Biological Monitoring of Indium by Means of Graphite Furnace Atomic Absorption Spectrophotometry in Workers Exposed to Particles of Indium Compounds

Exposure profiles of workers from indium tin oxide target manufacturing and recycling factories in Taiwan

Indium tin oxide exposure poses a potential health risk, but the exposure assessment in occupational setting remains incomplete and continues to be a significant challenge. To this end, we investigated the association of work type, airborne indium concentration, respirable fraction of total indium, and cumulative indium exposure index (CEI) with the levels of plasma indium (P-In) and urinary indium (U-In) among 302 indium tin oxide target manufacturing and recycling workers in Taiwan. We observed that recycling-crushing produced the highest concentrations of total indium (area: 2084.8 μg/m³; personal: 3494.5 μg/m³) and respirable indium (area: 533.4 μg/m³; personal: 742.0 μg/m³). Powdering produced the highest respirable fraction of total indium (area: 58.6%; personal: 81.5%), where the workers had the highest levels of P-In (geometric mean: 2.0 μg/L) and U-In (1.0 μg/g creatinine). After adjusting for the confounder, the CEIs of powdering (β_PR = 0.78; β_PR = 0.44), bonding (β_PT = 0.61; β_PT = 0.37), and processing workers (β_PT = 0.43; β_PT = 0.28) showed significant associations with P-In and U-In, validating its utility in monitoring the exposure. Also, the respirable fraction of total indium significantly contributed to the increased levels of P-In and U-In among workers. The varying levels of relationship noted between indium exposure and the levels of P-In and U-In among workers with different work types suggested that setting the exposure limits among different work types is warranted.

The toxicology of indium oxide

Indium oxide (In₂O₃) is a technologically important semiconductor essentially used, doped with tin oxide, to form indium tin oxide (ITO). It is poorly soluble in all so far tested physiologic media. After repeated inhalation, In₂O₃ particles accumulate in the lungs. Their mobilization can cause significant systemic exposure over long periods of time. An increasing number of cases of severe lung effects (characterized by pulmonary alveolar proteinosis, emphysema and/or interstitial fibrosis) in workers of the ITO industry warrants a review of the toxicological hazards also of In₂O₃. The database on acute and chronic toxicity/carcinogenicity/genotoxicity/reproductive toxicity as well skin/eye irritation and sensitization is very limited or even lacking. Short-term and subchronic inhalation studies in rats and mice revealed persistent alveolar proteinosis, inflammation and early indicators of fibrosis in the lungs down to concentrations of 1 mg/m³. Epidemiological and medical surveillance studies, serum/blood indium levels in workers as well as data on the exposure to airborne indium concentrations indicate a need for measures to reduce exposure at In₂O₃ workplaces.

Application of the ICRP respiratory tract model to estimate pulmonary retention of industrially sampled indium-containing dusts

Inhalation of indium-containing dusts is associated with the development of indium lung disease. Workers may be exposed to several different chemical forms of indium; however, their lung dosimetry is not fully understood. We characterized the physicochemical properties and measured the lung dissolution kinetics of eight indium-containing dusts. Indium dissolution rates in artificial lung fluids spanned two orders of magnitude. We used the International Commission on Radiological Protection (ICRP) human respiratory model (HRTM) to estimate pulmonary indium deposition, retention and biokinetic clearance to blood. For a two-year (median workforce tenure at facility) exposure to respirable-sized particles of the indium materials, modeled indium clearance (>99.99% removed) from the alveolar-interstitial compartment was slow for all dusts; salts would clear in 4 years, sintered indium-tin oxide (ITO) would clear in 9 years, and indium oxide would require 48 years. For this scenario, the ICRP HRTM predicted that indium translocated to blood would be present in that compartment for 3.5-18 years after cessation of exposure, depending on the chemical form. For a 40-year exposure (working lifetime), clearance from the alveolar-interstitial compartment would require 5, 10 and 60 years for indium salts, sintered ITO and indium oxide, respectively and indium would be present in blood for 5-53 years after exposure. Consideration of differences in chemical forms of indium, dissolution rates, alveolar clearance and residence time in blood should be included in exposure assessment and epidemiological studies that rely on measures of total indium in air or blood to derive risk estimates.

Comparing plasma, serum and whole blood indium concentrations from workers at an indium-tin oxide (ITO) production facility

OBJECTIVE

Occupational exposure to indium compounds including indium-tin oxide (ITO) can result in potentially fatal indium lung disease. We compared plasma, serum and whole blood indium concentrations (In_P, In_S and In_B) from workers at a single ITO production facility to assess the comparability of these matrices used for biological monitoring of indium exposure.

METHOD

In_P, In_S and In_B were measured using inductively coupled mass spectrometry from consenting workers at an ITO production facility with specimen collection occurring during June-July 2014. Matched pairs from workers were assessed to determine the matrix relationships using the Pearson correlation, paired t-tests, per cent difference, linear regression and κ statistics.

RESULTS

Indium matrices were collected from 80 workers. Mean (SD) In_P, In_S and In_B were 3.48 (3.84), 3.90 (4.15) and 4.66 (5.32) mcg/L, respectively. The In_S-In_P difference was 14%; In_S was higher in all but two workers. In_P and In_S were highly correlated (r=>0.99). The In_B-In_S difference was 19%; In_B was higher in 85% of workers. The In_B-In_P difference was 34%; In_B was higher in 66% of workers. In_B was highly correlated with both In_P and In_S (r=0.97 and 0.96, respectively). κ Statistics were 0.84, 0.83 and 0.82 for In_P, In_S and In_B, respectively, for individuals with each matrix ≥1 mcg/L (p<0.01).

CONCLUSIONS

While all matrices were highly correlated, we encourage the use of In_P and In_S to reliably compare studies across different populations using different matrices. The higher per cent difference and increased variability of In_B may limit its utility in comparisons with In_P and In_S in different populations.

The toxicology of indium tin oxide

Indium tin oxide (ITO) is a technologically important semiconductor. An increasing number of cases of severe lung effects (characterized by pulmonary alveolar proteinosis and/or interstitial fibrosis) in ITO-exposed workers warrants a review of the toxicological hazards. Short- and long-term inhalation studies in rats and mice revealed persistent alveolar proteinosis, inflammation and fibrosis in the lungs down to concentrations as low as 0.01mg/m(3). In rats, the incidences of bronchiolo-alveolar adenomas and carcinomas were significantly increased at all concentrations. In mice, ITO was not carcinogenic. A few bronchiolo-alveolar adenomas occurring after repeated intratracheal instillation of ITO to hamsters have to be interpreted as treatment-related. In vitro and in vivo studies on the formation of reactive oxygen species suggest epigenetic effects as cause of the lung tumor development. Repeated intratracheal instillation of ITO to hamsters slightly affected the male sexual organs, which might be interpreted as a secondary effect of the lung damage. Epidemiological and medical surveillance studies, serum/blood indium levels in workers as well as data on the exposure to airborne indium concentrations indicate a need for measures to reduce exposure at ITO workplaces.

Effects of a powered air-purifying respirator intervention on indium exposure reduction and indium related biomarkers among ITO sputter target manufacturing workers

This study aimed to evaluate the efficacy of powered air-purifying respirators (PAPRs) worn by the workers, and to investigate the effect of this application on exposure and preclinical effects in terms of workplace measuring and biomarker monitoring in ITO sputter target manufacturing plants and workers, respectively. Fifty-four workers were recruited and investigated from 2010-2012, during which PAPRs were provided to on-site workers in September 2011. Each worker completed questionnaires and provided blood and urine samples for analysis of biomarkers of indium exposure and preclinical effects. Area and personal indium air samples were randomly collected from selected worksites and from participants. The penetration percentage of the respirator (concentration inside respirator divided by concentration outside respirator) was 6.6%. Some biomarkers, such as S-In, SOD, GPx, GST, MDA, and TMOM, reflected the decrease in exposure and showed lower levels, after implementation of PAPRs. This study is the first to investigate the efficacy of PAPRs for reducing indium exposure. The measurement results clearly showed that the implementation of PAPRs reduces levels of indium-related biomarkers. These findings have practical applications for minimizing occupational exposure to indium and for managing the health of workers exposed to indium.

Macrophage solubilization and cytotoxicity of indium-containing particles as in vitro correlates to pulmonary toxicity in vivo

Macrophage-solubilized indium-containing particles (ICPs) were previously shown in vitro to be cytotoxic. In this study, we compared macrophage solubilization and cytotoxicity of indium phosphide (InP) and indium-tin oxide (ITO) with similar particle diameters (∼ 1.5 µm) and then determined if relative differences in these in vitro parameters correlated with pulmonary toxicity in vivo. RAW 264.7 macrophages were treated with InP or ITO particles and cytotoxicity was assayed at 24 h. Ionic indium was measured in 24 h culture supernatants. Macrophage cytotoxicity and particle solubilization in vitro were much greater for InP compared with ITO. To correlate changes in vivo, B6C3F1 mice were treated with InP or ITO by oropharyngeal aspiration. On Days 14 and 28, bronchoalveolar lavage (BAL) and pleural lavage (PL) fluids were collected and assayed for total leukocytes. Cell differentials, lactate dehydrogenase activity, and protein levels were also measured in BAL. All lavage parameters were greatly increased in mice treated with InP compared with ITO. These data suggest that macrophage solubilization and cytotoxicity of some ICPs in vitro are capable of predicting pulmonary toxicity in vivo. In addition, these differences in toxicity were observed despite the two particulate compounds containing similar amounts of indium suggesting that solubilization, not total indium content, better reflects the toxic potential of some ICPs. Soluble InCl3 was shown to be more cytotoxic than InP to macrophages and lung epithelial cells in vitro further suggesting that ionic indium is the primary cytotoxic component of InP.

Macrophage solubilization and cytotoxicity of indium-containing particles in vitro

Indium-containing particles (ICPs) are used extensively in the microelectronics industry. Pulmonary toxicity is observed after inhalation exposure to ICPs; however, the mechanism(s) of pathogenesis is unclear. ICPs are insoluble at physiological pH and are initially engulfed by alveolar macrophages (and likely airway epithelial cells). We hypothesized that uptake of ICPs by macrophages followed by phagolysosomal acidification results in the solubilization of ICPs into cytotoxic indium ions. To address this, we characterized the in vitro cytotoxicity of indium phosphide (InP) or indium tin oxide (ITO) particles with macrophages (RAW cells) and lung-derived epithelial (LA-4) cells at 24h using metabolic (3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide) and membrane integrity (lactate dehydrogenase) assays. InP and ITO were readily phagocytosed by RAW and LA-4 cells; however, the particles were much more cytotoxic to RAW cells and cytotoxicity was dose dependent. Treatment of RAW cells with cytochalasin D (CytoD) blocked particle phagocytosis and reduced cytotoxicity. Treatment of RAW cells with bafilomycin A1, a specific inhibitor of phagolysosomal acidification, also reduced cytotoxicity but did not block particle uptake. Based on direct indium measurements, the concentration of ionic indium was increased in culture medium from RAW but not LA-4 cells following 24-h treatment with particles. Ionic indium derived from RAW cells was significantly reduced by treatment with CytoD. These data implicate macrophage uptake and solubilization of InP and ITO via phagolysosomal acidification as requisite for particle-induced cytotoxicity and the release of indium ions. This may apply to other ICPs and strongly supports the notion that ICPs require solubilization in order to be toxic.

Use of and occupational exposure to indium in the United States

Indium use has increased greatly in the past decade in parallel with the growth of flat-panel displays, touchscreens, optoelectronic devices, and photovoltaic cells. Much of this growth has been in the use of indium tin oxide (ITO). This increased use has resulted in more frequent and intense exposure of workers to indium. Starting with case reports and followed by epidemiological studies, exposure to ITO has been linked to serious and sometimes fatal lung disease in workers. Much of this research was conducted in facilities that process sintered ITO, including manufacture, grinding, and indium reclamation from waste material. Little has been known about indium exposure to workers in downstream applications. In 2009-2011, the National Institute for Occupational Safety and Health (NIOSH) contacted 89 potential indium-using companies; 65 (73%) responded, and 43 of the 65 responders used an indium material. Our objective was to identify current workplace applications of indium materials, tasks with potential indium exposure, and exposure controls being used. Air sampling for indium was either conducted by NIOSH or companies provided their data for a total of 63 air samples (41 personal, 22 area) across 10 companies. Indium exposure exceeded the NIOSH recommended exposure limit (REL) of 0.1 mg/m(3) for certain methods of resurfacing ITO sputter targets, cleaning sputter chamber interiors, and in manufacturing some inorganic indium compounds. Indium air concentrations were low in sputter target bonding with indium solder, backside thinning and polishing of fabricated indium phosphide-based semiconductor devices, metal alloy production, and in making indium-based solder pastes. Exposure controls such as containment, local exhaust ventilation (LEV), and tool-mounted LEV can be effective at reducing exposure. In conclusion, occupational hygienists should be aware that the manufacture and use of indium materials can result in indium air concentrations that exceed the NIOSH REL. Given recent findings of adverse health effects in workers, research is needed to determine if the current REL sufficiently protects workers against indium-related diseases.

Occupational exposure to indium: what does biomonitoring tell us?

BACKGROUND

The industrial uses of indium, a rare metal with no known physiological role in humans, have increased dramatically over the past 15 years. The results of animal toxicity studies showing pulmonary and systemic effects as well as some reports in workers have created a growing concern about the possible occurrence of toxic effects in exposed workers. Validated biomarkers to assess exposure to indium are not available.

OBJECTIVES

This work aimed at investigating the kinetics of indium in urine (In-U) and plasma (In-Pl) in workers manufacturing In ingots and mainly exposed to hardly water-soluble In compounds. All nine workers from the In department of a large metallurgical concern participated in the study as well as 5 retired workers and 20 controls.

METHODS

Personal breathing zone air was collected throughout the work shift on Monday and Friday. Blood and urine samples were collected, before and after the shift, on the same day as the air sampling and on preshift the next Monday after a non-working week-end. Moreover, rats were given either InCl(3) by intraperitoneal injection or In(2)O(3) by pharyngeal aspiration, In was followed in plasma during 120 days and measured in tissues 120 days after exposure.

RESULTS

Higher In-Pl and In-U concentrations were found in both current (range 0.32-12.61 μg/L plasma; 0.22-3.50 μg/g creat) and former (0.03-4.38 μg/L plasma; 0.02-0.69 μg/g creat) workers compared with controls (<0.03 μg/L plasma; <0.02 μg/g creat). Both biological parameters were highly correlated but no correlation was found between In-air (10-1030 μg/m(3)) and In-Pl or In-U. Normalizing In-U by the urinary creatinine concentration reduced the inter- (from 90% to 70%) and intra-individual variability (from 54% to 35%). In-Pl remained remarkably stable along the working week (inter- and intra-individual variability: 89% and 10%, respectively). Neither In-U nor In-Pl significantly increased during the day or the week. A week-end without occupational exposure was not sufficient to reach the background In-Pl and In-U levels measured in controls. The results of the experimental investigations confirmed the hypothesis that inhalation of hardly soluble In compounds may cause accumulation of In in the body leading to a prolonged "endogenous exposure" from both a lung depot of "insoluble" particles that are progressively absorbed and from a retention depot in other internal organs.

CONCLUSION

This study shows that in workers exposed to hardly soluble In compounds, In-U and In-Pl are very sensitive to detect exposure and mainly reflect long-term exposure. In-Pl levels are particularly stable for a given individual. In-U might be more influenced than In-Pl by recent exposure. Both parameters remained high years after withdrawal from exposure, indicating a possible endogenous exposure and a prolonged risk of pulmonary and systemic diseases even after work exposure has ceased.

Inhalation carcinogenicity and chronic toxicity of indium-tin oxide in rats and mice

OBJECTIVES

Carcinogenicity and chronic toxicity of indium-tin oxide (ITO) were examined by inhalation exposure of rats and mice to ITO aerosol.

METHODS

Fifty mice of both sexes were exposed to ITO at 0 (control), 0.01, 0.03 or 0.1 mg/m(3) for 6 h/day, 5 day/wk for 104 wk, and 50 rats of both sexes were exposed to 0, 0.01 or 0.03 mg/m(3) ITO for the same time period. The repeated exposure of 50 rats of both sexes to 0.1 mg/m(3) ITO was discontinued at the 26th wk, followed by clean air exposure for the remaining 78 wk.

RESULTS

In rats, incidences of bronchiolo-alveolar adenomas and carcinomas, bronchiolo-alveolar hyperplasia, alveolar wall fibrosis and thickened pleural wall, alveolar proteinosis and infiltrations of alveolar macrophages and inflammatory cells were significantly increased. Combined incidences of malignant lung tumors in male rats and total lung tumors in male and female rats were significantly increased at exposure to 0.01 mg/m(3) ITO. In mice, no carcinogenic response occurred, but thickened pleural wall, alveolar proteinosis and alveolar macrophage infiltration were induced. Mice were less susceptible to ITO than rats. The lung content of indium was the greatest, followed by the spleen, kidney and liver. Blood indium levels increased dose-dependently.

CONCLUSIONS

There was clear evidence of carcinogenicity of inhaled ITO in male and female rats but not clear evidence in mice, together with occurrence of the chronic pulmonary lesions in both rats and mice.

Causal relationship between indium compound inhalation and effects on the lungs

BACKGROUND

Recent case reports and epidemiological studies suggest that inhalation of indium dust induces lung damage.

OBJECTIVES

To elucidate the dose-dependent effects of indium on the lungs and to prove a causal relationship more clearly.

METHODS

A baseline observation was conducted on 465 workers currently exposed to indium, 127 workers formerly exposed to indium and 169 workers without indium exposure in 12 factories and 1 research laboratory from 2003 to 2006. Indium in serum (In-S) was determined as an exposure parameter, and its effects on the lungs were examined.

RESULTS

The means of In-S in the current, former and no exposure workers were 8.35, 9.63 and 0.56 ng/ml, respectively. The current and former exposure workers had significantly higher levels of KL-6, and showed significant dose-dependent increases in KL-6, SP-D, and SP-A. Current exposure workers with In-S of 3 ng/ml or above demonstrated a significant increase of KL-6 in both GM and prevalence exceeding the reference value. Approximately a quarter of the former exposure workers had interstitial changes as seen on chest HRCT. In-S of exposed workers who had been working before improvements of the working environment (Group Bef) and those who started working after improvements (Group Aft) were 12.29 and 0.81 ng/ml, respectively. Adjusted odds ratios indicated 87%, 71% and 44% reductions among Group Aft workers who exceeded the reference values of KL-6, SP-D and SP-A, respectively.

CONCLUSION

Dose-dependent lung effects due to indium exposure were shown, and a decrease of indium exposure reduced the lung effects. An In-S value of 3 ng/ml may be a cut-off value which could be used to prevent early effects on the lungs.