can also be deduced that for successive dilution, finally only the bitumen fume gas phase will remain. The same behavior can be observed when examining the size distributions shown in Fig. 7. Since in a mixture containing particles of different sizes but of the same composition, small particles tend to evaporate preferentially, the involved size distributions with decreasing concentration should be shifted towards larger median diameters, as is the case in Fig. 7.


Figure 7
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  		Fig. 7 Averaged size distribution curves (number) of the aerosol phase for the four different inhalation units (NMMD = number median mobility diameter).

 


Figure 8
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  		Fig. 8 Number concentration as function of aerosol phase TOM.

 

   DISCUSSION AND SUMMARY
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION AND SUMMARY
REFERENCES
 
There are only few data available about the particle size distribution of bitumen fumes found at workplaces. Fabries et al. (2000) reported the mass size distribution of bitumen fumes generated by heating bitumen in a test rig. They measured mass median diameters between 1.3 and 3.2 µm (concentration 5–100 mg m–3), indicating a heterogeneous condensation process in the test rig with only few condensation nuclei, resulting in few big particles carrying the aerosol mass. Bonnet et al. (2000) found a mass median diameter of ~0.5 µm for TPM target concentrations of 5 mg m–3. Binet et al. (2002) described a laboratory setup where the mass median aerodynamic diameter of airborne particles varied from 1.4 µm at a fume concentration of 5 mg m–3 to 3.2 µm at 100 mg m–3. Kriech et al. (2004) report number median diameters measured with an aerodynamic particle sizer ranging from almost the lower size range of the instrument (0.68 µm) up to 1 µm. Brandt et al. (1985) and Brandt and de Groot (1999) have described work on bitumen fume generated in a test rig and compared this fume with fumes sampled at workplaces, but did not present results on size distribution. The aerosol generated by the Fraunhofer ITEM method, if expressed as mass size distribution, has a mass median diameter in the range of 0.13 µm without much variation with concentration.

At workplaces, size distribution and partitioning between gas and aerosol phases for semivolatiles is strongly dependent on environmental conditions. At different bitumen fume concentrations, caused for example, by changing wind speeds or convective flows, the ratio of aerosol phase to vapor phase can be quite different (compare e.g. Ekström et al. 2001). Hence, the size distribution might also change considerably. Another factor that noticeably influences the size distribution is the concentration of other environmental aerosols of non-bitumen origin, since these aerosols may serve as condensation nuclei. Starting out from the same bitumen fume concentration at low condensation nuclei concentrations, more mass per nucleus is present than for higher concentrations and hence the particles grow larger. On the other hand, if the number concentration of condensation nuclei is larger, the median diameter of the resulting particles will be smaller. In summary, therefore, no typical aerosol size distribution for the bitumen fume at workplaces can be expected.

Figure 9 shows the deposition fraction of aerosol in the different compartments of the rat lung as a function of the particle diameter. For particles with diameters above ~2.5 µm, the most dominant deposition is in the head compartment. The lowest deposition for all compartments is ~1 µm. For particles of less than ~0.3 µm, deposition increases for all compartments. Table 2 shows the relative amount of aerosol phase to gas phase in the inhalation atmosphere for the different dose groups. The values lie well within the range found for workplace atmospheres. Because of all reasons given, we assume that the fume generated for the inhalation study represents a good model for inhalation studies with rats.


Figure 9
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  		Fig. 9 Deposition of aerosols in the rat lung, calculated with MPPD V1.0 (© CIIT& RIVM, 2002). The TB curve represents the deposition probability in the tracheo-bronchial region, whereas P represents the pulmonary region.

 
In summary, it can be concluded that
  • the composition of the inhalation atmosphere in all inhalation units compares well with that of the bitumen fume condensate,
  • since the bitumen fume condensate compares quite well with workplace personal monitoring samples, the composition of the inhalation atmosphere is comparable to that part of the workplace atmosphere that workers are exposed to at paving sites, which is derived from use of bitumen,
  • the particle size range of the aerosol phase is such that sufficient deposition in the rat lung can be expected.

Received March 29, 2006; in final form June 27, 2006


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION AND SUMMARY
 REFERENCES
 

BIA Method 6305: Messverfahren für Gefahrstoffe (Analysenverfahren), Kennzahl 6305: Bitumen–Dämpfe und Aerosole.

Bonnet P, Binet S, Brandt H, et al. (2000) Inhalation study on exposure to bitumen fumes, Part 1: Development and validation of the equipment. Ann Occup Hyg 44:15–29.[Abstract/Free Full Text]

Binet S, Bonnet P, Brandt H, et al. (2002) Development and validation of a new bitumen fume generation system which generates polycyclic aromatic hydrocarbon concentrations proportional to fume concentrations. Ann Occup Hyg 46:617–29.[Abstract/Free Full Text]

Brandt HCA, De Groot PC, Molineux MKB, et al. (1985) Sampling and analysis of bitumen fume. Ann Occup Hyg 29:27–29.[Abstract/Free Full Text]

Brandt HCA and de Groot PC. (1999) A laboratory rig for studying aspects of worker exposure to bitumen fumes. Am Ind Hyg Assoc J 60:182–29.[Web of Science][Medline]

Ekström L-G, Kriech A, Bowen C, et al. (2001) International studies to compare methods for personal sampling of bitumen fumes. J Environ Monit 3:439–29.[CrossRef][Web of Science][Medline]

Fabries JF, Wrobel R, Gorner P, et al. (2000) Comparison of particle-size distributions of bitumen fumes measured by aerosizer® and QCM impactor techniques. J Aerosol Sci 31:SI011–29.

Koch W, Windt H, Carrothers T. (1993) Generation of submicron aerosols in a free turbulent jet in International Workshop on the Synthesis and Measurement of Ultrafine Particles. (Delftse Universitaire PersIn Marijnissen JCM, Pratsinis S, Friedlander S (Eds.). , Delft)S.51–59 ISBN 90-6275-896-7.

Kriech AJ, Osborn LV, Wissel HL, et al. (2004) Total versus inhalable sampler comparison study for the determination of asphalt fume exposures within the road paving industry. J Environ Monit 6:827–29.[CrossRef][Web of Science][Medline]

Preiss A, Koch W, Kock H, et al. (2006) Collection, validation and generation of bitumen fumes for inhalation studies in rats. Part 1: workplace samples and validation criteria. Ann Occup Hyg in press.

Yocum MW, Saltzman AR, Douglas MS, et al. (1976) Extrinsic allergic alveolitis after Aspergillus fumigatus inhalation. Am J Med 61:939–45.[CrossRef][Web of Science][Medline]


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This Article
Right arrow Abstract Freely available
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