A Review and Meta-Analysis of Occupational Titanium Dioxide Exposure and Lung Cancer Mortality

Ultra-processed food consumption and risk of chronic respiratory diseases mortality among adults: evidence from a prospective cohort study

PURPOSE

The purpose of the study was to determine the relationships between ultra-processed food (UPF) consumption and risk of mortality due to chronic respiratory diseases (CRDs) overall, chronic obstructive pulmonary disease (COPD), and lung cancer.

METHODS

A total of 96,607 participants aged 55 years and over were included from the Prostate, Lung, Colorectal and Ovarian (PLCO) cancer trial. Dietary intake was measured using food frequency questionnaire. Cox regression was fitted to estimate the risk of all-cause mortality and mortality due to CRDs overall, COPD and lung cancer associated with UPF intake. Competing risk regression was used to account for deaths from other causes and censoring.

RESULTS

During the follow-up of 1,379,655.5 person-years (median 16.8 years), 28,700 all-cause, 4092 CRDs, 2015 lung cancer and 1,536 COPD mortality occurred. A higher intake of UPF increased the risk of mortality from CRDs overall by 10% (HR 1.10; 95% CI 1.01, 1.22) and COPD by 26% (HR 1.26; 95% CI 1.06, 1.49) but not associated with lung cancer mortality risk (HR 0.97; 95% CI 0.84, 1.12). However, the risk of lung cancer increased by 16% (HR 1.16; 95% CI 1.01, 1.34) in the highest UPF intake after multiple imputation. Dose-response relationships existed for CRDs and COPD mortality but not lung cancer.

CONCLUSION

UPF consumption was associated with an increased risk of CRD mortality. The association between UPF consumption and lung cancer mortality is inconclusive and only significant when multiple imputation was applied.

Evaluation of Systemic Genotoxic/Oxidative and Proinflammatory Effects in Workers of a Titanium Dioxide Production Plant

This study is aimed at evaluating whether the occupational exposure to TiO₂ during the industrial production process is able to induce genotoxic, oxidative, and inflammatory effects on blood, biomonitoring the same workers that showed micronucleus induction in the exfoliated buccal cells, as previous published. The final aim was to find sensitive and suitable biomarkers to evaluate potential early toxicity of occupational exposure to TiO₂. On the same 40 workers involved in the manufacture of TiO₂ pigment, 5 office workers, and 18 controls previously studied, we used formamidopyrimidine glycosylase- (Fpg-) comet assay on lymphocytes to evaluate genotoxic/oxidative effects and detected cytokine (IL-6, IL-8, and TNFα) release by ELISA to evaluate proinflammation. Moreover, we studied the possible influence of single nucleotide polymorphisms of XRCC1 and hOGG1 DNA repair genes and of GST metabolism-related genes (GSTT1 and GSTM1) on the evaluated effects. We did not find statistically significant differences in the mean values of the analysed Fpg-comet assay parameters; only the percentage of DNA damaged cells appearing in the test as comets (% comets) resulted higher in the exposed workers compared to controls. Also, the data analysed taking into account the specific task (bagging, industrial cleaning, mobile operations, maintaining, and production) showed differences only for % comets which resulted higher in industrial cleaners compared to controls. We found variations of IL-6 and IL-8 levels in the exposed workers with concentrations that were lower for IL-6 and higher for IL-8 compared to the control group. XRCC1, hOGG1, and GSTT1 polymorphisms did not influence neither comet parameters nor cytokine release. These findings demonstrate that TiO₂ production process is able to induce slight proinflammatory effects in terms of IL-8 increased release but not significant genotoxic/oxidative effects on lymphocytes, which do not seem to be a target of TiO₂, prevalently inhalable particles, generated in the studied production site.

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.

Review of Lung Particle Overload, Rat Lung Cancer, and the Conclusions of the Edinburgh Expert Panel—It's Time to Revisit Cancer Hazard Classifications for Titanium Dioxide and Carbon Black

Chronic inhalation of titanium dioxide or carbon black by rats at concentrations which overload lung particle clearance can result in lung cancer. Based on this rat lung response, IARC, NIOSH, and ECHA classified titanium dioxide, and IARC classified carbon black, as potential human carcinogens. These classifications have been questioned based on an extensive data base demonstrating: the rat lung cancer occurred only under conditions of extreme lung particle overload; the lung cancer response in rats has not been seen in other animal species; and studies in titanium dioxide and carbon black exposed human populations have not shown an increased incidence of cancer. In 2019 an international panel of science and regulatory experts was convened to document the state of the science on lung particle overload and rat lung cancer after exposure to poorly soluble low toxicity particles. Regarding hazard identification, the expert panel concluded, in the absence of supporting data from other species, lung particle overload-associated rat lung cancer does not imply a cancer hazard for humans. Regarding high to low dose extrapolation, the expert panel concluded rat lung tumors occurring only under conditions of lung particle overload are not relevant to humans exposed under non-overloading conditions. The conclusions of the Edinburgh Expert Panel directly conflict with IARC, ECHA and NIOSH's extrapolation of lung particle overload associated rat lung cancer to hazard for humans. The hazard classifications for titanium dioxide and carbon black inhalation should be assessed considering the state-of-the-science on lung particle overload and rat lung cancer.

Carcinogenicity of Poorly Soluble Low Toxicity Particles: Commentary on Epidemiology as a Risk Assessment “Reality Check”

Inhaled particles that are poorly soluble or insoluble and of low toxicity (“poorly soluble low toxicity” or “PSLT” particles), can accumulate in the lung and at lung overload levels induce lung cancers in rats. The question of whether PSLT particles increase lung cancer risk in humans is complicated by large differences between rats and humans and the relatively large particle doses administered in animal studies even when compared with heavy human occupational exposures. We review the findings of epidemiological studies on occupational exposure to each of three different PSLT particles (carbon black, talc and taconite). The epidemiological evidence indicates that at even very high occupational exposure levels at which non-malignant respiratory diseases including pneumoconiosis and even talcosis are observed, lung cancer risks appear not to be elevated. Although positive human cancer risks might be predicted based on extrapolation from overload doses in rats to relevant exposures in humans, the epidemiological “reality check” based on the three examples indicates that these PSLT particles are unlikely to increase lung cancer risk in humans even at high occupational levels of exposure. Therefore, we propose that careful evaluation of the epidemiological evidence can serve as a “reality check” for human risk assessment and help balance the risk evaluation process.

Biomonitoring of workers employed in a titanium dioxide production plant: Use of buccal micronucleus cytome assay as noninvasive biomarker to evaluate genotoxic and cytotoxic effects

We aimed to evaluate whether TiO₂ production process induces genotoxic and cytotoxic effects on the first target organ of inhalable particles by a sensitive and noninvasive biomarker of effect. Final aim was to find a useful and suitable tool to assess and manage the risk of TiO₂ occupational exposure. We enrolled 40 workers employed in TiO₂ production, 5 office workers, and 18 external controls. Buccal micronucleus cytome assay (BMCyt assay) was applied because it allows to evaluate micronucleus (MN), nuclear buds (NB), and broken eggs (BE) indicating the presence of chromosomal instability and gene amplification and binucleated cells (BIN), karyolytic cells (KL), and condensed chromatin (CC) indicating cytokinesis defect or arrest, cell death and apoptosis respectively. We characterized the exposure measuring inhalable and respirable particles by personal monitoring. BMCyt-assay showed in exposed workers compared with external controls a higher value of MN frequency (2.57 vs. 0.05‰, p < .001) and MN positivity, evaluated as percentage of subjects with MN frequency higher than a 1.5‰ cut-off value (52.5 vs. 0%). We also found in exposed workers higher frequency of BE + NB (2.41 vs. 0.22‰, p = .002), BIN (9.45 vs. 8.44‰, p = .047) and CC (1.80 vs. 0.21, p = .001) than in controls. Moreover, we found a relationship between personal monitoring results and presence of MN and other cellular anomalies. This study demonstrates induction of genotoxic and cytotoxic effects on buccal cells of workers involved in TiO₂ production, suggesting the suitability of BMCyt assay as tool for risk assessment and management of TiO₂ exposure.

Comment on Saber et al. (2019), "Commentary: the chronic inhalation study in rats for assessing lung cancer risk may be better than its reputation"

In their Commentary Saber et al. (Part Fibre Toxicol 16: 44, 2019) argue that chronic inhalation studies in rats can be used for assessing the lung cancer risk of insoluble nanomaterials. The authors make several significant errors in their interpretation and representation of the underlying science. In this Letter to the Editor we discuss these inaccuracies to correct the scientific record. When the science is recounted accurately it does not support Saber et al's statements and conclusions.

A response to the letter to the editor by Driscoll et al

In response to the Letter to the Editor by Kevin Driscoll et al., we certainly agree that particle clearance halftimes are increased with increasing lung burden in rats, hamsters and mice, whereas complete inhibition of particle clearance has only been observed in rats, and only at high particle concentrations (50 mg/m³). Where we disagree with Kevin Driscoll and colleagues, is on the implications of the increased clearance halftimes observed at higher lung burden. We argue that it does not hamper the extrapolations from relatively high dose levels to lower dose levels.Furthermore, we highlight, again, the challenges of detecting particle-induced lung cancer in epidemiological studies where occupational, particle-induced lung cancer has to be detected on top of the background lung cancer incidence. Almost all available epidemiological studies on carbon black and titanium dioxide suffer from a number of limitations, including lack of control for smoking, the use of background population cancer rates as reference in the US studies, lack of information regarding particle size of the exposure, and incomplete follow-up for cause of death of the study population.

Acquisition of Cancer Stem Cell-like Properties in Human Small Airway Epithelial Cells after a Long-term Exposure to Carbon Nanomaterials

Cancer stem cells (CSCs) are a key driver of tumor formation and metastasis, but how they are affected by nanomaterials is largely unknown. The present study investigated the effects of different carbon-based nanomaterials (CNMs) on neoplastic and CSC-like transformation of human small airway epithelial cells and determined the underlying mechanisms. Using a physiologically relevant exposure model (long-term/low-dose) with system validation using a human carcinogen, asbestos, we demonstrated that single-walled carbon nanotubes, multi-walled carbon nanotubes, ultrafine carbon black, and crocidolite asbestos induced particle-specific anchorage-independent colony formation, DNA-strand break, and p53 downregulation, indicating genotoxicity and carcinogenic potential of CNMs. The chronic CNM-exposed cells exhibited CSC-like properties as indicated by 3D spheroid formation, anoikis resistance, and CSC markers expression. Mechanistic studies revealed specific self-renewal and epithelial-mesenchymal transition (EMT)-related transcription factors that are involved in the cellular transformation process. Pathway analysis of gene signaling networks supports the role of SOX2 and SNAI1 signaling in CNM-mediated transformation. These findings support the potential carcinogenicity of high aspect ratio CNMs and identified molecular targets and signaling pathways that may contribute to the disease development.