Ann. occup. Hyg., Vol. 46, No. 2, pp. 237-243, 2002
© 2002 British Occupational Hygiene Society
Published by Oxford University Press
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Effect on Blood Lead of Airborne Lead Particles Characterized by Size
1Department of Environmental Health, Korea National Open University, 169 Dongsung-Dong, Chongro-Ku, Seoul 110-791, Korea; 2Department of Environmental Health, School of Public Health, Seoul National University, 28 Yunkeun-Dong, Chongro-Ku, Seoul 110-799, Korea
Received 21 August 2000; ; in final form 4 December 2001;
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Worker exposure to airborne lead particles was evaluated for a total of 117 workers in 12 workplaces of four different industrial types in Korea. The particle sizes were measured using 8-stage cascade impactors worn by the workers. Mass median aerodynamic diameters (MMAD) were determined by type of industry and percentage of lead particles as a fraction of airborne lead (PbA) concentration was determined by particle size. Blood lead (PbB) levels of workers who matched airborne lead samples were also examined. A Scheffé’s pairwise comparison test showed that MMAD and the fractions of each of respirable particles and lead particles
1 µm relative to PbA varied greatly by the type of industry. The concentrations of lead particles 1 µm, which the Center for Policy Alternatives model assumes is relatively constant at 12.5 µg/m3, increased with increasing PbA concentration. In addition, a better correlation was detected between concentrations of particles 1 µm and concentrations of respirable lead particles (r = 0.82) than that between concentrations of small particles and PbA (r = 0.61). A simple linear regression indicated that PbB correlated better with respirable lead concentration (r2 = 0.35, P = 0.0001) than with PbA concentration and had a higher slope coefficient. Controlling for respirable lead concentration reduced the partial correlation coefficient between PbA concentration and PbB level from 0.56 to 0.20 (P = 0.053). The results indicate that the contribution of respirable lead particles to lead absorption would be greater than that of PbA. This study concludes that the measurement of PbA only may not properly reflect a worker’s exposure to lead particles with diverse characteristics. For the evaluation of a worker’s exposure to various types of lead particles, it is recommended that respirable lead particles as well as PbA be measured.
Keywords: lead particles; size distributions; lead standard; blood lead
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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In 1989 the Korean Ministry of Labor lowered the occupational health standard for airborne lead (PbA) from 150 to 50 µg/m3 as an 8 h, time-weighted average (TWA) concentration (Korean Ministry of Labor, 1989). In 1978 the US Occupational Safety and Health Administration (OSHA) established a permissible exposure limit (PEL) of 50 µg/m3 to limit worker blood lead (PbB) levels to 40 µg/100 g (Occupational Safety and Health Administration, 1978). The OSHA PEL was based on a model of the PbA concentration–PbB level relationship, modified by Ashford et al. of the MIT Center for Policy Alternatives (CPA) to include the effects of particle size and job tenure (Ashford et al., 1977). The CPA model assumes that the first 12.5 µg/m3 of PbA consists of lead particles
1 µm aerodynamic diameter (AD), while the remainder consists of particles 
1 µm, and that 37% of all lead particles 1 µm are deposited with 100% absorption efficiency. Based on these assumptions, the PbB distribution would result in 70% of lead-exposed workers having a PbB level <40 µg/100 g and only 6% would have >50 µg/100 g. These assumptions of the CPA model have been examined in several studies. Froines et al. (1986) reported that in the furnace area of primary lead smelters and the pouring area of brass foundries the mean PbB levels predicted from the actual size distribution of lead particles in the air were less than those predicted using the CPA model.
Hodgkins (1990) reported that in battery plants the cut-off concentration of PbA contributed by particles 1.0 µm AD was not 12.5 but 1 µg/m3 and that substantially lower PbB levels were predicted for a given PbA level.
Tsai et al. (1997) concluded that concentrations of lead particles 1 µm increased along with increasing PbA concentration. These results are contrary to the CPA assumption, which states that concentrations of lead particles 1 µm are relatively constant. The different findings among the studies cited above could be caused by dissimilarities between the industries or operations investigated, the sampling methods employed and/or the numbers of samples.
A better approach to resolve this issue would be to perform a large empirical study which correlates actual PbB level and lead concentration characterized by particle size in different work environments.
The specific objectives of this study were: (i) to investigate the size characteristics of lead particles present in worker breathing zones in different industries; (ii) to examine whether the CPA assumption that the concentration of lead particles 1 µm AD is constant is true; (iii) to examine whether the current occupational health standard, as PbA concentration, based on the CPA assumption, would be effective for monitoring worker exposure to lead.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Description of operations
Two secondary lead smelting plants, three radiator manufacturing plants, four lead-acid battery manufacturing plants and three lead powder manufacturing plants were studied.
In the secondary lead smelting plants, lead plates and paste are first separated by breaking up waste lead-acid batteries. Then they are conveyed to the top of the furnace and manually fed into the furnace, which is heated to 
1200°C for purification. Molten lead is poured semi-automatically into molds and made into lead ingots. All operations are conducted in the same area.
In the radiator manufacturing plants, a copper plate is first dipped into a tank that contains 15–30% lead in order to fill a gap between the cooling tube and the copper plate. Lead solution in the dipping tank is kept at >360°C to maintain the adhesive strength of the lead flux. The copper plate and the radiator head are attached by soldering with a stick containing 15–50% lead.
The process of manufacturing lead-acid batteries consists of several operations performed in separate departments. A lead plate is formed by casting molten metal. To form lead oxide, molten lead is added to a pot in which the lead is stirred by paddles. The plate is then pasted with a mixture composed of lead, lead oxide, sulfuric acid, water and expanders. The plates are dried and stacked to form the battery elements, with alternating negative and positive plates with an inert separator between each plate, welded to form groups, and finally charged.
The process of manufacturing lead powder (litharge, PbO) is similar to that described in Tsai et al. (1997). The litharge is formed in an isolated furnace heated to 600°C. Then it is dropped through a pipe and automatically weighed and packed. Workers in the furnace and packing operations were chosen for this study.
Sampling and analysis of airborne lead
Personal samples were taken from 117 workers randomly selected in the four types of lead industries. Samples were taken using a Marple personal cascade impactor (model 298; Anderson Sampler Inc., USA) with air drawn by personal pumps (model MSA 87004; MSA, USA). Sampling was carried out for the regular work duration of 8 h. The air sampling equipment was fitted to the subject on starting, removed or switched off during the break and finally removed at the end of the day. The cascade impactor had cut-off points (particle size for 50% collection) of 21.3, 14.8, 9.8, 6.0, 3.5, 1.55, 0.93 and 0.52 µm. The pump and sampling train were calibrated at 2 l/min before and after sampling. The Mylar substrate (Anderson stock C-290-MY; Anderson Sampler Inc.) was thinly coated with silicone grease (Dow Corning 316 Silicone Release Spray; Dow Chemicals, USA) to prevent lead particles bouncing off. The substrate and polyvinylchloride (PVC) back-up filter (5 µm pore size, Anderson stock F-290-p5; Anderson Sampler Inc.) for each stage was pretreated by microwave digestion (model MDS-2000; CEM Corp., USA). The lead mass was quantified using atomic absorption spectroscopy (model Spectra 300 plus; Varian Corp., Australia). The number of lead particles collected by each stage was corrected for internal particle loss using inlet sampling efficiencies as determined by Rubow et al. (1987). One field blank per 10 cascade impactor samples was taken to correct the results of the cascade impactor samples. In addition, a recovery test was performed to correct the analytical results. The recovery rates ranged from 98.6 to 102.5%. The Mylar substrates were spiked with an amount of lead corresponding to 0.1, 0.25, 0.5, 1 and 2 times the occupational health standard in an air volume of 400 l. External quality control was performed by participation in the inter-laboratory Proficiency Analytical Testing (PAT) quality program organized by the US National Institute for Occupational Safety and Health (NIOSH)/American Industrial Hygiene Association (AIHA). Our results were always evaluated as acceptable.
Lead particle size determination
From the size distribution of lead particles collected on the cascade impactor, the values of MMAD by industry were estimated by a log-probability plot of cumulative percentage versus AD. The PbA concentration was calculated by summing the collected lead particles at each stage and dividing the sum by the air volume sampled. The proportion of lead particles 1 µm was computed using a regression equation for the relationship between the cut-off diameter and the cumulative percentage of lead particles collected on each stage of the impactor.
Concentrations of respirable lead particles were calculated using the method proposed by Hinds (1986). The respirable mass fraction for each size interval of an impactor stage was estimated from the regression equation between collection efficiencies of respirable particulate mass (RPM) and AD defined by the American Conference of Governmental Industrial Hygienists (ACGIH) using a trapezoidal rule. The respirable concentration was calculated as a product of the respirable fraction and the PbA concentration.
Blood lead (PbB) test
Whole blood samples were taken in a lead-free vacuum tube (Monoject; Sherwood Medical), containing EDTA as anticoagulant, from 100 workers who matched the airborne lead samples. After pretreatment with a 1:4 solution of 0.1% Triton X-100 (scintillation grade; Merck) and 1.25% ammonium dihydrogen phosphate (puratronic grade; Merck) (Jacobson et al., 1991), lead concentration in whole blood (PbB) was analyzed by Zeeman effect graphite furnace atomic absorption spectrometry (Spectra AA-300/400 Zeeman; Varian). External quality control for blood lead was performed using the National Institute of Standards and Technology (NIST) standard reference material (SRM 955 B, 5.01–54.43 µg/dl). Accuracy ranged from 96.5 to 105.0%.
Data analysis
Statistical testing was carried out using SPSS 10.0 Standard Version. Scheffé’s pairwise comparison test was performed to classify each industry group by size characteristics and the fractions of lead concentrations in a given size range. Simple linear regression analyses were used to test the relationships between concentration of lead particles 1 µm, PbA and respirable lead concentrations. Relationships between PbB level and each of PbA, respirable particles and particles 1 µm were also tested. Partial correlation was evaluated to identify the relationship between PbA and PbB level, after controlling for respirable lead concentration.
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RESULTS AND DISCUSSION |
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Size distributions of lead particles
In furnace operation in the secondary lead smelting industry workers are exposed to lead particles with an average MMAD of 1.6 µm. Fine particles are generated when lead plates and the paste are smelted in the furnace and the molten lead is poured into molds.
Workers who solder radiators with air-propane gas in radiator manufacturing plants are exposed to lead with an average MMAD of 1.3 µm (see Table 1). The geometric standard deviation (GSD) of the MMAD was 5.0 in the secondary smelting plants and 9.6 in the radiator manufacturing plants, indicating a wide variation of lead particle sizes in the different industries. Froines et al. (1986) reported that the smelter furnace area and the pouring area of foundries have distinctly different lead size distributions, with high GSDs.
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In battery and lead powder manufacturing plants the average MMADs of lead particles are 14.1 and 15.1 µm, respectively. Tsai et al. (1997) reported that the ranges of MMAD in battery and lead powder manufacturing plants were 12.6–17.0 and 13–22 µm, respectively. The GSD of lead particles in all operations of the two industries ranged from 1.2 to 2.0. The results confirm that the size distribution of relatively coarse lead particles is narrower than that of fine particles (Froines et al., 1986; Tsai et al., 1997).
Our results are similar to those obtained in studies using cascade impactor-based instruments (Froines et al., 1986; Hodgkins, 1990; Tsai et al., 1997), but are relatively finer than those reported by Spear et al. (1998b), who investigated lead particle size distribution in a primary smelter using personal inhalable dust spectrometers (PIDSs). Spear et al. (1998b) concluded that PIDSs collect larger particles more efficiently than other cascade impactor-based instruments. A comparison study of samplers for size distribution seems to be necessary.
Employing Scheffé’s multiple comparison, a significant difference in MMAD between two industrial groups was detected. One group included secondary lead smelting and radiator manufacturing plants, which generate fine lead particles, including fumes. The other group included battery and lead powder manufacturing plants, which produce coarser lead particles. This finding is in agreement with the conclusions of Spear et al. (1998b), who found significant differences in lead particle size distribution between process areas, in particular that lead particles in blast furnace areas were generally finer than in the sinter plant.
It can be concluded that the MMAD of lead particles and the fractions of respirable and lead particles 1 µm relative to PbA vary greatly with the characteristics of the operation and type of industry.
PbA concentrations
Except for workers in radiator manufacturing plants, workers in secondary smelters, battery manufacturing plants and lead powder manufacturing plants were exposed to PbA concentrations >50 µg/m3, the Korean occupational health standard and OSHA PEL for airborne lead (see Table 1). In secondary smelters workers were exposed to an arithmetic mean (AM) of 653 µg/m3, which is over 10 times the standard. Since the workplaces were very small with 10 workers or less, control measures, such as local ventilation systems, were not properly installed and maintained. Papanek et al. (1992) reported that among the 112 lead-using companies in Los Angeles County, compliance with the OSHA PEL was particularly poor in small workplaces with 20 workers or less.
The PbA concentration AM in the radiator plants was 26 µg/m3, which is lower than the Korean occupational health standard. Because lead particles in these plants were very fine (see Table 1), the PbA concentration was not high. In Korea, since it was found that workers in radiator plants are exposed to PbA concentrations below the standard, inspections of the plants are usually ignored. Due to this fact, workers who repair or manufacture radiators are among the least monitored of lead-exposed workers: <1% receive routine PbB testing (Rudolph et al., 1990).
The nature and size of the lead particles will have significant implications for blood lead values (Froines et al., 1990). Because lead fumes generated from high temperature operations can be easily absorbed into the body, as compared with coarser lead particles, fumes may lead to an elevated PbB level.
Regression analysis showed that there was no relationship between particle size and PbA concentration in the four industries (r2 = 0.027, P > 0.05). This result is consistent with the work of Tsai et al. (1997), who reported that in battery, capacitor and lead powder plants MMAD was nearly independent of PbA concentration. The results are different from the assumption made in Froines et al. (1986) that the size distribution of a lead aerosol is constant with respect to PbA concentration over the range 50–200 µg/m3.
Lead particles 1.0 µm aerodynamic diameter
In high temperature operations, i.e. furnaces, soldering and dipping, the averages of the fractions were 37.4–44.0%. The results agree with those reported by Froines et al. (1986), who found that the average fraction of small lead particles 1.0 µm was 41.8% around the furnace of a primary smelter and 46.0% around the pouring operations of a brass foundry. This indicates that in high temperature operations fine lead particles make up a significant proportion of the PbA. On the other hand, the typical fraction of lead particles 1 µm in PbA in the battery and lead powder industries was <7%. These differences among industries were manifested in the results of a Scheffé’s pairwise comparisons test, that the fractions of lead particles 1.0 µm in secondary lead smelting and radiator manufacturing were significantly greater than those in the other two industries (P < 0.05).
The concentrations of lead particles 1.0 µm increased with increasing PbA concentration in the range 5.6–7741 µg/m3 (Fig. 1). Our finding from four industries is similar to that of Tsai et al. (1997)), who found that the concentration of lead particles 1.0 µm was not constant above a certain concentration but increased with increasing PbA concentration in the range 11–783 µg/m3. However, it differs from the study of Hodgkins (1990), who reported that lead particles 1.0 µm were relatively constant at 1.0 µg/m3 over the PbA concentration range 12.1–91.2 µg/m3 obtained from 40 personal cascade samples in the battery industry. Even the cut-off lead concentration point of 1.0 µg/m3 reported by Hodgkins is greatly different from the 12.5 µg/m3 assumed by the CPA model. In addition, lead particles 1.0 µm correlate better with respirable (r = 0.82) than with PbA concentation (r = 0.61) (Fig. 1). The results suggest that determination of the respirable fraction instead of PbA would be a better way to monitor worker exposure to fine lead particles.
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= 0.82) than with PbA (Respirable lead particles
Average fractions (%) of respirable lead particles in PbA by operation were 8.4–61.3%. High values, such as 61.3 and 48.6%, were found around the furnace operations of secondary smelters and the soldering operations of radiator manufacturing plants, respectively.
As discussed for MMAD and lead particles 1.0 µm, a Scheffé’s pairwise comparison test indicated that the fractions of respirable lead particles in PbA in secondary smelters and radiator manufacturing plants with high temperature operations were significantly higher than those in the other two industries, where coarse particles were generated (P < 0.05).
Respirable lead particles increased proportionately with increases in PbA concentration. In particular, battery and lead powder manufacturing plants, which generated relatively coarse lead particles with MMADs of 14.1 and 15.1 µm, respectively, the adjusted r2 values were 0.89 and 0.80, respectively. Concentrations of respirable lead particles were correlated with PbA concentrations for all data points (r2 = 0.77, P = 0.0001).
The effect of respirable lead particles on PbB level
The average PbB concentration was 38.6 µg/dl (range 7.3–113.5 µg/dl). The concentrations of PbA, respirable and small lead particle 1 µm were all regressed against PbB levels. All airborne lead concentrations were significantly associated with PbB level (P = 0.0001). The r2 for respirable lead particle concentrations was 0.35 and that for PbA concentration was 0.31 (see Table 2). In addition, the slope of the model for respirable concentration versus PbB (19.2) was >15.3 of airborne lead (see Table 2 and Fig. 2). The Pearson correlation coefficients between PbB and each of PbA and respirable lead particles were 0.55 and 0.56, respectively. Interestingly, the partial correlation coefficient between PbA and PbB decreased significantly to 0.20 (P = 0.053) after controlling for respirable lead concentration. Ignoring the linear effect as influenced by respirable lead particles could lead to a false conclusion. These results indicate that the contribution of respirable lead particles to lead absorption is greater than that of PbA.
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Welding, sand blasting, rivet busting, burning lead-painted structures and demolition work that generate fine lead particles have all been reported to cause an elevation in PbB levels in laborers and iron workers (Forst et al., 1997; Sokas et al., 1997). The health hazards associated with lead exposure may be appreciably enhanced in operations where finer particles are produced because finer sized particles appear to be more soluble, regardless of the bulk mineralogy of the dust (Ruby et al., 1992; Spear et al., 1998a). Recently, Reynolds et al. (1999) reported that participants who had ‘ever’ worked in demolition, burning paint and metal, and who had welded outdoors had higher PbB levels than those who had no experience in these activities.
The key assumption of the CPA model, that there is no deposition in the alveolar region of particles 1 µm, did not account for the 1–10 µm particles, so we assume instead that 1–10 µm particles are ingested and absorbed at 8% efficiency. It is generally accepted, however, that there is alveolar deposition of particles with AD in the range 1–10 µm (Froines et al., 1986; Hodgkins, 1990; American Conference of Governmental Industrial Hygienists, 2001), in particular, particles 3 µm in diameter. All species of lead compounds deposited in the alveoli are thought to be completely absorbed into the bloodstream (Barry, 1975; Morrow et al., 1980). A transition from occupational aerosol exposure limits based on an ill-defined airborne aerosol concept to new standards which reflect the particle size-selective nature of aerosol sampling and exposure has been suggested (Froines et al., 1986; Werner et al., 1996; Spear et al., 1998b; American Conference of Governmental Industrial Hygienists,, 2001).
The current method of sampling PbA may not effectively monitor worker exposure to fine lead particles that may be deposited in the gas exchange region of the lung and are absorbed with essentially 100% efficiency. Our study suggests that the occupational health standard for lead should be modified by including an assessment of respirable lead particles in addition to the assessment of PbA.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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The present occupational health standard employs sampling of PbA without consideration of particle size. In consideration of the diverse size characteristics of lead particles in various operations and types of industry, the measurement of respirable fractions is recommended. A further study is being performed to evaluate the effect of respirable lead particles on lead absorption; we are performing a multivariate analysis including person-related (e.g. hygiene behaviors) and work-related (e.g. work load, work practice) factors that might affect lead absorption.
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FOOTNOTES |
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* Author to whom correspondence should be addressed.
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