Triangulation of epidemiological evidence and risk of bias evaluation: A proposed framework and applied example using formaldehyde exposure and risk of myeloid leukemias.

Evidence triangulation may help identify the impact of study design elements on study findings and to tease out biased results when evaluating potential causal relationships; however, methods for triangulating epidemiologic evidence are evolving and have not been standardized. Building upon key principles of epidemiologic evidence triangulation and risk of bias assessment, and responding to the National Academies of Sciences, Engineering, and Medicine (NASEM) call for applied triangulation examples, the objective of this manuscript is to propose a triangulation framework and to apply it as an illustrative example to epidemiologic studies examining the possible relationship between occupational formaldehyde exposure and risk of myeloid leukemias (ML) including acute (AML) and chronic (CML) types. A nine-component triangulation framework for epidemiological evidence was developed incorporating study quality and ROB guidance from various federal health agencies (i.e., US EPA TSCA and NTP OHAT). Several components of the triangulation framework also drew from widely used epidemiological analytic tools such as stratified meta-analysis and sensitivity analysis. Regarding the applied example, fourteen studies were identified and assessed using the following primary study quality domains to explore potential key sources of bias: 1) study design and analysis; 2) study participation; 3) exposure assessment; 4) outcome assessment; and 5) potential confounding. Across studies, methodological limitations possibly contributing to biased results were observed within most domains. Interestingly, results from one study - often providing the largest and least-precise relative risk estimates, likely reflecting study biases, deviated from most primary study findings indicating no such associations. Triangulation of epidemiological evidence appears to be helpful in exploring inconsistent results for the identification of study results possibly reflecting various biases. Nonetheless, triangulation methodologies require additional development and application to real-world examples to enhance objectivity and reproducibility.

Systematic review of the association between talc and female reproductive tract cancers.

Talc is a hydrous magnesium sheet silicate used in cosmetic powders, ceramics, paints, rubber, and many other products. We conducted a systematic review of the potential carcinogenicity of genitally applied talc in humans. Our systematic review methods adhere to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and incorporated aspects from the US Institute of Medicine (IOM, now the National Academy of Medicine) and several US EPA frameworks for systematic reviews, evaluating and integrating the epidemiological, animal, and mechanistic literature on talc and cancer. We conducted a comprehensive literature search. Detailed data abstraction and study quality evaluation, adapting the Toxic Substances Control Act (TSCA) framework, were central to our analysis. The literature search and selection process identified 40 primary studies that assessed exposure to talc and female reproductive cancer risks in humans (n = 36) and animals (n = 4). The results of our evaluation emphasize the importance of considering biological plausibility and study quality in systematic review. Integrating all streams of evidence according to the IOM framework yielded classifications of suggestive evidence of no association between perineal application of talcum powders and risk of ovarian cancer at human-relevant exposure levels. We also concluded that there is suggestive evidence of no association between genital talc application and endometrial cancer, and insufficient evidence to determine whether a causal association exists between genital talc application and cervical cancer based on a smaller but largely null body of literature.

Systematic review of the human health hazards of propylene dichloride.

Propylene dichloride (PDC) is a chlorinated substance used primarily as an intermediate in basic organic chemical manufacturing. The United States Environmental Protection Agency (EPA) is currently evaluating PDC as a high-priority substance under the Toxic Substances Control Act (TSCA). We conducted a systematic review of the non-cancer and cancer hazards of PDC using the EPA TSCA and Integrated Risk Information System (IRIS) frameworks. We identified 12 epidemiological, 16 toxicokinetic, 34 experimental animal, and 49 mechanistic studies. Point-of-contact respiratory effects are the most sensitive non-cancer effects after inhalation exposure, and PDC is neither a reproductive nor a developmental toxicant. PDC is not mutagenic in vivo, and while in vitro evidence is mixed, DNA strand breaks consistently occur. Nasal tumors in rats and lung tumors in mice occurred after lifetime high-level inhalation exposure. Cholangiocarcinoma (CCA) was observed in Japanese print workers exposed to high concentrations of PDC. However, co-exposures, as well as liver parasites, hepatitis, and other risk factors, may also have contributed. The cancer mode of action (MOA) analysis revealed that PDC may act through multiple biological pathways occurring sequentially and/or simultaneously, although chronic tissue damage and inflammation likely dominate. Critically, health benchmarks protective of non-cancer effects are expected to protect against cancer in humans.

Mode of action assessment for propylene dichloride as a human carcinogen.

As part of a systematic review of the non-cancer and cancer hazards of propylene dichloride (PDC), with a focus on potential carcinogenicity in workers following inhalation exposures, we determined that a mode of action (MOA)-centric framing of cancer effects was warranted. In our MOA analysis, we systematically reviewed the available mechanistic evidence for PDC-induced carcinogenesis, and we mapped biologically plausible MOA pathways and key events (KEs), as guided by the International Programme on Chemical Safety (IPCS)-MOA framework. For the identified pathways and KEs, biological concordance, essentiality of KEs, concordance of empirical observations among KEs, consistency, and analogy were evaluated. The results of this analysis indicate that multiple biologically plausible pathways may contribute to the cancer MOA for PDC, but that the relevant pathways vary by exposure route and level, tissue type, and species; further, more than one pathway may occur concurrently at high exposure levels. While several important data gaps exist, evidence from in vitro mechanistic studies, in vivo experimental animal studies, and ex vivo human tumor tissue analyses indicates that the predominant MOA pathway likely involves saturation of cytochrome p450 2E1 (CYP2E1)-glutathione (GSH) detoxification (molecular initiating event; MIE), accumulation of CYP2E1-oxidative metabolites, cytotoxicity, chronic tissue damage and inflammation, and ultimately tumor formation. Tumors may occur through several subsets of inflammatory KEs, including inflammation-induced aberrant expression of activation-induced cytidine deaminase (AID), which causes DNA strand breaks and mutations and can lead to tumors with a characteristic mutational signature found in occupational cholangiocarcinoma. Dose concordance analysis showed that low-dose mutagenicity (from any pathway) is not a driving MOA, and that prevention of target tissue damage and inflammation (associated with saturation of CYP2E1-GSH detoxification) is expected to also prevent the cascade of processes responsible for tumor formation.

Strategies for refinement of occupational inhalation exposure evaluation in the EPA TSCA risk evaluation process.

The focus on occupational exposures in the first published risk evaluations of existing chemicals by the Environmental Protection Agency (EPA) under the amended Toxic Substances Control Act (TSCA) puts a welcome spotlight on protecting the health of workers in the United States. Because new, fit-for-purpose occupational exposure assessment methodologies were developed by EPA, the objective of this analysis was to evaluate these methodologies in light of other existing occupational risk assessment frameworks. We focused our analysis on three chlorinated chemicals (methylene chloride, carbon tetrachloride, perchloroethylene). The EPA's methods were evaluated relative to peer-reviewed and professional organizations' guidelines for conducting site- and facility-based exposure assessment. Analyses of several key phases in the EPA approach were conducted to evaluate the effect of alternative approaches on exposure estimates. The revised exposure estimates using these alternative approaches yielded substantially different exposure estimates from those in the TSCA risk evaluations for these chemicals. The results also demonstrated the importance of utilizing a tiered approach to exposure estimation that includes collecting qualitative data, defining similar exposure groups, and integrating well-parameterized models with empirical data. These approaches aid in preventing mischaracterization of exposures and generating exposure estimates representative of current industrial practices. Collaboration among industry, EPA, and other government agencies to develop a harmonized approach to exposure assessment would improve the methodological rigor of, and increase stakeholder confidence in, the results of TSCA risk evaluations.

Analysis of dermal exposure assessment in the US Environmental Protection Agency Toxic Substances Control Act risk evaluations of chemical manufacturing.

The United States Environmental Protection Agency (EPA) regulates chemical manufacture, import, processing, distribution, use, and disposal under the 2016 amended Toxic Substances Control Act (TSCA) for the purposes of protecting the public and sensitive populations-including workers-from chemical exposure risk. The publication of several TSCA risk evaluations provided a unique opportunity to evaluate the evolving regulatory approach for assessing the dermal exposure pathway in occupational settings. In this analysis, the occupational dermal exposure assessment methods employed in several TSCA risk evaluations were assessed. Specifically, a methodology review was conducted for the occupational dermal scenarios of manufacturing and feedstock use in the risk evaluations of three chlorinated organic chemicals: trichloroethylene, carbon tetrachloride, and perchloroethylene. Additionally, alternative exposure estimates were generated using the exposure model IH SkinPermTM. The review and alternative exposure analyses indicate that the current TSCA modeling approach may generate total dermal absorbed doses for chlorinated chemical manufacturing and feedstock use scenarios that are 2- to 20-fold higher than those generated by IH SkinPerm. Best-practice recommendations developed in the methodology review support a tiered, integrated approach to dermal exposure assessment that emphasizes collecting qualitative data; employing validated, peer-reviewed models that align with current industrial practices; and gathering empirical sampling data where needed. Collaboration among industry, EPA, and other stakeholders to share information and develop a standard approach to exposure assessment under TSCA would improve the methodological rigor of, and increase confidence in, the risk evaluation results.

Systematic review of the scientific evidence of the pulmonary carcinogenicity of talc.

We conducted a systematic review to assess the potential pulmonary carcinogenicity of inhaled talc in humans. Our systematic review methods adhere to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and incorporated aspects from the US Institute of Medicine (IOM) and several United States (US) Environmental Protection Agency (EPA) frameworks for systematic reviews. A comprehensive literature search was conducted. Detailed data abstraction and study quality evaluation, adapting the US Toxic Substances Control Act (TSCA) framework, were central to our analysis. The literature search and selection process identified 23 primary studies that assessed exposure to talc and pulmonary cancer risks in humans (n = 19) and animals (n = 3). Integrating all streams of evidence according to the IOM framework yielded classifications of suggestive evidence of no association between inhaled talc and lung cancer and pleural mesothelioma at human-relevant exposure levels.

Systematic review of the scientific evidence on ethylene oxide as a human carcinogen.

Ethylene oxide is a highly reactive chemical primarily used as an intermediate in chemical production and as a sterilant of medical equipment and food products; it also is produced endogenously as a result of physiological processes. We conducted a systematic review of the potential carcinogenicity of inhaled ethylene oxide in humans using methods that adhere to PRIMSA guidelines and that incorporate aspects from the Institute of Medicine (IOM) (now the National Academy of Medicine) as well as several US Environmental Protection Agency (EPA) frameworks for systematic reviews. After a comprehensive literature search and selection process, study quality was evaluated following a method adapted from the EPA Toxic Substances Control Act (TSCA) framework. The literature screening and selection process identified 24 primary studies in animals or humans and more than 50 mechanistic studies. Integrating epidemiological, animal, and mechanistic literature on ethylene oxide and cancer according to the IOM framework yielded classifications of suggestive evidence of no association between ethylene oxide and stomach cancer, breast cancer and lymphohematopoietic malignancies at human relevant exposures. However, we acknowledge that there is additional uncertainty in the classification for lymphohematopoietic malignancies owing to a paucity of evidence for specific types of these tumors, each of which is a distinct disease entity of possibly unique etiology.

Lost in the woods: Finding our way back to the scientific method in systematic review.

Systematic review has become the preferred approach to addressing causality and informing regulatory and other decision-making processes, including chemical risk assessments. While advocates of systematic reviews acknowledge that they hold great potential for increasing objectivity and transparency in assessments of chemicals and human health risks, standardizing and harmonizing systematic review methods have been challenging. This review provides a brief summary of the development of systematic review methods and some of the frameworks currently in use in the US and Europe. We also provide an in-depth evaluation and comparison of two "competing" US EPA systematic review frameworks, informed by the constructively critical recommendations from the US National Academies of Science, Engineering and Medicine. We conclude with suggestions for moving forward to harmonize systematic review methods, as we believe that further criticism of individual available frameworks likely will be unproductive. Specifically, we issue a call to action for an international collaboration to work toward a blueprint that embraces the most scientifically critical elements common to most systematic review frameworks, while necessarily accommodating adaptations for specific purposes. Despite the array of available systematic review methods, it is clear that there is a shared goal and desire to promote objective assessment and synthesis of scientific evidence informing globally important issues regarding disease causality and human health risk evaluation.

Lung physiology and controlled exposure study design.

Controlled human inhalation exposure ( CHIE) studies provide a unique opportunity to conduct formal experiments to examine the human health effects of airborne pollutants. Lung function, easily measured using spirometry, is a common physiological variable often utilized in these studies. By design, CHIE studies only induce mild and reversible acute effects, which may or may not predict adverse effects that may develop under chronic exposure conditions. There is substantial inter- and intra-individual variability in functional capacity and symptoms such as chest tightness and dyspnea, which are complex variables that are affected by individual perception, physiological lung impairment, and other variables (e.g., concomitant health conditions, and level of conditioning/fitness). Thus, the design of the CHIE study and physiological and environmental factors of study participants can affect each CHIE study's results. Researchers can address many of these critical issues in the problem formulation phase of CHIE studies, utilizing existing information on the expected effects of the substance of interest and possible modes of action. Thoughtful design and interpretation of CHIE studies will increase their utility for evaluating and setting environmental health policy.

Critique of the ACGIH 2016 derivation of toluene diisocyanate Threshold Limit Values.

In 2016, the American Conference of Governmental Industrial Hygienists (ACGIH) lowered the 8-hr Threshold Limit Value - time-weighted average (TLV-TWA) for toluene diisocyanate (TDI) from 5 ppb to 1 ppb, and the 15-min short-term exposure limit (STEL) from 20 ppb to 5 ppb. We evaluated ACGIH's basis for lowering these values. It is our opinion that the ACGIH's evaluation of the evidence for occupational asthma and respiratory effects from TDI exposure does not fully integrate the results of all the available human and animal studies. We found that some studies reported occupational asthma cases at TWAs less than 5 ppb, but these cases were likely caused by peak exposures above 20 ppb. Advances in industrial hygiene have reduced peak exposures and the incidence of upset conditions, such as spills and accidents, in modern TDI facilities. Taken together, the human evidence indicates that adherence to the previous 8-hr TLV-TWA and 15-min STEL (5 ppb and 20 ppb, respectively) prevents most, if not all, cases of occupational asthma, and eliminates or reduces the risk of lung function decrements and other respiratory effects. While limited, the animal literature supports the human evidence and indicates that TDI-induced asthma is a threshold phenomenon. We conclude that ACGIH's decision to lower the TLV-TWA and STEL values for TDI is not adequately supported.

Short-term ozone exposure and asthma severity: Weight-of-evidence analysis.

To determine whether evidence indicates that short-term exposure to ambient concentrations of ozone in the United States can affect asthma severity, we systematically reviewed published controlled human exposure, epidemiology, and animal toxicity studies. The strongest evidence for a potential causal relationship came from epidemiology studies reporting increased emergency department visits and hospital admissions for asthma following elevated ambient ozone concentrations. However, while controlled exposure studies reported lung function decrements and increased asthma symptoms following high ozone exposures 160-400 parts per billion [ppb]), epidemiology studies evaluating similar outcomes reported less consistent results. Animal studies showed changes in pulmonary function at high ozone concentrations (> 500ppb), although there is substantial uncertainty regarding the relevance of these animal models to human asthma. Taken together, the weight of evidence indicates that there is at least an equal likelihood that either explanation is true, i.e., the strength of the evidence for a causal relationship between short-term exposure to ambient ozone concentrations and asthma severity is "equipoise and above."

Dermal exposure to toluene diisocyanate and respiratory cancer risk.

Human exposure to toluene diisocyanate (TDI) occurs mainly through inhalation of vapors in occupational settings where TDI is produced or used, but dermal exposure to TDI is also possible during some operations. Because of a recent epidemiology study reporting a possible association with lung cancer risk in workers with potential dermal exposure to TDI, we evaluated the evidence from epidemiological, toxicological, and toxicokinetic studies to assess whether it is likely that dermal exposure to TDI can cause human respiratory cancers. We found that the reported associations with respiratory cancers in the epidemiology studies do not support TDI as a causal factor, as there are other explanations that are more likely than causation, such as confounding by smoking and low socioeconomic status. Experimental animal and genotoxicity studies indicate that the carcinogenic potential of TDI depends on its conversion to toluene diamine (TDA), and there is no evidence of systemic availability of TDA after dermal or inhalation exposure to TDI. Also, systemic uptake of TDI is very low after dermal exposure, and any absorbed TDI is more likely to react with biomolecules on or below the skin surface than to form TDA. Even if some TDA formation occurred after dermal exposure to TDI, TDA does not induce respiratory tract tumors in experimental animals after either dermal or oral exposure. We conclude that the available evidence indicates that dermal TDI exposure does not cause respiratory cancers in humans.

Quantitative assessment of lung and bladder cancer risk and oral exposure to inorganic arsenic: Meta-regression analyses of epidemiological data.

Inorganic arsenic (iAs) in drinking water varies geographically and is prevalent worldwide. While exposures in the US are generally low, there are some areas with higher levels of naturally occurring iAs (potentially >100µg/L) where residents rely on unregulated drinking water wells. Much of the evidence on the association between iAs and cancer comes from epidemiological studies conducted in South American and Asian populations. These populations have generally been exposed to much higher levels of iAs and have differing underlying characteristics, both of which make comparing them to Western populations difficult. A key question is whether and how one should extrapolate from these high exposure studies to estimate cancer risk at lower exposures. We conducted an independent analysis to determine the most appropriate cancer endpoints, studies, and models to support an oral carcinogenicity assessment of iAs, taking into consideration factors that affect the apparent potency of iAs across geographically and culturally distinct populations. We identified bladder and lung cancer as high-priority endpoints and used meta-regression to pool data across studies from different regions of the world to derive oral cancer slope factors (CSFs) and unit risks (excess risk per µg/L) for iAs based on the background risks of bladder and lung cancer in the US. We also calculated concentrations of iAs in water that are not likely to result in cancer risk above what is considered acceptable by the United States Environmental Protection Agency (US EPA). While we derived these factors assuming a linear, no-threshold relationship between iAs and cancer risk, we also evaluated the shape of the dose-response curves and assessed the evidence for overall nonlinearity. Overall, we found that the incremental risks of bladder and lung cancer associated with iAs were relatively low. The sensitivity analyses we conducted suggested that populations with relatively high iAs exposures appeared to drive the pooled cancer risk estimates, but many of our other tested assumptions did not substantially alter these estimates. Finally, we found that the mode of action evidence supports there being a threshold, but making a robust quantitative demonstration of a threshold using epidemiological data is difficult. When considered in the context of typical exposure levels in the US, our potency estimates indicate that iAs-induced cancer risk is much lower than observed bladder and lung cancer incidences. This suggests that the low iAs levels to which much of the general US population is exposed likely do not result in substantial additional cancer risk.

Systematic comparison of study quality criteria.

Approaches for the systematic review and evaluation of chemical toxicity are currently being reconsidered, with a specific focus on the evaluation of individual studies and their integration into the overall body of evidence. This renewed interest has arisen, in part, as a result of several prominent reviews of these approaches by special committees of the National Research Council (NRC), among others. We conducted a critical evaluation of several available frameworks for evaluating study quality. We assessed the criteria separately for human, animal, and in vitro studies as well as for systematic reviews. We then evaluated commonalities across disciplines. We also considered the potential implications of applying criteria frameworks and how they bear on fundamental risk assessment questions. We found that the available frameworks within each discipline differed in terms of their intended purpose and level of guidance for decision making. All the frameworks across disciplines shared common themes, however, including the adequate reporting of specific details of study conditions and design/protocol, selection and randomization of study groups (where applicable), outcome assessment methods and applicability (e.g., validity and reliability), avoidance of selective reporting, and the consideration of potential confounders or bias. We identified the most informative study quality considerations, which will enable researchers to implement more objective and standardized methods for evaluating studies and, ultimately, improve risk assessment methods.

Ozone exposure and systemic biomarkers: Evaluation of evidence for adverse cardiovascular health impacts.

The US Environmental Protection Agency (EPA) recently concluded that there is likely to be a causal relationship between short-term (< 30 days) ozone exposure and cardiovascular (CV) effects; however, biological mechanisms to link transient effects with chronic cardiovascular disease (CVD) have not been established. Some studies assessed changes in circulating levels of biomarkers associated with inflammation, oxidative stress, coagulation, vasoreactivity, lipidology, and glucose metabolism after ozone exposure to elucidate a biological mechanism. We conducted a weight-of-evidence (WoE) analysis to determine if there is evidence supporting an association between changes in these biomarkers and short-term ozone exposure that would indicate a biological mechanism for CVD below the ozone National Ambient Air Quality Standard (NAAQS) of 75 parts per billion (ppb). Epidemiology findings were mixed for all biomarker categories, with only a few studies reporting statistically significant changes and with no consistency in the direction of the reported effects. Controlled human exposure studies of 2 to 5 hours conducted at ozone concentrations above 75 ppb reported small elevations in biomarkers for inflammation and oxidative stress that were of uncertain clinical relevance. Experimental animal studies reported more consistent results among certain biomarkers, although these were also conducted at ozone exposures well above 75 ppb and provided limited information on ozone exposure-response relationships. Overall, the current WoE does not provide a convincing case for a causal relationship between short-term ozone exposure below the NAAQS and adverse changes in levels of biomarkers within and across categories, but, because of study limitations, they cannot not provide definitive evidence of a lack of causation.

Mechanisms of action for arsenic in cardiovascular toxicity and implications for risk assessment.

The possibility of an association between inorganic arsenic (iAs) exposure and cardiovascular outcomes has received increasing attention in the literature over the past decade. The United States Environmental Protection Agency (US EPA) is currently revising its Integrated Risk Assessment System (IRIS) review of iAs, and one of the non-cancer endpoints of interest is cardiovascular disease (CVD). Despite the increased interest in this area, substantial gaps remain in the available information, particularly regarding the mechanism of action (MOA) by which iAs could cause or exacerbate CVD. Few studies specifically address the plausibility of an association between iAs and CVD at the low exposure levels which are typical in the United States (i.e., below 100 µg As/L in drinking water). We have conducted a review and evaluation of the animal, mechanistic, and human data relevant to the potential MOAs of iAs and CVD. Specifically, we evaluated the most common proposed MOAs, which include disturbance of endothelial function and hepatic dysfunction. Our analysis of the available evidence indicates that there is not a well-established MOA for iAs in the development or progression of CVD. Few human studies of the potential MOAs have addressed plausibility at low doses and the applicability of extrapolation from animal studies to humans is questionable. However, the available evidence indicates that regardless of the specific MOA, the effects of iAs on physiological processes at the cellular level appear to operate via a threshold mechanism. This finding is consistent with the lack of association of CVD with iAs exposure in humans at levels below 100 µg/L, particularly when considering important exposure and risk modifiers such as nutrition and genetics. Based on this analysis, we conclude that there are no data supporting a linear dose-response relationship between iAs and CVD, indicating this relationship has a threshold.