Toxicological evaluation of carcinogenicity of the pyrethroid imiprothrin in rats and mice.

The carcinogenic potential of a non-genotoxic pyrethroid imiprothrin was examined in rats and mice. There was no carcinogenicity in rats up to a maximum dose of 5000 ppm of the diet. There was a higher (p = 0.03) incidence of lung adenocarcinomas at 7000 ppm in males, and females showed an increasing (p < 0.01) trend in the incidence of lung adenomas and combined lung adenoma/adenocarcinomas. Additional step sections of lung demonstrated no significant increases in any tumor at p < 0.05, although an increasing trend with dose was observed among females. We argue that, the 7000 ppm dose exceeded the Maximum Tolerated Dose (MTD) for both sexes, based on systemic toxicity as evidenced by body weight gain reduction (both sexes) and high mortality (females). If the 7000 ppm dose is therefore removed from consideration, there are not significant (p < 0.05) increases in tumor formation. Moreover, a consideration of multiple comparisons reveals that the lung tumor increases observed are totally consistent with what would be expected by chance alone. Based on high susceptibility of this mouse strain for the appearance of lung tumors and the lack of a statistically significant increase in tumors by appropriate analysis, the mouse study does not indicate a carcinogenic effect of imiprothrin, and thus no classification for carcinogenicity is warranted.

A bounding quantitative cancer risk assessment for occupational exposures to asphalt emissions during road paving operations.

The International Agency for Research on Cancer recently classified straight-run bitumens and associated emissions during road paving as possibly carcinogenic to humans (Group 2B), owing to potential exposures to polycyclic aromatic hydrocarbons. We examine existing chemistry, exposure, epidemiology, and animal toxicity data to explore quantitative cancer risk implications for paving workers exposed to asphalt emissions from the data used in identifying this qualitative hazard. Epidemiology studies show no consistent cancer risk elevation. One skin-painting mouse study of paving asphalt emission condensate found a single tumor at only the highest tested dose, as did one rat inhalation study. These studies were used to develop an upper bound on possible carcinogenic potency of emissions that are inhaled or dermally deposited. Extending earlier work on roofing asphalt, we conducted time-to-tumor modeling using the dose-time-response shape for several dose levels of benzo[a]pyrene (B[a]P) in concurrent bioassay controls to infer presumed parallel dose-time-response curves for paving-asphalt-emission condensate. In addition, we developed a scientific rationale, based on general scaling considerations and on dermal uptake, for the chosen means to scale observed dermal cancer potencies in mice to apply to dermal exposures in humans. The results indicate that paving asphalt emissions have a reduced dermal cancer potency compared to roofing asphalt, consistent with the lower levels of the multi-ringed PAHs implicated in cancer risks. Based on existing occupational exposure studies, cancer risks to pavers from both dermal and inhalation exposure to asphalt emissions is within a range typically acceptable within regulatory frameworks.

Hypothesis-based weight-of-evidence evaluation and risk assessment for naphthalene carcinogenesis.

Inhalation of naphthalene causes olfactory epithelial nasal tumors in rats (but not in mice) and benign lung adenomas in mice (but not in rats). The limited available human data have not identified an association between naphthalene exposure and increased respiratory cancer risk. Assessing naphthalene's carcinogenicity in humans, therefore, depends entirely on experimental evidence from rodents. We evaluated the respiratory carcinogenicity of naphthalene in rodents, and its potential relevance to humans, using our Hypothesis-Based Weight-of-Evidence (HBWoE) approach. We systematically and comparatively reviewed data relevant to key elements in the hypothesized modes of action (MoA) to determine which is best supported by the available data, allowing all of the data from each realm of investigation to inform interpretation of one another. Our analysis supports a mechanism that involves initial metabolism of naphthalene to the epoxide, followed by GSH depletion, cytotoxicity, chronic inflammation, regenerative hyperplasia, and tumor formation, with possible weak genotoxicity from downstream metabolites occurring only at high cytotoxic doses, strongly supporting a non-mutagenic threshold MoA in the rat nose. We also conducted a dose-response analysis, based on the likely MoA, which suggests that the rat nasal MoA is not relevant in human respiratory tissues at typical environmental exposures. Our analysis illustrates how a thorough WoE evaluation can be used to support a MoA, even when a mechanism of action cannot be fully elucidated. A non-mutagenic threshold MoA for naphthalene-induced rat nasal tumors should be considered as a basis to determine human relevance and to guide regulatory and risk-management decisions.

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.

Quantitative cancer risk assessment for occupational exposures to asphalt fumes during built-up roofing asphalt (BURA) operations.

The International Agency for Research on Cancer qualitatively characterized occupational exposure to oxidized bitumen emissions during roofing as probably carcinogenic to humans (Group 2A). We examine chemistry, exposure, epidemiology and animal toxicity data to explore quantitative risks for roofing workers applying built-up roofing asphalt (BURA). Epidemiology studies do not consistently report elevated risks, and generally do not have sufficient exposure information or adequately control for confounders, precluding their use for dose-response analysis. Dermal carcinogenicity bioassays using mice report increased tumor incidence with single high doses. In order to quantify potential cancer risks, we develop time-to-tumor model methods [consistent with U.S. Environmental Protection Agency (EPA) dose-response analysis and mixtures guidelines] using the dose-time-response shape of concurrent exposures to benzo[a]pyrene (B[a]P) as concurrent controls (which had several exposure levels) to infer presumed parallel dose-time-response curves for BURA-fume condensate. We compare EPA relative potency factor approaches, based on observed relative potency of BURA to B[a]P in similar experiments, and direct observation of the inferred BURA dose-time-response (scaled to humans) as means for characterizing a dermal unit risk factor. We apply similar approaches to limited data on asphalt-fume inhalation and respiratory cancers in rats. We also develop a method for adjusting potency estimates for asphalts that vary in composition using measured fluorescence. Overall, the various methods indicate that cancer risks to roofers from both dermal and inhalation exposure to BURA are within a range typically deemed acceptable within regulatory frameworks. The approaches developed may be useful in assessing carcinogenic potency of other complex mixtures of polycyclic aromatic compounds.

Are the elements of the proposed ozone National Ambient Air Quality Standards informed by the best available science?

The United States Environmental Protection Agency (US EPA) issues National Ambient Air Quality Standards (NAAQS) for six criteria pollutants, including ozone. Each standard has four elements: an indicator, level, averaging time, and form. Ozone levels (i.e., air concentrations) alone in scientific studies are not directly comparable to the "level" element of the NAAQS because the standard considers the level in the context of its relation to the remaining elements. Failure to appreciate this has led to misunderstandings regarding NAAQS that would be health-protective. This can be seen with controlled human ozone exposure studies, which often involved small numbers of people exercising quasi-continuously for a long duration at an intensity not common in the general population (and unlikely achievable by most sensitive individuals), under worst-case exposure profiles. In addition, epidemiology studies have used different averaging times and have had methodological limitations that may have biased results. Such considerations can make it difficult to compare ozone levels and results across studies and to appropriately apply them in a NAAQS evaluation. Relating patterns and circumstances of exposure, and exposure measurements, to all elements of the NAAQS can be challenging, but if US EPA fully undertook this, it would be evident that available evidence does not indicate that proposed lower ozone standards would be more health protective than the current one.

Comments on the opinions published by Bergman et al. (2015) on Critical Comments on the WHO-UNEP State of the Science of Endocrine Disrupting Chemicals (Lamb et al., 2014).

Recently Bergman et al. (2015) took issue with our comments (Lamb et al., 2014) on the WHO-UNEP(1) report entitled the "State of the Science of Endocrine Disrupting Chemicals - 2012" (WHO 2013a). We find several key differences between their view and ours regarding the selection of studies and presentation of data related to endocrine disrupting chemicals (EDCs) under the WHO-IPCS(2) definition (2002). In this response we address the factors that we think are most important: 1. the difference between hazard and risk; 2. the different approaches for hazard identification (weight of the evidence [WOE] vs. emphasizing positive findings over null results); and 3. the lack of a justification for conceptual or practical differences between EDCs and other groups of agents.

Strengthening the foundation of next generation risk assessment.

In a recent draft report, Next Generation Risk Assessment: Incorporation of Recent Advances in Molecular, Computational, and Systems Biology, the US Environmental Protection Agency presents valuable contributions to understanding the roles that evolving toxicity testing methods and associated interpretative techniques can play in assessing the risks associated with chemical exposures. However, the evaluations presented in the NexGen report would benefit from more thorough consideration of several essential components of a critical review of toxicity data, e.g., data quality, data relevance, and the extent to which the test endpoints reflect adverse effects. Such considerations are necessary to ensure that the NexGen report evaluations--and the resulting conclusions and recommendations--are grounded in scientifically sound, representative data reviews. We illustrate these concerns with a critique of the report's prototype ozone evaluation. Although substantial additional research is needed before new toxicity data types can be used reliably in rigorous risk assessment applications, they clearly offer exciting opportunities for advancing toxicological science and risk assessment. By explicitly identifying limitations still to be addressed and providing stronger guideposts for future research needs, the NexGen report could serve an influential role in achieving the promise of these new research approaches.

Critical comments on the WHO-UNEP State of the Science of Endocrine Disrupting Chemicals - 2012.

Early in 2013, the World Health Organization (WHO) released a 2012 update to the 2002 State of the Science of Endocrine Disrupting Chemicals. Several significant concerns have been identified that raise questions about conclusions reached in this report regarding endocrine disruption. First, the report is not a state-of-the-science review and does not follow the 2002 WHO recommended weight-of-evidence approach. Second, endocrine disruption is often presumed to occur based on exposure or a potential mechanism despite a lack of evidence to show that chemicals are causally established as endocrine disruptors. Additionally, causation is often inferred by the presentation of a series of unrelated facts, which collectively do not demonstrate causation. Third, trends in disease incidence or prevalence are discussed without regard to known causes or risk factors; endocrine disruption is implicated as the reason for such trends in the absence of evidence. Fourth, dose and potency are ignored for most chemicals discussed. Finally, controversial topics (i.e., low dose effects, non-monotonic dose response) are presented in a one-sided manner and these topics are important to understanding endocrine disruption. Overall, the 2012 report does not provide a balanced perspective, nor does it accurately reflect the state of the science on endocrine disruption.

Hypothesis-based weight-of-evidence evaluation of the human carcinogenicity of toluene diisocyanate.

Humans are exposed to toluene diisocyanate (TDI) primarily through inhalation in workplaces where TDI is produced or used. It is classified as a possible human carcinogen, based primarily on increased tumor incidences in rodents treated with TDI by oral gavage. We used the hypothesis-based weight-of-evidence (HBWoE) method to evaluate whether the available data support the hypothesis that TDI is a human carcinogen. The epidemiology data are not sufficiently robust to support TDI as a human carcinogen; the few positive associations are more likely attributable to alternative explanations than causation. The experimental animal studies indicate that inhalation exposure to TDI does not induce tumors in rats or mice. Tumors observed after oral gavage exposure are most likely due to the conversion of approximately 5% of the administered TDI to toluene diamine (TDA), a known rodent tumorigen. This contention is supported by the observations that TDA is rapidly formed from TDI during in vitro genotoxicity assays, the spectra of responses to TDA and TDI in these assays and in oral bioassays are essentially the same, and TDI is not genotoxic in rodents or humans in vivo after inhalation exposure, when TDA is not formed to a biologically significant degree. We conclude that the weight of the evidence indicates that the conversion of TDI to TDA does not occur in mammalian species under physiological exposure conditions (i.e. inhalation), but is necessary for carcinogenesis to occur. Thus, a causal association between TDI exposure and carcinogenic effects is not plausible in humans.

Evaluation of the causal framework used for setting national ambient air quality standards.

Abstract A scientifically sound assessment of the potential hazards associated with a substance requires a systematic, objective and transparent evaluation of the weight of evidence (WoE) for causality of health effects. We critically evaluated the current WoE framework for causal determination used in the United States Environmental Protection Agency's (EPA's) assessments of the scientific data on air pollutants for the National Ambient Air Quality Standards (NAAQS) review process, including its methods for literature searches; study selection, evaluation and integration; and causal judgments. The causal framework used in recent NAAQS evaluations has many valuable features, but it could be more explicit in some cases, and some features are missing that should be included in every WoE evaluation. Because of this, it has not always been applied consistently in evaluations of causality, leading to conclusions that are not always supported by the overall WoE, as we demonstrate using EPA's ozone Integrated Science Assessment as a case study. We propose additions to the NAAQS causal framework based on best practices gleaned from a previously conducted survey of available WoE frameworks. A revision of the NAAQS causal framework so that it more closely aligns with these best practices and the full and consistent application of the framework will improve future assessments of the potential health effects of criteria air pollutants by making the assessments more thorough, transparent, and scientifically sound.

A survey of frameworks for best practices in weight-of-evidence analyses.

The National Academy of Sciences (NAS) Review of the Environmental Protection Agency's Draft IRIS Assessment of Formaldehyde proposed a "roadmap" for reform and improvement of the Agency's risk assessment process. Specifically, it called for development of a transparent and defensible methodology for weight-of-evidence (WoE) assessments. To facilitate development of an improved process, we developed a white paper that reviewed approximately 50 existing WoE frameworks, seeking insights from their variations and nominating best practices for WoE analyses of causation of chemical risks. Four phases of WoE analysis were identified and evaluated in each framework: (1) defining the causal question and developing criteria for study selection, (2) developing and applying criteria for review of individual studies, (3) evaluating and integrating evidence and (4) drawing conclusions based on inferences. We circulated the draft white paper to stakeholders and then held a facilitated, multi-disciplinary invited stakeholder workshop to broaden and deepen the discussion on methods, rationales, utility and limitations among the surveyed WoE frameworks. The workshop developed recommendations for improving the conduct of WoE evaluations. Based on the analysis of the 50 frameworks and discussions at the workshop, best practices in conducting WoE analyses were identified for each of the four phases. Many of these best practices noted from the analysis and workshop could be implemented immediately, while others may require additional refinement as part of the ongoing discussions for improving the scientific basis of chemical risk assessments.

Hypothesis-based weight-of-evidence evaluation of methyl methacrylate olfactory effects in humans and derivation of an occupational exposure level.

Over 40 years of scientific evidence indicates that methyl methacrylate (MMA) causes olfactory effects in rodents that are relevant to humans. More recent scientific studies have focused on understanding the apparent lack of species concordance between the rodent and human studies. Toxicokinetic studies and a physiologically based pharmacokinetic (PBPK) model describing inhalation dosimetry of MMA in the upper respiratory tract (URT) of rats and humans point to differences in nasal morphology and biochemistry that could explain and reconcile these differences as species-specific manifestations of a common toxicological process. We have applied the hypothesis-based weight-of-evidence (HBWoE) approach to evaluate the concordance of the available data and the hypothesis that the observed difference in sensitivity between rats and humans may be the expected result of physiological and biochemical differences. Our WoE analysis indicates that when the several lines of evidence (i.e., animal, human, mode-of-action, and toxicokinetics data) are integrated, they inform interpretation of one another and, overall, support use of the human data for derivation of an MMA occupational exposure level (OEL) of 50 ppm.

A critique of the European Commission document, "State of the Art Assessment of Endocrine Disrupters".

In this commentary, we critique a recently finalized document titled "State of the Art Assessment of Endocrine Disrupters" (SOA Assessment). The SOA Assessment was commissioned by the European Union Directorate-General for the Environment to provide a basis for developing scientific criteria for identifying endocrine disruptors and reviewing and possibly revising the European Community Strategy on Endocrine Disrupters. In our view, the SOA Assessment takes an anecdotal approach rather than attempting a comprehensive assessment of the state of the art or synthesis of current knowledge. To do the latter, the document would have had to (i) distinguish between apparent associations of outcomes with exposure and the inference of an endocrine-disruption (ED) basis for those outcomes; (ii) constitute a complete and unbiased survey of new literature since 2002 (when the WHO/IPCS document, "Global Assessment of the State-of-the-Science of Endocrine Disruptors" was published); (iii) consider strengths and weaknesses and issues in interpretation of the cited literature; (iv) follow a weight-of-evidence methodology to evaluate evidence of ED; (v) document the evidence for its conclusions or the reasoning behind them; and (vi) present the evidence for or reasoning behind why conclusions that differ from those drawn in the 2002 WHO/IPCS document need to be changed. In its present form, the SOA Assessment fails to provide a balanced and critical assessment or synthesis of literature relevant to ED. We urge further evidence-based evaluations to develop the needed scientific basis to support future policy decisions.

Hypothesis-Based Weight-of-Evidence evaluation of methanol as a human carcinogen.

Recent scientific debate has focused on the potential for exposure to methanol to cause lymphomas in humans. The concern stems from a few animal studies reporting an association, although evidence suggests the studies may have been confounded by chronic respiratory infection. Although the toxicological evidence for methanol carcinogenesis is weak, two modes of action have been put forth, one involving metabolism of methanol to formaldehyde, followed by formaldehyde induction of lymphoma, and another involving oxidative stress caused by hydrogen peroxide release during catalase-induced metabolism of methanol to formaldehyde. In this article, we apply our Hypothesis-Based Weight-of-Evidence (HBWoE) approach to evaluate the evidence regarding methanol exposure and lymphoma, attending to how human, animal, and mode-of-action results inform one another, tracing the logic of inference within and across all studies, and articulating how one could account for the suite of available observations. Upon comparison of alternative proposals regarding what causal processes may have led to the array of observations as we see them, we conclude that the apparent association between methanol exposure and lymphoma in some animal studies is weak and strains biological plausibility, and is better interpreted as due to confounding or to a mechanism not relevant in humans.

Low-dose effects and nonmonotonic dose-responses of endocrine disrupting chemicals: has the case been made?

Vandenberg et al. (2012) claim that "most if not all [endocrine-disrupting chemicals (EDCs)] are likely to have low-dose effects" and "nonmonotonicity is a common occurrence after exposures to hormones and EDCs in cell culture and animals and across human populations." They present examples as anecdotes without attempting to review all available pertinent data, selectively citing studies without evaluating most of them or examining whether their putative examples are consistent and coherent with other relevant information. They assume that any statistically significant association indicates causation of an adverse effect, and their limited evaluation of specific studies is not done uniformly (i.e., studies with positive results are evaluated differently than those with null results). They also do not evaluate whether exposures in studies are truly "low-dose" and relevant to humans. They propose a number of different nonmonotonic dose-response curves, but do not consider reasons for why they should be expected to apply generally across species. Many of their examples would be - and indeed have been - questioned by many scientists. Overall, Vandenberg et al. put forth many asserted illustrations of their two conclusions without providing sufficient evidence to make the case for either and while overlooking evidence that suggests the contrary.

Is exposure to formaldehyde in air causally associated with leukemia?--A hypothesis-based weight-of-evidence analysis.

Recent scientific debate has focused on the potential for inhaled formaldehyde to cause lymphohematopoietic cancers, particularly leukemias, in humans. The concern stems from certain epidemiology studies reporting an association, although particulars of endpoints and dosimetry are inconsistent across studies and several other studies show no such effects. Animal studies generally report neither hematotoxicity nor leukemia associated with formaldehyde inhalation, and hematotoxicity studies in humans are inconsistent. Formaldehyde's reactivity has been thought to preclude systemic exposure following inhalation, and its apparent inability to reach and affect the target tissues attacked by known leukemogens has, heretofore, led to skepticism regarding its potential to cause human lymphohematopoietic cancers. Recently, however, potential modes of action for formaldehyde leukemogenesis have been hypothesized, and it has been suggested that formaldehyde be identified as a known human leukemogen. In this article, we apply our hypothesis-based weight-of-evidence (HBWoE) approach to evaluate the large body of evidence regarding formaldehyde and leukemogenesis, attending to how human, animal, and mode-of-action results inform one another. We trace the logic of inference within and across all studies, and articulate how one could account for the suite of available observations under the various proposed hypotheses. Upon comparison of alternative proposals regarding what causal processes may have led to the array of observations as we see them, we conclude that the case for a causal association is weak and strains biological plausibility. Instead, apparent association between formaldehyde inhalation and leukemia in some human studies is better interpreted as due to chance or confounding.

Measurement error in environmental epidemiology and the shape of exposure-response curves.

Both classical and Berkson exposure measurement errors as encountered in environmental epidemiology data can result in biases in fitted exposure-response relationships that are large enough to affect the interpretation and use of the apparent exposure-response shapes in risk assessment applications. A variety of sources of potential measurement error exist in the process of estimating individual exposures to environmental contaminants, and the authors review the evaluation in the literature of the magnitudes and patterns of exposure measurement errors that prevail in actual practice. It is well known among statisticians that random errors in the values of independent variables (such as exposure in exposure-response curves) may tend to bias regression results. For increasing curves, this effect tends to flatten and apparently linearize what is in truth a steeper and perhaps more curvilinear or even threshold-bearing relationship. The degree of bias is tied to the magnitude of the measurement error in the independent variables. It has been shown that the degree of bias known to apply to actual studies is sufficient to produce a false linear result, and that although nonparametric smoothing and other error-mitigating techniques may assist in identifying a threshold, they do not guarantee detection of a threshold. The consequences of this could be great, as it could lead to a misallocation of resources towards regulations that do not offer any benefit to public health.

Hypothesis-based weight-of-evidence evaluation of the neurodevelopmental effects of chlorpyrifos.

We used a hypothesis-based weight-of-evidence (HBWoE) approach to analyze the evidence regarding the hypothesis that chlorpyrifos can cause neurodevelopmental effects below the threshold for inhibition of acetylcholinesterase activity in the nervous system, which is an established mode of action for chlorpyrifos neurotoxicity. The epidemiology data do not consistently demonstrate associations between chlorpyrifos exposure and neurodevelopmental toxicity, and the animal toxicity data do not provide clear evidence that neurodevelopmental effects occur at doses below the threshold for acetylcholinesterase inhibition. The alternative mechanisms proposed to underlie potential neurodevelopmental effects in humans have been observed in the absence of acetylcholinesterase inhibition in a few in vitro studies but not in the developing brain in vivo. We provide perspective on the HBWoE approach compared with frameworks developed by the United States Environmental Protection Agency and the European Center for Ecotoxicology and Toxicology of Chemicals. We suggest that our HBWoE approach offers advantages over these frameworks in providing a better perspective on how to integrate all of the relevant data and how to use each line of evidence to inform the integration of other kinds of data or compare alternative hypotheses. Based on an HBWoE analysis, we conclude that a causal association between chlorpyrifos exposure and neurodevelopmental effects in the absence of acetylcholinesterase inhibition in the brain is not plausible in humans, and the few positive associations observed in epidemiology studies are most likely attributable to alternative explanations.