Annals of Occupational Hygiene Advance Access originally published online on December 3, 2007
Annals of Occupational Hygiene 2008 52(1):9-21; doi:10.1093/annhyg/mem056
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Published by Oxford University Press on behalf of the British Occupational Hygiene Society
Ultrafine and Respirable Particles in an Automotive Grey Iron Foundry
1 Division of Applied Research and Technology, National Institute for Occupational Safety and Health, 4676 Columbia Parkway, MS-R3, Cincinnati, OH 45226, USA
2 Department of Occupational and Environmental Health, University of Iowa, 102 IREH, 100 Oakdale Campus, Iowa City, IA 52242-5000, USA
3 International Truck and Engine Corporation, 4201 Winfield Road, Warrenville, IL 60555, USA
* Author to whom correspondence should be addressed. Tel: +1-319-335-4213; fax: +1-319-335-4225; e-mail: william-heitbrink{at}uiowa.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTS AND FINDINGSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Ultrafine particle number and respirable particle mass concentrations were measured throughout an automotive grey iron foundry during winter, spring and summer using a particle concentration mapping procedure. Substantial temporal and spatial variability was observed in all seasons and attributed, in part, to the batch nature of operations, process emission variability and frequent work interruptions. The need for fine mapping grids was demonstrated, where elevations in particle concentrations were highly localized. Ultrafine particle concentrations were generally greatest during winter when incoming make-up air was heated with direct fire, natural gas burners. Make-up air drawn from roof level had elevated respirable mass and ultrafine number concentrations above ambient outdoor levels, suggesting inadvertent recirculation of foundry process emissions. Elevated respirable mass concentrations were highly localized on occasions (e.g. abrasive blasting and grinding), depended on the area within the facility where measurements were obtained, but were largely unaffected by season. Particle sources were further characterized by measuring their respective number and mass concentrations by particle size. Sources that contributed to ultrafine particles included process-specific sources (e.g. melting and pouring operations), and non-process sources (e.g. direct fire natural gas heating units, a liquid propane-fuelled sweeper and cigarette smoking) were additionally identified.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND FINDINGSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Increased cardiopulmonary morbidity and mortality has been associated with exposure to ambient fine particulate matter, usually measured as the mass concentration of particles <2.5 µm (Dockery et al., 1993; Samet et al., 2000; Brook et al., 2004). Evidence suggests that ultrafine particles (diameter < 100 nm) may drive this relationship because they may be considerably more toxic than larger particles found in ambient particulate matter (Oberdörster, 2001; Donaldson et al., 2002; Oberdorster et al., 2002; Englert, 2004; Gilmour et al., 2004; Nel, 2005). Although number concentration is often dominated by ultrafine particles, their mass is often negligible compared to that of larger particles (Maynard, 2003). Consequently, regulatory standards based on mass concentration may not be protective of individuals from either an environmental or an occupational health perspective.
Ultrafine particle exposures may be generated by ‘hot’ processes (Vincent and Clement, 2000), such as welding (Brouwer et al., 2004), and by combustion, such as burning of fuel in internal combustion engines (Ramachandran et al., 2005). Consequently, exposure to ultrafine particles may be common in industrial workplaces where these processes occur. Möhlmann and Riediger (Riediger and Möhlmann 2001; Möhlmann, 2005) observed elevated ultrafine particle concentrations ranging from 2 x 104 to 4 x 107 particles cm–3 near various processes including plasma cutting, metal inert gas and tungsten inert gas welding, metal grinding, aircraft maintenance, brazing, food preservation (smoking), smelting and laser ablation. During primary aluminium production, Thomassen et al. (2006) identified ultrafine particle concentrations ranging anywhere from 1.5 x 103 to 8.0 x 104 particles cm–3 in a crane operator's cab receiving clean filtered air, to 1.0 x 107 particles cm–3 adjacent to anode changing operations within a pre-bake potroom.
The concentration, composition and size of particles may differ substantially depending on their underlying source. For example, in automotive machining environments, wet machining, dry machining and grinding with straight oils may produce markedly different aerosol properties (Dasch et al., 2005). In addition, ultrafine particles produced at relatively high concentration from non-process sources, such as direct-fired natural gas heating systems (Peters et al., 2006; Heitbrink et al., 2007), may mask process-related sources. Foundry operations include numerous potential sources of ultrafine particles which may range from melting, pouring and shakeout (Chang et al., 2005) to metalworking activities such as grinding (Zimmer and Maynard, 2003) and welding (Zimmer and Biswas, 2001; Brouwer et al., 2004). In consequence, adverse health outcomes from potentially diverse aerosol exposures may therefore vary substantially. A careful assessment of aerosol emissions and underlying sources is therefore required within these workplace environments.
Objectives
With automated processes and manufacturing operations, an understanding of contaminant emission sources is required before planning to assess worker exposure to ultrafine and respirable particles. Understanding emission sources is also critical to implementing effective and targeted control strategies to reduce potential exposures. The primary objective of this work was to identify notable emission sources in a grey iron foundry. This objective was achieved through the mapping of ultrafine particle number and respirable mass concentrations. A secondary objective was to obtain number and mass concentrations according to particle size for identified sources.
Facility and process descriptions
Measurements were conducted within a 0.5 million square feet automotive grey iron foundry located in Indianapolis, IN, USA. Three work shifts poured >700 tons of iron alloy per day, sufficient to supply three diesel engine machining and assembly facilities with cast cylinder blocks and heads. Although not physically partitioned, the foundry was divided into the following three distinct operational areas.
Core and mould area.
Cores and moulds functioned as temporary internal voids and the outermost form of a casting, respectively, as hot molten alloy was poured into the assembly. Thus, the final shape of the casting was determined by these internal and external components. As castings sufficiently cooled and solidified, both mould and core materials were disintegrated and removed. Cores were made of new silica sand, coated with a resin, mechanically pressed into shape and cured by forcing a catalyst gas through it. External moulds were formed of ‘green sand’ composed of reconditioned sand, sea coal, clay binder and sufficient moisture to hold its shape when compressed. Presses were occasionally cleaned with compressed air and patterns were sometimes sprayed with a mould release agent. Cores were coated on appropriate surfaces in automated processes, with a water-based wash containing aluminium and magnesium silicates in addition to small amounts of silica and carbon. Components were subsequently conveyed through an electrically heated oven to bake the core wash, which primarily served to prevent burn-in of the sand by the molten alloy during pouring. Core and mould components were fully assembled and transported to the pour operations by conveyors.
A worker's breakroom was situated next to the core–mould area. During breaks, distinctive cigarette smoke aromas were evident on occasions in this locale. Furthermore, several small gas-supplied heating units were also noted close to the roof within this area and served to temper incoming make-up air during cold periods.
Melt and pour area.
Iron ingots, scrap metal and additives were charged into any of eight induction furnaces and melted to produce molten grey iron alloy. The molten alloy from the furnace was decanted into vessels, or ladles, mounted to overhead cranes that transported the molten material to holding furnaces, which in turn transferred the iron into either an automated pouring machine (the rotopour) or manually manipulated ladles for hand pouring. Alloy was poured into mould assemblies to form engine blocks, cylinder heads and other engine components. Upon pouring, components were moved along enclosed, ventilated conveyers to the shakeout–cleaning area.
Although exact alloy composition was proprietary information, cast components met specifications for ASTM (American Society for Testing and Materials) class 35 grey iron designation. SAE (Society of Automotive Engineers) grades G3500 or G4000 grey cast iron may achieve ASTM class 35. With iron as the matrix, base compositions for these grades may range from 3.0 to 3.5% total carbon, 0.6 to 0.9% manganese, 1.3 to 1.8% silicon, together with relatively minor components of phosphorus and sulphur (ASM, 1983).
During furnace charging, fugitive emissions were observed to rise with thermal plumes, towards large exhaust vents within a cupola at roof level. Emissions were particularly noticeable during furnace charging and during pouring when furnaces were tilted to transfer molten alloy into crane-mounted ladles. During normal operation, some fume avoided capture by local exhaust ventilation shrouds mounted to the lid of the furnaces. However, tilting the furnace during pouring caused obvious emissions into the workplace. Upon pouring the molten alloy (1430–1445°C) into the mould assembly, visible fume and a lazy flame were observed from combustion and thermal degradation of components within the core–mould assemblies. Although local exhaust ventilation was situated to capture most of these emissions from the pouring process, some fugitive emissions occurred.
Shakeout and cleaning area.
Once iron was poured into the mould assembly, it was conveyed along a lengthy enclosed path where it cooled while continuing to give off fugitive emissions. Although most sections of ventilated enclosure appeared to control emissions, some small sections provided incomplete dust control and noticeable emissions. When sufficiently solidified and cooled, the mould assembly was moved to a vibrating conveyor that separated the mould and core material from the casting. However, the ventilation system for the conveyor did not completely capture emissions. Emissions appeared to consist of partial combustion products (observed as smoke) and dust generated by mechanically separating the mould and core materials from the castings. In addition, several small gas-supplied heating units were also noted close to the roof within the shakeout–cleaning area. Castings underwent further cleaning by automated shot blasting followed by an automated coarse, dry grinding to remove unwanted cast material. Finally, cast components were visually inspected and, if needed, additional manual chipping and grinding was performed.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND FINDINGSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Particle concentration mapping
Ultrafine particle number and respirable mass concentrations were measured throughout the facility following the mapping procedures of Peters et al. (2006). Briefly, a condensation particle counter (CPC, Model 3007, TSI Inc., Shoreview, MN, USA) was used to measure the number concentration of particles with diameters from 0.01 to 1 µm. The upper concentration limit of the CPC was extended by using a dilutor consisting of a modified high efficiency particulate air filter cartridge with a pre-drilled end cap (Peters et al., 2006). An optical particle counter (OPC, PDM-1108, Grimm, Ainring, Germany) was used to measure particle number concentration of particles from 0.3 to 20 µm. Respirable particle mass concentrations CResp were computed as follows:
 | (1) |
- fi = fraction of respirable particles in channel i computed per American Conference of Governmental Industrial Hygienists (ACGIH) criteria (ACGIH, 2007),
- di = geometric mean of the upper and lower boundaries for channel i (µm),
- Ci = particle number concentration in channel i of the OPC and
= assumed unit density of 1 g cm–3.
- di = geometric mean of the upper and lower boundaries for channel i (µm),
Ultrafine particle number concentrations (NUFP) were computed as follows:
 | (2) |
- NCPC = particle number concentration measured with the CPC.
As the lower boundary from the OPC's first channel was 0.3 µm and the upper boundary of the fifth channel was 1 µm, NUFP is strictly the number concentration of particles in the 0.01- to 0.3-µm range. However, subsequent measurements of particle number concentration by size presented here illustrate that particle counts are generally dominated by ultrafine particles (<0.1 µm), with particles in the 0.1- to 0.3-µm range subsequently comprising a relatively small fraction. Therefore as a practical matter, NUFP, is essentially the number concentration of ultrafine particles (<0.1 µm).
In addition to the aerosol instruments described, an indoor air quality monitor (Q-Track Plus, Model 8554, TSI Inc.) was used to provide temperature, relative humidity, CO2 and CO data during spring and summer.
During the winter mapping in December 2004, data for the maps were collected over two consecutive days. This consisted of two coarse sampling grids on the first day with between 59 and 102 sampling locations and one fine sampling grid on the second day, with 192 locations. During the summer mapping in August 2005, one mapping session was conducted per day, for three consecutive days and the three mapping exercises involved between 54 and 247 locations. One additional coarse mapping event (98 locations) took place during late March 2005, and provided useful insights into particle source identification. However, the spring mapping event was not included in any statistical analyses, as one single-mapping event in spring cannot be statistically compared with equal weighting to three mapping events in winter and summer.
Comparison of OPC-estimated to gravimetrically measured respirable mass concentration
Gravimetric samples were collected concurrently with the real-time aerosol measurements during mapping events and at fixed locations in the foundry with a respirable sampler (4.2 Lpm, GK2.69 Respirable Cyclone, BGI, Waltham, MA, USA). Battery-operated pumps (Universal Sampler pumps, Model 224-PCXR4, SKC, Eighty-Four, PA, USA) were used to draw air through the filter-based mass samplers at their specified flow rates. Pumps were pre-calibrated with a flow calibrator (tri-Cal, BGI). If the sampling flow was deviated by >5% from the initial set point, the pump ceased sampling and displayed the elapsed time. Filters from these samplers were analysed gravimetrically using National Institute for Occupational Safety and Health (NIOSH) method 0600 (NIOSH, 1998).
Particle size distribution measurements
During summer, particle size distributions were measured with an electrical low pressure impactor (ELPI, Dekati Ltd, Tampere, Finland) and the OPC, together with a CPC for ultrafine particle concentration measurements, at a select number of locations according to Fig. 4 and described in Table 3. ELPI particle number concentrations by size were measured every second in 12 channels from 0.007 to 10 µm over a period of at least 5 min, as a single event at each location. Particle mass concentration by size was derived from OPC data, as the OPC had a larger upper particle size limit than the ELPI and the OPC had been routinely used to derive respirable mass concentrations in the prior mapping events and in our prior studies (Peters et al., 2006; Heitbrink et al., 2007). Furthermore, mass concentrations obtained from the ELPI may be adversely influenced by the diffusion of smaller particles to the uppermost stages of the ELPI cascade impactor, since by particle mass, particles of larger diameter carry a much greater weighting. This phenomenon has been noted when monitoring combustion aerosols of small size, but at relatively high concentrations (Maricq et al., 2000, 2006; Gulijk et al., 2004; Kinsey et al., 2006).
Statistical analyses
Mapping exercises resulted in multiple concentration measurements within each area. Analyses of variance (ANOVAs) (Proc GLM, SAS, Cary, NC) were conducted to determine whether season (winter or summer) or area (core–mould, melt–pour and shakeout–cleaning) or their interaction influenced ultrafine particle number or respirable mass concentration. All statistical procedures were conducted with the geometric mean of log-transformed concentrations within each area for each mapping event to avoid issues of spatial and temporal autocorrelation. Season was subsequently found not to influence respirable mass concentration; therefore, winter and summer data were combined for these analyses and Tukey's studentized range test multiple comparison test, using the variance estimated from the ANOVA, was used to examine differences between the mean log-transformed concentrations in the different areas. However, season was the only variable found to influence ultrafine particle number concentration; therefore, Tukey's test was not required.
The geometric mean ratio of OPC-derived estimates, divided by the measured gravimetric mass concentration, was computed along with upper and lower confidence limits. Regression analyses were used to evaluate whether OPC-estimated mass concentrations were predictive of those measured gravimetrically, according to the following model and regression coefficients and R2, the fraction of variability in CGrav that is explained by CResp was obtained:
 | (3) |
- Y = ln(CGrav) and CGrav is the mass concentration measured gravimetrically (mg m–3),
- X = ln(CResp) and CResp is the mass concentration estimated from the OPC (mg m–3),
- a and b are regression coefficients and
is the residual, the difference between the observed and modelled value of gravimetric concentration.
- X = ln(CResp) and CResp is the mass concentration estimated from the OPC (mg m–3),
The SAS General Linear Models Procedure was used to compute 95% confidence intervals about the values of the regression coefficients. This model was selected to evaluate whether the OPC estimates of respirable dust concentration were directly proportional to the gravimetric measurements.
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RESULTS AND FINDINGS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND FINDINGSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Comparison of OPC-estimated to gravimetrically measured respirable mass
Figure 1 provides a plot comparing respirable mass concentration derived from OPC estimates to those of gravimetric filter-based samples. In Fig. 1, the regression equation is expressed in terms of a power law model. From the regression analysis, the value of b and its 95% confidence interval is 0.96 ± 0.34. This indicates that the OPC response is, within the limits of experimental error, proportional to the gravimetrically measured concentration. The value of the regression coefficient a, and its 95% confidence interval was 0.74 ± 0.79. The value ea, of 2.1, is essentially the proportionality constant between the gravimetrically measured respirable dust concentration and the estimate of respirable dust concentration obtained from the OPC. The geometric mean ratio of respirable mass concentration obtained from the OPC to gravimetric measurements was 2.3 with upper and lower 95% confidence limits of 2.8 and 1.7, respectively. Thus, OPC response is proportional to respirable mass concentration, although the OPC underestimates the respirable mass concentration by a factor of approximately two.
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Particle concentration mapping
Tables 1 and 2 summarize data for ultrafine particle number and respirable mass concentration data, respectively, and Figs 2 and 3 present concentration maps of these metrics. Maps were constructed from data obtained during the winter, spring and summer mapping events as noted. Each map corresponded to a single data collection event, during which both particle number and respirable mass concentration were simultaneously measured at numerous locations.
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Ultrafine particle number concentration.
Ultrafine particle number concentration maps (Fig. 2) illustrate the high degree of temporal variability of ultrafine particle concentration within the foundry. These concentration maps have different peak concentrations during each mapping event. Ultrafine particle number concentration was significantly less in summer than in winter (P = 0.002), as visually evident in Fig. 2. However, process area and the interaction between process area and season did not significantly affect geometric mean number concentration (P = 0.24 and 0.14, respectively) and therefore, the results of Tukey's test were not required in Table 1, which presents the geometric mean of ultrafine particle number concentration by season and area.
Respirable mass concentration.
Respirable mass concentration maps (Fig. 3) indicated considerable variability between individual mapping events, with largely inconsistent localized elevated concentrations noted in the core–mould, melt–pour and shakeout–cleaning areas of the facility. ANOVA determined that both the season and the interaction between season and process area (P = 0.12 and 0.62, respectively) did not influence respirable mass concentration significantly. However, process area did influence respirable aerosol concentration (P = 0.007). A multiple comparison test on the combined data and the results of the multiple comparison tests are presented in Table 2 with the summary statistics for respirable mass concentration by season and area. Overall, the melt–pour area had a significantly higher respirable mass concentration than either the core–mould or the shakeout–cleaning areas.
Occupational exposure to respirable crystalline silica is a potential health concern and it is an important dust control consideration within this foundry. ACGIH (ACGIH, 2007) has stated a threshold limit value of 0.025 mg m–3 for respirable crystalline silica. However, this study was not conducted with an aim to assess worker's exposure to crystalline silica. Respirable mass concentrations presented in Table 2 and Fig. 3 ranged anywhere from 0.001 to 1.07 mg m–3, but it is unclear how crystalline silica or any other single constituent contributed to these estimated concentrations.
Particle size distribution measurements
Figure 4 provides measurement locations further described in Table 3. Figure 5a provides number concentration by size and Fig. 5b provides mass concentration by size for general locations, outdoor backgrounds and incoming make-up air. Figure 6a provides number concentration by size and Fig. 6b provides mass concentration by size for processes, sources and incoming make-up air. In general, particle number concentration by size differed according to the underlying particle generation process or location. Most mass distributions exhibited two modes, with one centred near 0.4 µm and the other at 
5 µm (Figs 5b and 6b). The measurement obtained near the abrasive blasting process (Fig. 6b) was an exception, with an additional mass mode centred on 1 µm, whereas other measurements tended to indicate particle mass concentration minima close to this particular particle size. This abrasive blasting process also exhibited elevated particle number concentrations (Fig. 6a) at larger particle diameters (i.e. >1 µm) compared to many other processes.
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Particle number and mass concentrations measured outdoors were substantially lower than any measurements taken within the foundry (see locations c and d in Table 3 and Fig. 5). Although the facility make-up air (location a in Table 3 and Figs 5 and 6) was the cleanest air sampled within the foundry, it appeared contaminated when compared to that measured outdoors at ground level (locations c and d in Fig. 5a,b).
Measurements obtained above the rotopour and within the plume of the propane sweeper had elevated particle number concentrations in the smallest channel (0.007–0.023 µm) measured by the ELPI (Fig. 6a). The sweeper contributed elevated ultrafine concentrations in the core–mould and melt–pour areas on occasions (see elevation in Fig. 2a winter map 1, top left hand corner). The rotopour operation generated the greatest particle concentrations by number or by mass, with three possible modes (<0.02, 0.035 and 0.2 µm) evident in the particle number by size distribution (Fig. 6a). In contrast, near other foundry processes, the smallest mode in the particle number by size distribution occurred in the next largest size channel (0.023–0.031 µm). The rotopour operation generated the greatest particle concentrations by number or by mass, with three possible modes (<0.023, 0.035 and 0.2 µm) evident in the particle number by size distribution (Fig. 6a). Compared to particle number concentrations in the incoming make-up air, the concentrations measured on the melt deck were substantially elevated for all particles sizes (Fig. 6a).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND FINDINGSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Comparison of OPC-estimated to gravimetrically measured respirable mass
Respirable mass concentration estimated with the OPC was substantially less than, but proportional to, that measured with the gravimetric samplers. Simply stated, the OPC underestimated respirable mass concentration by a factor of 2.3 based upon the geometric mean ratio of OPC estimate of respirable mass concentration to gravimetrical measured respirable mass concentration. A similar 2-fold underestimate was observed in an engine machining and assembly facility, where metalworking fluid mists were encountered (Heitbrink et al., 2007). Optical aerosol instrument response is known to vary with particle properties (Gebhart, 2001), including density, morphology, refractive index and other optical properties. These particle properties were likely to have varied widely throughout the foundry, influenced by predominant particle sources. Nevertheless, OPC estimates of respirable mass concentration provide useful indications of ‘relative’ mass concentration within workplaces, where their cost, portability and rapid measurement capability make them amenable to use in applications, where relative concentrations are needed.
Particle concentration mapping
Concentration mapping is an exploratory and measurement intensive procedure to illustrate contaminant variability within a workplace but is unlikely to find routine practice in occupational hygiene, due to the extensive data collection and subsequent processing involved. However, concentration maps may be useful as powerful visual aids in informing facility staff and management of high concentration areas where control measures need improvement. Concentration maps may also indicate the relative magnitude and the extent that contaminants migrate from their original point of emission to other areas of the plant. Thus, concentration maps may be useful for planning exposure-assessment studies and for estimating personal exposures to ultrafine particles in the absence of exposure measurements and where worker location is known. Presently, personal exposure monitors for ultrafine particles are not widely available and workplace ultrafine particle concentrations may span perhaps four orders of magnitude (i.e. 1 x 103 to 1 x 107 particles cm–3).
The utility of mapping will largely depend upon the consistency of the plant's operations. For intermittent batch operations, different mapping events revealed noticeably different areas of peak concentration as illustrated by the respirable mass concentration maps in summer (see Fig. 3) and ultrafine particle concentration maps in Fig. 2. In addition, others have reported that foundry emissions vary considerably even within the same process (Chang et al., 2005). A combination of several mapping events reported here aided in identifying the majority of particle emission sources within the foundry facility, their relative positions and magnitudes and extent of contaminant migration from the point of emission. Although with hindsight, this may have been improved with finer sampling grids, particularly at source locations.
Emission sources
Concentration mapping revealed several obvious and less obvious sources of both ultrafine and respirable particulate. Melting and pouring of alloy into moulds created obvious emissions of respirable and ultrafine particulate matter. Pouring molten alloy into transfer ladles created a visible plume of freshly generated fume which rose towards the roof of the foundry into a cupola, where exhaust fans were located. The pouring of molten alloy into moulds caused combustible materials in the mould assembly to burn, releasing thermal decomposition products. Although engineering controls (enclosures with exhaust ventilation) were present, these did not appear to completely capture fugitive emissions. For example, carbon monoxide measurements taken upon the rotopour deck (20 ppm, the highest reading attained) suggested that particle plumes were likely influenced by partial combustion. Chang et al. (2005) reported that a pouring operation together with subsequent shakeout contributed to the highest concentration emissions for both gases and particles. The ultrafine particle concentration in the plume from the rotopour was 8.4 x 105 particles cm–3, which is likely an underestimate, since the ambient temperature of 45°C exceeded the maximum operating temperature of 35°C for the CPC (TSI, 2004). In addition, the number and mass concentrations by size measured at the rotopour exceeded all others. In Fig. 6a, the particle number by size distribution for the fume generated at the rotopour appeared to possess three modes. The smallest and dominant mode (<0.023 µm) was likely composed of freshly nucleated particles, some indication of a possible intermediate mode, observed at 0.04 µm, and the largest size mode (0.2 µm) likely formed by condensation of volatiles onto pre-existing particles.
Alloy melting was an obvious source of ultrafine particles. In Table 3, this resulted in the highest ultrafine particle number concentration observed, 1.6 x 106 particles cm–3, as measured with CPC and OPC. On the melt deck, the mode in the particle number by size distribution occurred on the 0.023- to 0.0301-µm impaction stage as opposed to the 0.007–0.023 µm for the rotopour operation. On the melt deck, however, measurements could not be conducted directly within the thermal plume, for safety considerations. Two major modes were present in the mass distributions measured on the melt deck (Fig. 6b). The mass mode associated with particles <1 µm closely matched that of the incoming make-up air. However, the mass mode composed of particles > 1 µm was substantially elevated above that of the incoming make-up air and, therefore, these particles were likely generated within the foundry facility. It is unclear whether these particles may have been generated by the melting and furnace operations alone or by other nearby processes, as no physical barriers existed between processes or operations.
Other less obvious sources of ultrafine particles were not directly related to foundry processes. Significant non-process-related particle sources included the intermittent operation of small roof-level direct gas-fired heating units during cooler months, to warm incoming make-up air, the use of a liquefied propane gas-fuelled sweeper and cigarette smoking. Direct gas-fired heating has been shown to generate substantial quantities of ultrafine particles (Peters et al., 2006; Heitbrink et al., 2007). The particles generated by these gas heaters were generally dispersed throughout the facility with prevailing air currents through forced ventilation, resulting in ultrafine particle number concentrations of 2 x 105 particles cm–3 during winter throughout the foundry, as compared to summer measurements of 0.7 x 105 particles cm–3 in the core–mould and in the shakeout–cleaning areas (see Table 1). In addition, the elevated particle number concentration spot, in the centre of the core–mould area (Fig. 2, winter mapping event 3), was due to a series of measurements taken in close proximity to an air duct, bringing contaminated air directly from a heating unit. In contrast, respirable mass concentrations did not vary with season with the operation of the gas-fired heaters, in this, or in our previous studies (Peters et al., 2006; Heitbrink et al., 2007) (see Table 2). Particle number and mass concentration by size measurements, for similar natural gas-fired heating units, are reported in Heitbrink et al. (2007).
The sweeper was periodically used to remove dust and debris deposits from accessible floor spaces and walkways. Together with high ultrafine particle number concentrations, the sweeper exhaust plume additionally contained elevated CO concentrations, with 105 ppm being the peak reading attained. Although an important non-process source of ultrafines, the sweeper was encountered only occasionally (for example, Fig. 2, winter mapping event 1, ultrafine elevation in top left hand corner). For particles <0.05 µm, ultrafine particle number concentrations in the sweeper's exhaust plume were much greater than prior background measurements (see Fig. 6a). In addition, small elevations in particle mass concentration by size also indicated that larger particle resuspension from the functioning sweeper may have also occurred. Staff at the facility are currently considering an electrical sweeper when a replacement for this vehicle is due.
In the vicinity of a worker rest area and nearby, a distinct aroma of cigarette smoke was evident. Ultrafine particle number concentrations were also simultaneously elevated. It is highly likely that this source was responsible for the ultrafine particle number concentration rise, in the area noted (Fig. 2, spring map, top centre). No other obvious ultrafine particle sources were apparent. Smoking has since been banned throughout this facility.
Particle size distributions measured in make-up air (Fig. 6a,b) were similar to those observed indoors and different from those measured outside the facility. This observation suggests that the make-up air may have been contaminated (both ultrafine and respirable mass) by inadvertent recirculation of air exhausted from the facility. The geometric mean of ultrafine particle number concentration in the mould/core area and in the shakeout–cleaning area were 7.0 x 104 and 7.1 x 104 particles cm–3, respectively, as compared to 6.7 x 104 particles cm–3 in the incoming fresh air (Table 4). In relatively clean areas of the plant, ultrafine aerosol concentration may be explained by the aerosol present in the supplied make-up air, in addition to the ingress of cleaner outdoor air, particularly during summer, due to open doors and windows.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND FINDINGSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Ultrafine particle number and respirable mass concentration varied both spatially and temporally within an automotive grey iron foundry, in contrast to virtual steady-state conditions encountered in an adjacent engine machining facility. Respirable mass concentrations ranged from 0.001 to 1.07 mg m–3 and the ultrafine particle number concentration ranged from 1.9 x 104 to 3.5 x 106 particles cm–3. Temporal variability was attributed to the batch nature of particle-generating activities such as melting and pouring, variability in process emissions and frequent work interruptions. Spatial and temporal variability require that particle mapping efforts be conducted on a more frequent and spatially resolved basis to accurately determine particle concentrations, identify sources and indicate the extent of contaminant migration from them.
Alloy melting and subsequent pouring operations were the primary sources of ultrafine particles within the foundry. Non-process foundry sources substantially influenced indoor ultrafine particle number concentration. Specific non-process aerosol sources included direct fire natural gas heaters, liquefied propane-fuelled sweeper trucks and cigarette smoking. Elevated ultrafine particle number concentrations in winter, when compared to summer, were primarily attributed to direct fire natural gas heaters. Specific operations contributing to respirable mass elevation included pouring, abrasive blasting/grinding and melting, although many non-specific operations appeared to contribute to elevating respirable mass concentrations in all three operational areas. In addition, inadvertent recirculation of foundry process emissions appeared to play a role in ultrafine particle number and respirable mass concentration elevations for facility make-up air drawn from roof level, when compared to ambient outdoor background.
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FUNDING |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND FINDINGSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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International Union, United Automobile, Aerospace and Agricultural Implement Workers of America (UAW); International Truck and Engine Corporation (to W.A.H. and T.M.P.).
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND FINDINGSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Additional logistical support and monitoring equipment was provided by NIOSH. The cooperation of staff at the foundry and in particular the authors’ primary point of contact, Sonny Painter, was greatly appreciated.
NIOSH Disclaimer—The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the NIOSH. Mention of product or company name does not constitute endorsement by the Centers for Disease Control and Prevention.
Received April 6, 2007; in final form October 9, 2007
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TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND FINDINGSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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