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Original Research: COPD |

Five-Year Cohort StudyFive-Year Cohort Study of Indium-Exposed Workers: Emphysematous Progression of Indium-Exposed Workers FREE TO VIEW

Makiko Nakano, MD, PhD; Kazuyuki Omae, MD, PhD; Kazuhiko Uchida, MD, PhD; Takehiro Michikawa, MD, PhD; Noriyuki Yoshioka, DVM; Miyuki Hirata, PhD; Akiyo Tanaka, DVM, PhD
Author and Funding Information

From the Department of Preventive Medicine and Public Health (Drs Nakano, Omae, Uchida, Michikawa, and Yoshioka), School of Medicine, Keio University, Tokyo; Environmental Epidemiology Section (Dr Michikawa), Center for Environmental Health Sciences, National Institute for Environmental Studies, Tsukuba; and Environmental Medicine (Drs Hirata and Tanaka), Graduate School of Medical Science, Kyushu University, Fukuoka, Japan.

CORRESPONDENCE TO: Makiko Nakano, MD, PhD, Department of Preventive Medicine and Public Health, School of Medicine, Keio University, 35 Shinanomachi, Shinjuku-ku, Tokyo 1608582, Japan; e-mail: nakano.makiko@z8.keio.jp


FUNDING/SUPPORT: This study was supported by Grants-in-aid for Scientific Research (Project Nos. 15390191, 17390179, 20249039, and 23249033) from the Ministry of Education, Culture, Sports, Science and Technology of Japan [2003-4, 2005-6, 2008-10, and 2011] and in part by donations for research in preventive and environmental medicine from two of the surveyed companies.

Reproduction of this article is prohibited without written permission from the American College of Chest Physicians. See online for more details.


Chest. 2014;146(5):1166-1175. doi:10.1378/chest.13-2484
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BACKGROUND:  Dose-dependent adverse lung effects due to indium exposure have been reported in a cross-sectional study. This is a 5-year longitudinal cohort study of indium-exposed and unexposed workers, assessing indium exposure levels and its clinical lung effects.

METHODS:  From 2008 to 2011, a 5-year follow-up study was conducted on 40 unexposed and 240 workers formerly or currently exposed to indium at 11 factories. Indium exposure was assessed by serum indium (In-S) (μg/L). Lung effects were assessed by subjective symptoms, serum biomarkers, spirometry, and chest high-resolution CT scan. Effect biomarkers used were Krebs von den Lungen and surfactant protein D.

RESULTS:  Mean values of In-S, Krebs von den Lungen, and surfactant protein D among the workers exposed to indium at baseline declined during the 5-year follow-up by 29.8%, 27.2%, and 27.5%, respectively. Of the exposed subjects with In-S levels > 20 μg/L, 26.3% experienced emphysematous progression on high-resolution CT scan. Ninety percent (18 of 20) of workers with emphysematous progression during follow-up were current smokers at baseline, and a trend of increasing incidence of emphysematous progression at higher In-S levels was observed among the smokers (P = .005). Emphysematous changes among subjects with In-S levels > 20 μg/L were likely to progress, after adjusting for age, mean duration since initial indium exposure, and smoking history (OR = 10.49, 95% CI = 1.54-71.36).

CONCLUSIONS:  Long-term adverse effects on emphysematous changes were observed. The results suggest workers exposed to indium with In-S levels > 20 μg/L should be immediately removed from exposure.

Figures in this Article

Indium is a rare metal used in the form of indium-tin oxide (ITO) as the electrode in flat panel displays. Japan is the largest consumer of indium accounting for 85% of global demand.1 To our knowledge, the first (and fatal) case of indium-related lung disease (indium lung) was reported in Japan in 2003.2 As of 2011,3 seven more cases have been reported in Japan,4,5 two in the United States,6 and one in China.7 In Japan, a study of indium-exposed workers was conducted at the ITO-processing factory where, to our knowledge, the first case was reported,8 as well as a multicenter study of indium-exposed and unexposed workers in other ITO-processing and ITO-recycling plants.9 The multicenter cross-sectional cohort study was later expanded and found dose-dependent adverse lung effects due to indium exposure.10 The harmful effect of indium was further brought to light by a 2-year ITO inhalation experiment, revealing ITO as a lung carcinogen in rats.11 Based on these findings, in 2010, the Japanese Ministry of Health, Labor and Welfare established a prevention guideline for ITO-processing workers12 and, in 2013, added indium to the list of substances regulated by the Ordinance on Prevention of Hazards due to Specified Chemical Substances.13,14

The long-term effects of indium exposure on the lungs remain largely unknown. This is a 5-year follow-up study of the largest cohort of indium-exposed and unexposed workers. Our objective is to assess the association between exposure levels of indium and its clinical effects on the lungs.

This study was approved by the ethical committee of the School of Medicine at Keio University (approval numbers 15-46 and 20110268). Written informed consent was obtained from all subjects.

Study Design and Subjects

In comparison with the multicenter baseline study conducted at 12 factories and one research laboratory between 2003 and 2006,10 this longitudinal study added workers from an additional factory and removed cohort members from three factories due to logistics. The resulting dataset covered 11 plants, including 383 exposed and 159 unexposed workers. Approximately 5 years after the baseline study (2003-2006), we conducted a follow-up study at these 11 plants between 2008 and 2011, involving 247 exposed and 63 unexposed workers (follow-up rates 64.5% and 39.6%). Among the subjects with high baseline serum indium (In-S) levels (≥ 20 μg/L), the follow-up rate was 82.6% (Fig 1).

Figure Jump LinkFigure 1 –  Study population. The flow of the subjects. aNakano et al.10Grahic Jump Location

At the 5-year follow-up, in accordance with the baseline study, a medical interview, questionnaire, blood test, spirometry, and high-resolution CT (HRCT) scan examination of the lungs were all conducted on 240 exposed and 40 unexposed subjects, excluding seven subjects with undetermined exposure duration and 23 unexposed workers exposed during follow-up. One unexposed and 10 exposed workers were excluded from final analysis of the lung function test results due to inadequate test maneuvers or a medical history of surgical lung resection. Of the 280 subjects, baseline HRCT scans for 207 workers were obtained, allowing direct comparison with the follow-up scans. Thirty-five unexposed and 172 exposed workers had HRCTs from both the baseline and follow-up.

Categorization of exposed workers into currently or formerly exposed groups was based on their exposure status at baseline. Job history was based on the job records at the plants, or if unavailable, based on physician’s interview about occupational history.

Exposure Indexes

In-S (μg/L) was measured by inductively coupled plasma mass spectrometry at the Center of Advanced Instrumental Analysis, Kyushu University.9 In-S below the detection limit (0.1 μg/L) was ascribed an arbitrary value of 0.05 μg/L for statistical analysis.

Effect Indexes and Confounding Factors

Medical examinations conducted at the 5-year follow-up were the same as those at the baseline study. Serum Krebs von den Lungen-6 (KL-6) (EIDIA Co, Ltd)15,16 and serum surfactant protein D (SP-D) (Yamasa Corporation)17 were used as biomarkers for interstitial changes in the lungs and evaluated at a major commercial clinical laboratory (Special Reference Laboratory).

Spirometry was performed using electronic spirometers (HI-701 or HI-801; CHEST M.I. Inc) based on American Thoracic Society guidelines. Age and height-adjusted predicted values of FVC and FEV1 were determined by sex, using the regression formula recommended by the Japanese Respiratory Society18; and percentages of predicted FVC and FEV1 were calculated.

At nine of the plants, HRCT scanning was performed in a specially assembled vehicle, using the same multislice CT scanner as the baseline study10 at 120 kV, 200 mA, and a slice thickness of 1 mm. For the other two plants, HRCT scanning was performed at nearby hospitals with a helical or multislice CT scanner. All HRCT scans were carried out at three lung levels (the upper, middle, and lower lung fields) as recommended by the Japanese Respiratory Society. The same technique was used in the baseline study.10

In accordance with the Japanese Respiratory Society guideline for the diagnosis and management of COPD,19 interstitial changes, including interlobular septal thickening, ground-glass appearance, and nodular infiltrate, as well as emphysematous changes in the upper, middle, and lower bilateral lung fields20 were jointly assessed by a Japan Radiologic Society-certified radiologist and a Japanese Respiratory Society-certified pulmonologist. The two experts assessed all the scans together, comparing side by side the clearly defined lung fields on the baseline and follow-up HRCT scans for each subject. Emphysematous change was defined as an emergence of a new or enlarged low attenuation area on any one of the six HRCT scan slices. Worsening of the follow-up CT scan compared with the baseline CT scan was labeled “progression of interstitial changes” or “progression of emphysematous changes,” and an improvement or no change was labeled “no progression.”

Using the Japanese version21 of the American Thoracic Society-Division of Lung Disease questionnaire22 and supplementary questions, the following were investigated: respiratory symptoms, smoking history, and confounding factors including sex, age, medical history, and history of exposure to other materials.

Statistical Analysis

Nonnormally distributed data were transformed to an approximately normal distribution before analysis. The Student t test or the Mann-Whitney U test was used to compare continuous variables between exposed and unexposed groups. The χ2 test or Fisher exact method was used to compare proportions, prevalence, or incidence.

Based on the classification criteria of In-S adopted in our baseline study,10 the exposed subjects were stratified into six In-S categories: In-S level < 1.0 μg/L, 1.0 to 2.9 μg/L, 3.0 to 4.9 μg/L, 5.0 to 9.9 μg/L, 10.0 to 19.9 μg/L, and > 20.0 μg/L. These six categories were used to assess the risk of indium exposure on the effect variables, as well as their dose-response relationship.

Mean values of 5-year differences in biomarkers and lung functions among the exposed subjects were stratified by the aforementioned six In-S categories and compared with the unexposed subjects using the Dunnett test. Incidence of abnormalities (change from normal to abnormal values) by exposure group for the biomarkers, lung function, and HRCT scan progression was analyzed, using the following cutoff for abnormal values: KL-6 ≥ 500 U/mL, SP-D ≥ 110 ng/mL, FEV1/FVC < 70%, %FVC < 80%, and %FEV1 < 80%. Test for trend in the In-S categories was performed using the Cochran-Armitage test for categorical data.

Based on the test for trend in the In-S categories, as well as the analysis of exposure and effect indexes with respect to HRCT scan progression, the relationship between In-S and HRCT scan progression was further analyzed, using a logistic regression model. Adjusted variables were age, mean duration since initial indium exposure, and smoking.

Statistical significance was assessed by two-tailed analysis with P < .05. All statistical analyses were performed using SPSS, version 19 (IBM) and JMP, version 10.0.2 (SAS Institute Inc).

Table 1 shows the characteristics of the study subjects and the pulmonary effects of indium at baseline and at 5-year follow-up. The mean duration since first indium exposure was 5.5 years for the currently exposed group at baseline and 12.1 years for the formerly exposed. The currently exposed subjects were younger than the unexposed workers (P < .05). No difference in the proportion of male subjects and smoking history was observed between the exposed and unexposed subjects.

Table Graphic Jump Location
TABLE 1 ]  Characteristics, Exposure Levels, Biomarkers of Effect, and Lung Function of Unexposed and Indium-Exposed Subjects at Baseline and at 5-Year Follow-up

In-S = serum indium; KL-6 = Krebs von den Lungen; SP-D = surfactant protein D.

a 

P < .05 by Student t test, χ2 test, or Fisher exact method with respect to unexposed subjects.

b 

P < .01 by Student t test, χ2 test, or Fisher exact method with respect to unexposed subjects.

At baseline, the mean values for KL-6, SP-D, and pulmonary symptoms in the exposed group were significantly higher than in the unexposed group. For the mean values of pulmonary function test results, no difference was observed between the two groups.

At follow-up, the mean values of In-S, KL-6, and SP-D among the currently exposed workers declined from baseline by 29.8%, 27.2%, and 27.5%; those among the formerly exposed declined by 39.4%, 24.7%, and 21.9%, respectively. The significant difference observed at baseline in KL-6 between the unexposed and the exposed groups disappeared at follow-up. Mean values of FEV1/FVC, %FVC, and %FEV1 in the exposed group slightly decreased during follow-up.

Table 2 shows the 5-year differences and incidence of abnormal values in serum biomarkers and lung function, stratified by In-S levels. Those with baseline In-S levels above 3 μg/L displayed a notable 5-year decline in In-S. With increasing baseline In-S levels, increasing incidence of abnormal lung functions (FEV1/FVC and %FEV1) was observed (P < .05).

Table Graphic Jump Location
TABLE 2 ]  Five-Year Differences and Incidence of Abnormal Values in Biomarkers and Lung Function, Stratified by In-S Categories

See Table 1 legend for expansion of abbreviations.

a 

P < .05 by Dunnett test (for categorized In-S) with respect to unexposed subjects.

b 

P < .01 by Dunnett test (for categorized In-S) with respect to unexposed subjects.

c 

Cutoff for abnormal values: KL-6 ≥ 500 U/mL, SP-D ≥ 110 ng/mL, FEV1/FVC < 70%, %FVC < 80%, %FEV1 < 80%.

Table 3 shows the incidence of progression on HRCT scan findings during 5-year follow-up stratified by In-S categories. Incidences of emphysematous progression in unexposed and exposed workers were two of 35 (5.7%) and 20 of 172 (11.6%). Dose-dependent increase of the incidence was observed (P for trend = .002). Among the exposed subjects with In-S levels above 20 μg/L, 26.3% experienced emphysematous progression, and the crude OR was 5.89 (95% CI = 1.19-29.17). Of 20 exposed workers with emphysematous progression, 18 were ex- or current smokers at baseline. A statistically significant trend of increasing incidence was observed for smokers (P = .005). When exposed to high levels of indium (In-S ≥ 20 μg/L), nine of 31 smokers (29.0%) experienced emphysematous progression, compared with one of seven (14.3%) never smokers. Meanwhile, incidence of interstitial progression was two of 35 (5.7%) among the unexposed and 10 of 172 (5.8%) among the exposed workers. No statistically significant trend was observed.

Table Graphic Jump Location
TABLE 3 ]  Incidence of Progression of HRCT Scan Findings During 5-Year Follow-up, Stratified by In-S Categories

Adjusted OR: adjusted variables are age, mean duration since initial indium exposure, and smoking history at baseline. In-S categories with zero incidence were combined to assess the ORs using logistic regression models. See Table 1 legend for expansion of abbreviation.

Table 4 compares, within the exposed workers, the baseline indium exposure levels, serum biomarkers, and lung functions between those with and without HRCT scan progression. Compared with those without an emphysematous deterioration, those who experienced emphysematous progression exhibited significantly higher In-S levels, were older in age, had higher KL-6 levels, and lower %FEV1. Regarding those with and without progression in interstitial changes, there was no statistical difference in In-S and in KL-6, while those with interstitial changes had statistically higher SP-D levels (P < .01) and lower FEV1/FVC (P < .05).

Table Graphic Jump Location
TABLE 4 ]  Exposure and Effect Indexes at Baseline Among Exposed Workers With or Without Progression of HRCT Scan Findings During Follow-up Period

See Table 1 legend for expansion of abbreviations.

a 

P < .05 by Student t test, Mann-Whitney U test, χ2 test, or Fisher exact method, with respect to subjects with no progression.

b 

P < .01 by Student t test, Mann-Whitney U test, χ2 test, or Fisher exact method, with respect to subjects with no progression.

The 5-year follow-up study of indium-exposed and unexposed workers revealed a long-term effect of indium on the lungs in the form of HRCT scan progression in emphysematous changes, despite a significant decline in In-S likely due to the workplace improvements enforced after our baseline study. The dose-response trend between In-S levels and incidence of emphysematous progression indicated that indium inhalation is a serious risk factor for emphysematous progression, especially among the highly exposed workers (In-S ≥ 20 μg/L). Due to the very slow clearance of hardly soluble indium particles from the lungs,11,23,24 both the currently and formerly exposed workers were being continually exposed to indium by the particles in their lungs, even after the reduction of occupational exposures to indium. Indium particles in the lungs perpetuated the phagocytosis and phagolysosomal acidification25 cycle by the alveolar macrophages. The proteases released by the macrophages and the cytotoxicity of indium may have promoted macrophage-mediated elastolysis, which is known to cause inflammation and destruction of the lung parenchyma,26 leading to emphysematous deterioration.

Smoking is a major risk factor of COPD,27 and the current study showed that smoking is also an important effect modifier of the risk of emphysematous progression among indium-exposed workers. Due to the small number of emphysematous progression in the unexposed workers, we could not assess the interaction between indium exposure and smoking using the logistic regression model. However, in addition to the statistically significant dose-response trend of the incidence among smokers, the incidence ratio of emphysematous progression between smokers and nonsmokers among the highly exposed workers (In-S ≥ 20) was 2.03 (95% CI = 0.31-13.5) (data not shown).

In contrast to the emphysematous progression, the incidence rate of progression in interstitial changes was not detected, although a causal relationship between indium exposure and interstitial changes has been previously identified.10 KL-6 is known to increase in the active phase of interstitial pneumonia and decrease in the inactive phase.16 In the current study, KL-6 among the indium-exposed subjects declined by 25% after the baseline study and subsequent intervention. These results imply that once indium exposure is eliminated or drastically reduced, KL-6 may significantly decline, although not to the level of the unexposed. Despite no statistically significant exposure-response relationship for interstitial changes across the categories of In-S, 10 exposed workers did have progression of interstitial change, of which four also experienced emphysematous progression. When restricted to In-S ≥ 10 μg/L, three of five had both interstitial and emphysematous progression. This raises the concern that interstitial changes and emphysematous changes coexist among highly exposed workers with interstitial progression.

The response rate of the unexposed group was low (40 of 159, 25.2%). Of the 119 workers who contribute to the low response rate, 23 were removed from the analysis because they switched to an exposed environment during follow-up. The remaining 96 unexposed workers lost to follow-up were due to the decision made by a few factories that further follow-up of the unexposed workers was unnecessary. This suggests that self-selection on ill health was unlikely, but the implication of selection bias remains a substantial limitation of the study.

The 5-year follow-up rate was also low at 57.2%. Subanalysis showed that the baseline mean In-S of the subjects lost to follow-up was significantly lower than that of the workers retained at follow-up (2.6 μg/L vs 9.1 μg/L). Additionally, 73 of the 280 followed-up subjects had no baseline HRCT scans. It is not possible to conclude whether including those lost to follow-up and those without baseline scans would have lowered or increased the risk reported in the current study.

Some of the unexposed workers exhibited measurable In-S up to 1.5 μg/L at baseline. The possibility of contamination of unexposed subjects’ workplace and exposure misclassification remains.

While a 5-year assessment is insufficient to predict clinical outcomes after longer periods, the current study results provide significant rationale to immediately remove highly exposed workers (In-S ≥ 20 μg/L) from indium exposure. The increased prevalence of progression was also evident at lower In-S levels and suggest that an In-S level between 5 μg/L and 19.9 μg/L is a warning sign for action. A larger sample size may have provided the statistical power to lower the preventive recommendation below 20 μg/L. Additionally, high levels of biomarker and abnormal lung function warrant attention as potential predictors of emphysematous changes.

The agenda for future research includes monitoring new clinical cases of indium lung, following further progression of emphysematous and interstitial changes in the current cohort, and studying the occurrence of lung cancer.

This 5-year cohort study of indium workers suggests that In-S burden is a risk factor for progression of emphysematous changes, particularly in smokers, and persists after In-S decreases. The study results provide the basis to immediately remove indium workers with In-S above 20 μg/L from indium exposure and monitor their lung conditions, as well as to consider In-S levels between 5 μg/L and 19.9 μg/L as a warning sign for action.

Author contributions: M. N. takes responsibility for the integrity and accuracy of the data and manuscript, had primary responsibility for the design of the study and drafting of the article, and contributed to fieldwork management, acquisition, analysis, and interpretation of data, and critical revision of the manuscript. K. O. had primary responsibility for the conception and design of the study and contributed to fieldwork management; K. U. contributed to analysis and interpretation of data; T. M. contributed to data acquisition; N. Y. contributed to data acquisition; M. H. assisted in the study design and contributed to acquisition, analysis, and interpretation of data; and A. T. assisted in the study design and contributed to fieldwork management and acquisition, analysis, and interpretation of data.

Financial/nonfinancial disclosures: The authors have reported to CHEST that no potential conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.

Role of sponsors: The sponsor had no role in the design of the study, the collection and analysis of the data, or the preparation of the manuscript.

Other contributions: We thank the staff members and participants at all factories for their helpful and cordial cooperation. We also thank Mutsuko Yamada, PhD, for assistance with critical revision of the manuscript.

HRCT

high-resolution CT

In-S

serum indium

ITO

indium-tin oxide

KL-6

Krebs von den Lungen

SP-D

surfactant protein D

Minami H. Demand, Supply, and Price Trend of Indium and Gallium [in Japanese]. Tokyo, Japan: Japan Oil, Gas and Metals National Corporation; 2010.
 
Homma T, Ueno T, Sekizawa K, Tanaka A, Hirata M. Interstitial pneumonia developed in a worker dealing with particles containing indium-tin oxide. J Occup Health. 2003;45(3):137-139. [CrossRef] [PubMed]
 
Omae K, Nakano M, Tanaka A, Hirata M, Hamaguchi T, Chonan T. Indium lung—case reports and epidemiology. Int Arch Occup Environ Health. 2011;84(5):471-477. [CrossRef] [PubMed]
 
Cummings KJ, Nakano M, Omae K, et al. Indium lung disease. Chest. 2012;141(6):1512-1521. [CrossRef] [PubMed]
 
Chonan T, Amano A, Modera H, Kanehara A, Honma K. Interstitial pneumonitis due to an inhalation of indium compounds [in Japanese]. Japanese J Chest Dis. 2010;69(9):851-855.
 
Cummings KJ, Donat WE, Ettensohn DB, Roggli VL, Ingram P, Kreiss K. Pulmonary alveolar proteinosis in workers at an indium processing facility. Am J Respir Crit Care Med. 2010;181(5):458-464. [CrossRef] [PubMed]
 
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Figures

Figure Jump LinkFigure 1 –  Study population. The flow of the subjects. aNakano et al.10Grahic Jump Location

Tables

Table Graphic Jump Location
TABLE 1 ]  Characteristics, Exposure Levels, Biomarkers of Effect, and Lung Function of Unexposed and Indium-Exposed Subjects at Baseline and at 5-Year Follow-up

In-S = serum indium; KL-6 = Krebs von den Lungen; SP-D = surfactant protein D.

a 

P < .05 by Student t test, χ2 test, or Fisher exact method with respect to unexposed subjects.

b 

P < .01 by Student t test, χ2 test, or Fisher exact method with respect to unexposed subjects.

Table Graphic Jump Location
TABLE 2 ]  Five-Year Differences and Incidence of Abnormal Values in Biomarkers and Lung Function, Stratified by In-S Categories

See Table 1 legend for expansion of abbreviations.

a 

P < .05 by Dunnett test (for categorized In-S) with respect to unexposed subjects.

b 

P < .01 by Dunnett test (for categorized In-S) with respect to unexposed subjects.

c 

Cutoff for abnormal values: KL-6 ≥ 500 U/mL, SP-D ≥ 110 ng/mL, FEV1/FVC < 70%, %FVC < 80%, %FEV1 < 80%.

Table Graphic Jump Location
TABLE 3 ]  Incidence of Progression of HRCT Scan Findings During 5-Year Follow-up, Stratified by In-S Categories

Adjusted OR: adjusted variables are age, mean duration since initial indium exposure, and smoking history at baseline. In-S categories with zero incidence were combined to assess the ORs using logistic regression models. See Table 1 legend for expansion of abbreviation.

Table Graphic Jump Location
TABLE 4 ]  Exposure and Effect Indexes at Baseline Among Exposed Workers With or Without Progression of HRCT Scan Findings During Follow-up Period

See Table 1 legend for expansion of abbreviations.

a 

P < .05 by Student t test, Mann-Whitney U test, χ2 test, or Fisher exact method, with respect to subjects with no progression.

b 

P < .01 by Student t test, Mann-Whitney U test, χ2 test, or Fisher exact method, with respect to subjects with no progression.

References

Minami H. Demand, Supply, and Price Trend of Indium and Gallium [in Japanese]. Tokyo, Japan: Japan Oil, Gas and Metals National Corporation; 2010.
 
Homma T, Ueno T, Sekizawa K, Tanaka A, Hirata M. Interstitial pneumonia developed in a worker dealing with particles containing indium-tin oxide. J Occup Health. 2003;45(3):137-139. [CrossRef] [PubMed]
 
Omae K, Nakano M, Tanaka A, Hirata M, Hamaguchi T, Chonan T. Indium lung—case reports and epidemiology. Int Arch Occup Environ Health. 2011;84(5):471-477. [CrossRef] [PubMed]
 
Cummings KJ, Nakano M, Omae K, et al. Indium lung disease. Chest. 2012;141(6):1512-1521. [CrossRef] [PubMed]
 
Chonan T, Amano A, Modera H, Kanehara A, Honma K. Interstitial pneumonitis due to an inhalation of indium compounds [in Japanese]. Japanese J Chest Dis. 2010;69(9):851-855.
 
Cummings KJ, Donat WE, Ettensohn DB, Roggli VL, Ingram P, Kreiss K. Pulmonary alveolar proteinosis in workers at an indium processing facility. Am J Respir Crit Care Med. 2010;181(5):458-464. [CrossRef] [PubMed]
 
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