Occupational exposure and mortality among workers at three titanium dioxide plants

Review on fate, transport, toxicity and health risk of nanoparticles in natural ecosystems: Emerging challenges in the modern age and solutions toward a sustainable environment

In today's era, nanoparticles (NPs) have become an integral part of human life, finding extensive applications in various fields of science, pharmacy, medicine, industry, electronics, and communication. The increasing popularity of NP usage worldwide is a testament to their tremendous potential. However, the widespread deployment of NPs unavoidably leads to their release into the environmental matrices, resulting in persistence in ecosystems and bioaccumulation in organisms. Understanding the environmental behavior of NPs poses a significant challenge due to their nanoscale size. Given the current environmental releases of NPs, known negative consequences, and the limited knowledge available for risk management, comprehending the toxicity of NPs in ecosystems is both awaiting and crucial. The present review aims to unravel the potential environmental influences of nano-scaled materials, and provides in-depth inferences of the current knowledge and understanding in this field. The review comprehensively summarizes the sources, fate, transport, toxicity, health risks, and remediation solutions associated with NP pollution in aquatic and soil ecosystems. Furthermore, it addresses the knowledge gaps and outlines further investigation priorities for the sustainable control of NP pollution in these environments. By gaining a holistic understanding of these aspects, we can work toward ensuring the responsible and sustainable use of NPs in today's fast-growing world.

Research Recommendations for Selected IARC-Classified Agents: Impact and Lessons Learned

BACKGROUND

The International Agency for Research on Cancer (IARC) Monographs program assembles expert working groups who publish a critical review and evaluation of data on agents of interest. These comprehensive reviews provide a unique opportunity to identify research needs to address classification uncertainties. A multidisciplinary expert review and workshop held in 2009 identified research gaps and needs for 20 priority occupational chemicals, metals, dusts, and physical agents, with the goal of stimulating advances in epidemiological studies of cancer and carcinogen mechanisms. Overarching issues were also described.

OBJECTIVES

In this commentary we review the current status of the evidence for the 20 priority agents identified in 2009. We examine whether identified Research Recommendations for each agent were addressed and their potential impact on resolving classification uncertainties.

METHODS

We reviewed the IARC classifications of each of the 20 priority agents and identified major new epidemiological and human mechanistic studies published since the last evaluation. Information sources were either the published Monograph for agents that have been reevaluated or, for agents not yet reevaluated, Advisory Group reports and literature searches. Findings are described in view of recent methodological developments in Monographs evidence evaluation processes.

DISCUSSION

The majority of the 20 priority agents were reevaluated by IARC since 2009. The overall carcinogen classifications of 9 agents advanced, and new cancer sites with either "sufficient" or "limited" evidence of carcinogenicity were also identified for 9 agents. Examination of published findings revealed whether evidence gaps and Research Recommendations have been addressed and highlighted remaining uncertainties. During the past decade, new research addressed a range of the 2009 recommendations and supported updated classifications for priority agents. This supports future efforts to systematically apply findings of Monograph reviews to identify research gaps and priorities relevant to evaluation criteria established in the updated IARC Monograph Preamble. https://doi.org/10.1289/EHP12547.

Health risks of titanium dioxide (TiO2) dust exposure in occupational settings - A scoping review

Titanium dioxide (TiO₂) is an inorganic compound with many applications, for example in paint, sunscreen or as food coloring. There have been concerns regarding its safety and according to IARC, the existing evidence is not substantial enough to rule them out, leading to the substance being classified as possibly carcinogenic to humans (2B). This work aims to provide a comprehensible overview about epidemiological studies on occupational health risks and methodological aspects. A literature search was conducted in two databases (MEDLINE and Web of Science). The search focused on occupational exposure since this setting provides the highest amounts of TiO₂ exposure. Of 443 unique search results, ten were included in this study, with publication dates ranging from 1988 to 2022. Seven of them are retrospective cohort studies and three have a case-control study design. Main outcomes of most studies were all-cause mortality and lung cancer mortality. For all-cause mortality, most cohort studies reported no association with TiO₂ exposure. For lung cancer mortality, a significantly increased risk was found in a study population from Europe. The analysis results of working cohorts from the US comparing exposed workers' mortality rates with those of the general population were unobtrusive. However, one US cohort found an elevated mortality risk for all causes and lung cancer based on a reference population of company workers unexposed to TiO₂. Case-control studies did not indicate an increased risk for cancer related to TiO₂. Recent publications partly questioned the validity of those earlier findings, claiming insufficient confounder analysis, most notably for smoking, as well as the presence of the healthy worker effect, masking a potential health risk. In conclusion, the associations between occupational TiO₂ exposure and mortality are unclear, but concerns regarding possible health risks recently re-emerged based on new analytical approaches, highlighting methodological difficulties that could have limited the inferential value of previously conducted studies.

Occupational Exposure to Poorly Soluble Low Toxicity Particles and Cardiac Disease: A Look at Carbon Black and Titanium Dioxide

Environmental particulate exposure and the potential risk to people with various types of cardiac diseases, most notably cardiovascular disease, have aroused scientific and regulatory interest worldwide. Epidemiological studies have shown associations between exposure to airborne environmental particulate matter (PM) and mortality from cardiovascular disease (CVD). The associations reported, however, are complex and may not involve a direct role for PM, since air pollutants are diverse and highly correlated. This study examines the potential role of occupational exposure to two types of particles, namely, manufactured carbon black (CB) and titanium dioxide (TiO₂), on the risk of cardiovascular disease. To address the risk of cardiovascular disease from exposure to carbon black and titanium dioxide, as reflective of poorly soluble low toxicity particles, we reviewed the published cohort mortality studies of occupational exposure to carbon black and titanium dioxide. Mortality studies of carbon black have been conducted in the United States, Germany, and the United Kingdom. Five mortality studies related to workers involved in the manufacture of titanium dioxide in the United States and Europe have also been conducted. In addition, a meta-analysis of the three-carbon black mortality studies was performed. In the random-effects meta-analysis, full cohort meta-SMRs were 1.01 (95% confidence interval (CI): 0.79–1.29) for heart disease; 1.02 (95% CI: 0.80–1.30) for ischemic heart disease; and 1.08 (95% CI: 0.74–1.59) for acute myocardial infarction (AMI) mortality. A small but imprecise increased AMI mortality risk was suggested for cumulative exposure by a meta-HR = 1.10 per 100 mg/m³-years (95% CI: 0.92–1.31) but not for lugged exposures, that is, for recent exposures. Results of five cohort mortality studies of titanium dioxide workers in the United States and Europe showed no excess in all heart disease or cardiovascular disease. In the most recent study in the United States, an internal analysis, that is, within the cohort itself, with no lag time, showed that the exposure group 15–35 mg/m³-years yielded a significantly increased risk for heart disease; however, there was no evidence of increasing risk with increasing exposure for any of the exposure categories. In contrast to environmental studies, the results of cohort mortality studies do not demonstrate that airborne occupational exposure to carbon black and titanium dioxide particulates increases cardiovascular disease mortality. The lack of a relationship between carbon black and titanium dioxide and CVD mortality suggests that the associations reported in air pollution studies may not be driven by the particulate component.

Metabolomics screening of serum biomarkers for occupational exposure of titanium dioxide nanoparticles

Although nanotoxicology studies have shown that respiratory exposure of titanium dioxide nanoparticles (TiO₂ NPs) could induce adverse health effects, limited biomarkers associated with occupational exposure of TiO₂ NPs were reported. The purpose of this study is to screen serum biomarkers among workers occupationally exposed to TiO₂ NPs using metabolomics. Compared with the control group, a total of 296 serum metabolites were differentially expressed in the TiO₂ NPs-exposed group, of which the relative expression of 265 metabolites increased, and the remaining 31 decreased. Three machine learning methods including random forest (RF), support vector machines (SVM), and boruta screened eight potential biomarkers and simultaneously selected a metabolite, Liquoric acid. Through multiple linear regression analysis to adjust the influence of confounding factors such as gender, age, BMI, smoking and drinking, occupational exposure to TiO₂ NPs was significantly related to the relative expression of the eight potential biomarkers. Meanwhile, the receiver operating characteristic curves (ROCs) of these potential biomarkers had good sensitivity and specificity. These potential biomarkers were related to lipid peroxidation, and had biological basis for occupational exposure to TiO₂ NPs. Therefore, it was demonstrated that the serum metabolites represented by Liquoric acid were good biomarkers of occupational exposure to TiO₂ NPs.

A Weight-of-the-Evidence Approach for Evaluating, in Lieu of Animal Studies, the Potential of a Novel Polysaccharide Polymer to Produce Lung Overload

The United States Environmental Protection Agency (EPA) is concerned about the respiratory effects caused by respirable particles of water-insoluble high molecular weight polymers. The EPA has proposed a tiered approach to evaluate polymer lung overload, a kinetic event. Kinetic polymer lung overload in itself is not necessarily adverse, however, inhalation of respirable particulate matter can have adverse effects (i.e., inflammation, fibrosis, etc.). If Tier I testing demonstrates that particles may reach the distal lung (i.e., a non-negligible amount of respirable particles/droplets ≤10 μm in diameter and lack of biosolubility), then animal inhalation testing in Tiers II-IV would be requested. In silico, in chemico, and in vitro alternatives should be considered versus in vivo testing for animal welfare purposes. An in chemico measure of biosolubility was used to demonstrate that a novel α-1,3-glucan polysaccharide, made by enzymatic polymerization of glucose from sucrose, is biosoluble and fits a simple exponential decay model with a half-life on the order of 66 days. The multiple-path particle dosimetry (MPPD) in silico model was used to predict lung burden for the novel α-1,3-glucan polysaccharide. MPPD was validated with measurements in rats exposed to a toner particulate and showed good agreement with lung burden measurements. A simulated 24 month rat exposure yielded 10-20 times less lung burden for the polysaccharide compared to the toner at equivalent exposure concentrations. The MPPD model was refined to include biosolubility data for the polysaccharide polymer. Data for amorphous silica were used to validate the clearance model, and the model incorporating dissolution predicted the amorphous silica lung burden within 20% of measured values. Human equivalent concentrations (HECs) were calculated for each toner rat exposure concentration. HECs were also determined for the polysaccharide at exposure concentrations yielding the same predicted internal doses as the toner. The in vitro, in chemico and in silico studies described here for the novel polysaccharide provide a useful weight of evidence approach in the absence of animal studies for the evaluation of polymer substances where polymer lung overload may be a concern.

Exploring urine biomarkers of early health effects for occupational exposure to titanium dioxide nanoparticles using metabolomics

Many experimental studies have demonstrated that titanium dioxide nanoparticles (TiO2 NPs) could induce adverse health effects in vivo and in vitro. But epidemiological evidence and biomarkers related to early health effects are still lacking. This study aimed to explore biomarkers in the urine samples of workers occupationally exposed to a relatively low concentration of TiO2 NPs. A cross-sectional study was conducted in Jinan, China, involving 132 employees of a TiO2 NP manufacturing plant, among which the exposed group and control group were 1 : 1 matched by confounding factors such as gender, age, BMI, smoking and drinking. Untargeted metabolomics was performed in urine samples using high performance liquid chromatography-mass spectrometry (HPLC-MS) technology. The differential metabolites between the TiO2 NP exposed group and the control group were analyzed and then screened for potential biomarkers using bioinformatics methods. Metabolomics found a total of 1760 differentially expressed metabolites in the TiO2 NP exposed group, of which 60 differential metabolites were simultaneously confirmed by one-dimensional and multi-dimensional statistical analysis. Among these 60 differential metabolites, the relative expression of 27 metabolites increased, and the remaining 33 decreased. Pathway enrichment analysis further found that the metabolic pathway of long chain acyl-coa dehydrogenase deficiency (Lcad) was significantly enriched. Ten differential metabolites were selected as potential biomarkers of occupational exposure to TiO2 NPs using machine learning methods, including dibenzyl ether, quassimarin, tryptophan, etc. The receiver operating characteristic curves (ROCs) of these potential biomarkers showed good sensitivity and specificity. These potential biomarkers also had biological basis for occupational exposure to TiO2 NPs. Therefore, urine metabolites represented by dibenzyl ether are considered as good biomarkers of early health effects for occupational exposure to TiO2 NPs.

Lung cancer mortality in the French cohort of titanium dioxide workers: some aetiological insights

OBJECTIVES

Titanium dioxide (TiO2) is widely used in construction, food, cosmetic and medical industry. The current evidence on TiO2 carcinogenicity in humans is considered inadequate. As French participants of the European cohort of TiO2 workers exhibited an increase in mortality from lung cancer, we aimed at investigating whether TiO2 exposure, co-exposures or smoking can explain this increase.

METHODS

We reanalysed the data of 833 French male workers (follow-up period 1968-1997) and used multiple imputation to complete their smoking status. We considered respirable TiO2 dust as primary exposure of interest, estimated as continuous cumulative (mg/m3-year) and annual average (mg/m3) concentrations and binary and 4-class categorical variables, with cut-off values of 0.3 and 2.4 mg/m3 (the German and American occupational exposure limits, respectively). For each exposure metric, we estimated HRs and associated 95% CIs, using Cox regression models adjusted for calendar period, exposure duration and smoking.

RESULTS

The fully adjusted model yielded a HR=3.7 (95% CI=0.79 to 17.95) for TiO2-exposed workers vs unexposed and a HR=27.33 (95% CI=4.35 to 171.84) for those exposed to >2.4 mg/m3 as annual average concentration. Employment duration was negatively related with lung cancer mortality, therefore cumulative exposure had a small effect on mortality (HR=1.03 (95% CI=0.99 to 1.08) per mg/m3-year).

CONCLUSION

This study suggests a positive relationship between TiO2 exposure and lung cancer mortality in TiO2 workers, whatever the exposure variable used, despite a limited statistical power in some models. The results question the current evidence on TiO2 carcinogenicity in humans but need to be confirmed in other cohorts, using different statistical approaches.

The long-term oral exposure to titanium dioxide impaired immune functions and triggered cytotoxic and genotoxic impacts in rats

BACKGROUND

Titanium dioxide "TiO₂, E171″ is a widely used food additive that exists in various everyday food products all over the world together with vast applications in cosmetics and industry. However, many toxicological aspects particularly following oral exposure still unclear.

METHODS

Hence, this study was planned to examine the effect of oral exposure of male Wistar rats to two doses of TiO₂ (20 or 40 mg/kg b.wt.) through oral gavage once daily for 90 consecutive days on the blood components, immunity, cytotoxic, and genotoxic indicators.

RESULTS

A dose-dependent leukopenia, eosinophilia, neutrophilia, and thrombocytopenia were noted. Also, the immunoglobins G (IgG) and IgM were significantly elevated in TiO₂ treated rats. The phagocytic activities, lysozyme, nitric oxide, and immunoglobulin levels were significantly depleted following TiO₂ exposure. A significantly reduced lymphocyte proliferation but elevated LDH activity was prominent in TiO₂ treated rats. Different pathological perturbations were observed in both splenic tissue and bone marrow. A marked increase in CD4⁺ and CD8⁺ immunolabeling was evident. A significant increase in the comet variables was recorded in response to the exposure of rats to the increasing level of TiO₂ at both levels.

CONCLUSION

Overall, these results indicated that TiO₂ could induce hematotoxicity, genotoxic, and immunotoxic alterations with exposure for long durations.

Titanium Dioxide Nanoparticles in Food and Personal Care Products-What Do We Know about Their Safety?

Titanium dioxide (TiO2) is a material of diverse applications commonly used as a food additive or cosmetic ingredient. Its prevalence in products of everyday use, especially in nanosize, raises concerns about safety. Current findings on the safety of titanium dioxide nanoparticles (TiO2 NPs) used as a food additive or a sunscreen compound are reviewed and systematized in this publication. Although some studies state that TiO2 NPs are not harmful to humans through ingestion or via dermal exposure, there is a considerable number of data that demonstrated their toxic effects in animal models. The final agreement on the safety of this nanomaterial has not yet been reached among researchers. There is also a lack of official, standardized guidelines for thorough characterization of TiO2 NPs in food and cosmetic products, provided by international authorities. Recent advances in the application of 'green-synthesized' TiO2 NPs, as well as comparative studies of the properties of 'biogenic' and 'traditional' nanoparticles, are presented. To conclude, perspectives and directions for further studies on the toxicity of TiO2 NPs are proposed.

Weight of epidemiological evidence for titanium dioxide risk assessment: current state and further needs

We address here the importance of epidemiological evidence in risk assessment and decision-making in Europe. To illustrate this, titanium dioxide (TiO₂) was used as a model compound. TiO₂ is widely used as an odorless white pigment and opacifying agent. A recent systematic review assessing the weight of evidence on the relationship between exposure to TiO₂ (all forms) and cancer in humans questions the assumptions that TiO₂ is an inert material of low toxicity. Based on this new data, France submitted a proposal to classify TiO₂ as a possible human carcinogen under the European regulation. The European Chemicals Agency Risk assessment committee concluded that TiO₂ (all forms) warrants a classification as a suspected human carcinogen via inhalation (Category-2) under the CLP regulation (for Classification, Labeling and Packaging of chemicals). No considerations was given to TiO₂ particle size, which may affect human health effects. Consequently, further epidemiological studies are needed to assess possible associations between different physical-chemical characteristics of TiO₂ exposures and their impact on human health. This would allow strengthening the evidence on which to build the most appropriate regulation and to guaranty safe use given any exposure route of any TiO₂ particle shape or size.

Mechanism of Action of TiO2: Recommendations to Reduce Uncertainties Related to Carcinogenic Potential

The Risk Assessment Committee of the European Chemicals Agency issued an opinion on classifying titanium dioxide (TiO₂) as a suspected human carcinogen upon inhalation. Recent animal studies indicate that TiO₂ may be carcinogenic through the oral route. There is considerable uncertainty on the carcinogenicity of TiO₂, which may be decreased if its mechanism of action becomes clearer. Here we consider adverse outcome pathways and present the available information on each of the key events (KEs). Inhalation exposure to TiO₂ can induce lung tumors in rats via a mechanism that is also applicable to other poorly soluble, low-toxicity particles. To reduce uncertainties regarding human relevance, we recommend gathering information on earlier KEs such as oxidative stress in humans. For oral exposure, insufficient information is available to conclude whether TiO₂ can induce intestinal tumors. An oral carcinogenicity study with well-characterized (food-grade) TiO₂ is needed, including an assessment of toxicokinetics and early KEs.

Cardiopulmonary effects induced by occupational exposure to titanium dioxide nanoparticles

Although some toxicological studies have reported that exposure to titanium dioxide nanoparticles (nano-TiO₂) may elicit adverse cardiopulmonary effects, related data collected from human are currently limited. The purpose of this study is to explore cardiopulmonary effects among workers who were exposed to nano-TiO₂ and to identify biomarkers associated with exposure. A cross-sectional study was conducted in a nano-TiO₂ manufacturing plant in eastern China. Exposure assessment and characterization of TiO₂ particles were performed in a packaging workshop. Physical examination and possible biomarkers for cardiopulmonary effects were examined among 83 exposed workers and 85 controls. In packaging workshop, the total mass concentration of particles was 3.17 mg/m³. The mass concentration of nanoparticles was 1.22 mg/m³ accounting for 39% of the total mass. Lung damage markers (SP-D and pulmonary function), cardiovascular disease markers (VCAM-1, ICAM-1, LDL, and TC), oxidative stress markers (SOD and MDA), and inflammation markers (IL-8, IL-6, IL-1β, TNF-α, and IL-10) were associated with occupational exposure to nano-TiO₂. Among those markers, SP-D showed a time (dose)-response pattern within exposed workers. The data strongly suggest that nano-TiO₂ could contribute, at least in part, to the cardiopulmonary effects observed in workers. The studied markers and pulmonary function tests may be useful in health surveillance for workers exposed to nanomaterials.

Relevance of the rat lung tumor response to particle overload for human risk assessment-Update and interpretation of new data since ILSI 2000

The relevance of particle-overload related lung tumors in rats for human risk assessment following chronic inhalation exposures to poorly soluble particulates (PSP) has been a controversial issue for more than three decades. In 1998, an ILSI (International Life Sciences) Working Group of health scientists was convened to address this issue of applicability of experimental study findings of lung neoplasms in rats for lifetime-exposed production workers to PSPs. A full consensus view was not reached by the Workshop participants, although it was generally acknowledged that the findings of lung tumors in rats following chronic inhalation, particle-overload PSP exposures occurred only in rats and no other tested species; and that there was an absence of lung cancers in PSP-exposed production workers. Since the publication of the ILSI Workshop report in 2000, there have been important new data published on the human relevance issue. A thorough and comprehensive review of the health effects literature on poorly soluble particles/lung overload was undertaken and published by an ECETOC (European Centre for Ecotoxicology and Toxicology of Chemicals) Task Force in 2013. One of the significant conclusions derived from that technical report was that the rat is unique amongst all species in developing lung tumors under chronic inhalation overload exposures to PSPs. Accordingly, the objective of this review is to provide important insights on the fundamental differences in pulmonary responses between experimentally-exposed rats, other experimental species and occupationally-exposed humans. Briefly, five central factors are described by the following issues. Focusing on these five interrelated/convergent factors clearly demonstrate an inappropriateness in concluding that the findings of lung tumors in rats exposed chronically to high concentrations of PSPs are accurate representations of the risks of lung cancer in PSP-exposed production workers. The most plausible conclusion that can be reached is that results from chronic particle-overload inhalation studies with PSPs in rats have no relevance for determining lung cancer risks in production workers exposed for a working lifetime to these poorly soluble particulate-types.

How to measure hazards/risks following exposures to nanoscale or pigment-grade titanium dioxide particles

Due to its multifunctional applications, titanium dioxide particles have widespread use in commerce. The particle-types function as sources of pigment color, in food products, anti-bacterial components, ultraviolet radiation scavengers, catalysts, as well as in cosmetics. Because of its inherent properties in a diverse number of products, exposures may occur via any of the major point-of-entry routes, i.e., inhalation, oral or dermal. Although the majority of TiO2 applications are known to exist in the pigment-grade form, nanoscale forms of TiO2 are also common components in several products. This brief review is designed to identify relevant toxicology and risk-related issues which inform health effects assessments on the various forms of titanium dioxide particles. While there has been an abundance of hazard data generated on titanium dioxide particulates, many of the published reports have limited informational value for assessing health effects due, in large part, to shortcomings in experimental design issues, such as: (1) inadequate material characterization of test samples; (2) questionable relevance of experimental systems employed to simulate human exposures; (3) applications of generally high doses, exclusive focus on acute toxicity endpoints, and a lack of reference benchmark control materials, to afford interpretation of measured results; and/or (4) failure to recognize fundamental differences between hazard and risk concepts. Accordingly, a number of important toxicology issues are identified and integrated herein to provide a more comprehensive assessment of the health risks of different forms of pigment-grade and nanoscale titanium dioxide particles. It is important to note that particle-types of different TiO2 compositions may have variable toxicity potencies, depending upon crystal structure, particle size, particle surface characteristics and surface coatings. In order to develop a more robust health risk evaluation of TiO2 particle exposures, this review focuses on the following issues: (1) Introduction to TiO2 particle chemistry/functionality and importance of robust material characterization of test samples; (2) Implementation of meaningful hazard studies for gauging EHS safety issues – pulmonary bioassay data and development of the Nano Risk Framework for developmental nano TiO2 compounds; (3) Epidemiological study findings on titanium dioxide workers – the most heavily-exposed populations; (4) Methodologies for setting occupational exposure limits including benchmarking or bridging comparisons; and (5) The importance of particle overload data in the lungs of rats as it relates to gauging the relevance of health effects for humans. A comprehensive evaluation of the existing animal and human health data is a necessary prerequisite for facilitating accurate assessments of human health risks to TiO2 exposures.