Derivation of endogenous equivalent values to support risk assessment and risk management decisions for an endogenous carcinogen: Ethylene oxide

Positive association between blood ethylene oxide levels and metabolic syndrome: NHANES 2013-2020

Purpose

The exposure of Ethylene oxide (EO) is linked to systemic inflammatory response and various cardiovascular risk factors. Hemoglobin's binding to ethylene oxide (HbEO) was used to measure serum EO level. This research aims to explore the association between metabolic syndrome (MetS) and HbEO, and between HbEO and components of metabolic syndrome.

Method

This research included 1842 participants from 2013 to 2020 in National Health and Nutrition Examination Survey (NHANES) database. Weighted logistic regression models were used to analyze the relationship between HbEO and metabolic syndrome risk, using odds ratio (OR) and 95% confidence interval (CI). The restricted cubic spline plot explores whether there is a dose-response relationship between HbEO and MetS risk. Subgroup analysis was performed to analyze study heterogeneity.

Results

Significant differences were found in gender, educational level, marital status, diabetes status and hypertension among different groups (P < 0.001, P = 0.007, P = 0.003, P < 0.001, P < 0.001, respectively). The serum HbEO level exhibited positive correlation with metabolic syndrome risk in Q2 level (OR=1.64, 1.04~2.48), Q3 level (OR=1.99, 1.29~3.08), and Q4 level (OR=2.89, 1.92~4.34). The dose-response association suggested a possible linear association between serum HbEO and metabolic syndrome risk (P-overall=0.0359, P-non-linear=0.179). L-shaped association was found between HbEO and the risk of MetS in female population, obese population and mid-age and elder population (P-overall<0.001, P-non-linear=0.0024; P-overall=0.0107, P-non-linear=0.0055 P-overall<0.001 P-non-linear=0.0157).

Conclusion

This study indicates a linear correlation between MetS and HbEO, with MetS risk escalating as HbEO levels increase. The prevalence of MetS varies depending on BMI, age and gender, and these factors can also influence MetS prevalence when exposed to EO.

Sex difference in the non-linear relationship between ethylene oxide exposure and depressive symptoms: A cross-sectional study

BACKGROUND

Ethylene oxide (EO) has been recognized as an animal carcinogen and environmental EO exposure was linked to several diseases. However, the association of EO exposure with depression prevalence is still not clear.

METHODS

We included 6016 participants with complete data on HbEO concentrations, depression diagnosis, and necessary covariates using the 2013-2020 National Health and Nutrition Examination Survey. Weighted multivariable logistic model was applied to examine the association of HbEO concentrations with depression risk. Weighted restricted cubic spline model was applied to draw the dose-response curve.

RESULTS

In the total population, individuals in the second, third, and fourth quartile of HbEO respectively had an adjusted OR of 0.99 (95%CI: 0.60, 1.63), 1.13 (95%CI: 0.73, 1.75), and 2.87 (95%CI: 1.86, 4.45) (Ptrend < 0.001) for depression with a significant "J" shaped non-linear dose-response relationship (Pnon-linear < 0.001). Females, drinkers, and smokers were susceptible to the depressive effect of EO. Doubling the HbEO concentrations was respectively associated with a 1.50-fold (95%CI: 1.25, 1.79), 1.29-fold (1.15, 1.44), and 1.17-fold (1.04, 1.33) increased risk of depression for females, drinkers, and smokers.

LIMITATIONS

Cross-sectional study design and self-reported depressive symptoms.

CONCLUSIONS

Environmental EO exposure was associated with increased depression risk, especially among females, drinkers, and smokers. Further prospective studies are required to affirm these findings.

Impact of hemoglobin adducts of ethylene oxide on the prevalence and prognosis of chronic kidney disease in US adults: an analysis from NHANES 2013-2016

Animal experiments have shown that high exposure to ethylene oxide (EO) can cause multiple system damages including the renal system. Recent studies have reported associations between exposure to EO and cancer, dyslipidemia, diabetes, and cardiovascular disease. However, the impact of exposure to EO on the prevalence and prognosis of chronic kidney disease (CKD) in humans is scarcely investigated. The study was designed to investigate the associations between EO exposure and incidence and prognosis of CKD among 2900 US adults. Exposure to EO was measured by detecting the levels of hemoglobin adducts of EO (HbEO). The diagnosis of CKD was made according to an estimated glomerular filtration rate (eGFR) < 60 mL/min/1.73 m2 and/or a urinary albumin-to-creatinine ratio (UACR) > 30 mg/g. Prognosis of CKD was assessed based on the evaluation system initiated by KDIGO that consists of eGFR and UACR. Survey-weighted generalized linear models and proportional odds models were constructed to assess the associations between HbEO and prevalence and prognosis of CKD, with odds ratios (ORs) and proportional odds ratios (PORs) and their 95% confidence intervals (CIs) reported, respectively. Restricted cubic spline (RCS) function was performed to depict the correlation between HbEO and CKD. The weighted median (interquartile range) of HbEO was 31.3 (23.1-60.3) pmol/g Hb. A total of 491 participants (16.9%) were diagnosed with CKD, and 153 participants (5.31%) were identified to be at high or very high risk. Referred to the first tertile of HbEO, the adjusted ORs (95% CIs) for CKD in the second and third tertile were 1.46 (0.85, 2.50) and 1.69 (1.00, 2.85), and the adjusted PORs (95% CIs) for prognosis of CKD in the second and third tertile were 1.37 (0.94, 1.99) and 1.58 (1.10, 2.26). When HbEO was analyzed as a continuous variable, the adjusted OR (95% CI) for CKD and POR (95% CI for prognosis of CKD were 1.24 (0.97, 1.58) and 1.22 (1.01, 1.47), respectively. RCS analysis revealed a non-linear positive correlation between HbEO and prevalence of CKD (P for nonlinearity < 0.05). Subgroup analysis indicated smoking status had a significant impact on this association, which remained significant among never smokers but lost significance among smokers. Among US adults, increased EO exposure was independently related to increased CKD prevalence and poor CKD outcomes, which was established in never smokers but not among ever smokers.

Suitability of new and existing ambient ethylene oxide measurement techniques for cancer inhalation risk assessment

Ethylene oxide (EtO) is an industrial gas that was recently reassessed to pose significant additional cancer risk at low ambient concentrations. The objective of this study is to evaluate the capabilities of existing and novel techniques to measure ambient EtO at concentrations relevant for assessing cancer risk. We present the first comparison of background ambient EtO measurements between the standard offline TO-15 techniques and two new cavity ringdown spectroscopy (CRDS) instruments, the Picarro G2920 Ethylene Oxide Gas Analyzer and the Entanglement Technologies AROMA-ETO, at a site in Atlanta, GA. Then, we analyzed background EtO measured at EPA NATTS sites across the US. Finally, we used TO-15 measurement data to assess EtO cancer risk at three near-source sites. We find that the TO-15 method has low precision for collocated samples (NME ranges from 24% to 63%), and measurements made with TO-15 pressurized samplers are biased 27% low compared to those from TO-15 passive samplers. Both CRDS methods are biased low compared to TO-15 methods (88% and 31% low bias for Picarro and AROMA, respectively), and TO-15 methods observe a seasonal peak during summer (June to September) whereas Picarro observes no seasonal trend. From our near-source assessment, we find only one site with notable elevation in cancer risk prior to EtO controls installation. Our results suggest that measurement techniques need further development to accurately assess near-source EtO cancer risk. Because different techniques measure distinct EtO trends, EtO cancer risk studies that rely on current measurement capabilities should subtract simultaneous background observations from near-source observations measured by the same method to account for these real or artificial background trends.

The association between ethylene oxide exposure and asthma risk: a population-based study

Ethylene oxide (EO) is a reactive epoxide. However, the association between EO exposure and the risk of developing asthma in humans is unknown. The aim of this study was to investigate the relationship between EO exposure and the risk of developing asthma in the general US population. In this cross-sectional study, data of 2542 patients from the National Health and Nutrition Examination Survey (NHANES) between 2013 and 2016 were obtained and analyzed. Hemoglobin adducts of EO (HbEO) level be used as the main factor for predicting EO exposure. The association between the level of EO exposure and the risk of developing asthma was evaluated with logistic regression models and dose-response analysis curves of restricted cubic spline function. Mediation analysis and linear regression analysis were utilized to evaluate the association between EO exposure and inflammation indicators. According to the quartiles of HbEO level, the patients were divided into four groups. The results indicated that an increased HbEO level was associated with a higher risk of asthma onset. Compared with the lowest quartile, the odds ratio (OR) with the 95% confidence interval (CI) for the highest quartile was 1.960 (95% CI: 1.348-2.849, P = 0.003). After being adjusted for numerous potential confounders, the OR of quartile 4 relative to quartile 1 was 1.991 (95% CI: 1.359-2.916, P = 0.001). Consistent results were also obtained in most subgroup analyses and dose-response analysis curves. In addition, EO levels were positively correlated with the inflammatory indicators (P = 0.006 for WBC, P = 0.015 for lymphocyte, and P = 0.015 for neutrophil). This study revealed a positive correlation between the level of EO exposure and the risk of asthma in a representative US population. In addition, inflammatory response may prove to be a potential biological mechanism underlying EO-induced asthma.

Food-Borne Chemical Carcinogens and the Evidence for Human Cancer Risk

Commonly consumed foods and beverages can contain chemicals with reported carcinogenic activity in rodent models. Moreover, exposures to some of these substances have been associated with increased cancer risks in humans. Food-borne carcinogens span a range of chemical classes and can arise from natural or anthropogenic sources, as well as form endogenously. Important considerations include the mechanism(s) of action (MoA), their relevance to human biology, and the level of exposure in diet. The MoAs of carcinogens have been classified as either DNA-reactive (genotoxic), involving covalent reaction with nuclear DNA, or epigenetic, involving molecular and cellular effects other than DNA reactivity. Carcinogens are generally present in food at low levels, resulting in low daily intakes, although there are some exceptions. Carcinogens of the DNA-reactive type produce effects at lower dosages than epigenetic carcinogens. Several food-related DNA-reactive carcinogens, including aflatoxins, aristolochic acid, benzene, benzo[a]pyrene and ethylene oxide, are recognized by the International Agency for Research on Cancer (IARC) as causes of human cancer. Of the epigenetic type, the only carcinogen considered to be associated with increased cancer in humans, although not from low-level food exposure, is dioxin (TCDD). Thus, DNA-reactive carcinogens in food represent a much greater risk than epigenetic carcinogens.

Blood ethylene oxide, systemic inflammation, and serum lipid profiles: Results from NHANES 2013-2016

BACKGROUND

Using data from the National Health and Nutrition Examination Survey (NHANES), this study aimed to explore the relationship between ethylene oxide (EO) exposure and serum lipid profiles as well as the mediation effect of systemic inflammation among the general adult population.

METHODS

This cross-sectional study analyzed NHANES data from 2013 to 2016, examining a total of 2721 participants. The EO biomarker (hemoglobin adduct of EO [HbEO]) was quantified in blood using a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method. The association among HbEO levels, inflammatory biomarkers, and four serum lipids was evaluated using a multivariable linear regression model. Mediating analysis was performed to examine the effect of inflammatory biomarkers on the relationship between HbEO levels and serum lipid profiles.

RESULTS

As the quartiles of HbEO increased, high-density lipoprotein cholesterol (HDL-C) monotonically decreased (p for trend <0.001). Using the lowest quartile of HbEO as a reference, the percent change for HDL-C was 6.30% (95% CI: 3.89%, 8.71%) in the highest quartile of HbEO. HbEO levels were dose-dependently associated with triglycerides (TG) (p for trend = 0.001). The percent change in TG in the fourth quartile of HbEO was 17.24% (95% CI: 2.01%, 32.48%) compared to the first quartile. Overall, inflammatory biomarkers (hs-CRP, alkaline phosphatase, white blood cell count, neutrophil count, and lymphocyte count) increased monotonically in correlation with increasing HbEO levels (all p for trend <0.01); were positively correlated with total cholesterol (TC), TG, and low-density lipoprotein cholesterol (LDL-C); and were negatively associated with HDL-C. Additionally, inflammatory biomarkers strongly mediated the relationships between HbEO and HDL-C and TG with maximum mediated proportions of 21.40% and 33.40%, respectively.

CONCLUSIONS

These findings suggest that HbEO is closely linked to serum lipid profiles and that systemic inflammation may be a key mediator of this association.

Monitored and Modeled Ambient Air Concentrations of Ethylene Oxide: Contextualizing Health Risk for Potentially Exposed Populations in Georgia

Recent studies have monitored and modeled long-term ambient air concentrations of ethylene oxide (EO) around emitting facilities in Georgia with the intent of informing risk management of potentially exposed nearby residential populations. Providing health context for these data is challenging because the U.S. Environmental Protection Agency’s risk-specific concentrations lack practical utility in distinguishing a health significant increase in exposure. This study analyzes EO data for eight emitting facilities, using a previously published alternative exposure metric, the total equivalent concentration, which is based on U.S. Centers for Disease Control biomarker data for the non-smoking U.S. population. Mean concentrations for monitoring sites were compared to mean background concentrations to assess whether emissions contribute significantly to environmental concentrations. To assess the health significance of potential exposure at nearby residential locations, the 50th percentile concentration was added to the 50th percentile endogenous equivalent concentration and compared to the total equivalent concentration distribution for the non-smoking U.S. population. The findings demonstrate that impacts from nearby emission sources are small compared to mean background concentrations at nearby locations, and the total equivalent concentrations for exposed populations are generally indistinguishable from that of the 50th percentile for the non-smoking U.S. population.

The role of endogenous versus exogenous sources in the exposome of putative genotoxins and consequences for risk assessment

The “totality” of the human exposure is conceived to encompass life-associated endogenous and exogenous aggregate exposures. Process-related contaminants (PRCs) are not only formed in foods by heat processing, but also occur endogenously in the organism as physiological components of energy metabolism, potentially also generated by the human microbiome. To arrive at a comprehensive risk assessment, it is necessary to understand the contribution of in vivo background occurrence as compared to the ingestion from exogenous sources. Hence, this review provides an overview of the knowledge on the contribution of endogenous exposure to the overall exposure to putative genotoxic food contaminants, namely ethanol, acetaldehyde, formaldehyde, acrylamide, acrolein, α,β-unsaturated alkenals, glycation compounds, N-nitroso compounds, ethylene oxide, furans, 2- and 3-MCPD, and glycidyl esters. The evidence discussed herein allows to conclude that endogenous formation of some contaminants appears to contribute substantially to the exposome. This is of critical importance for risk assessment in the cases where endogenous exposure is suspected to outweigh the exogenous one (e.g. formaldehyde and acrolein).

Association of exposure to ethylene oxide with risk of diabetes mellitus: results from NHANES 2013-2016

Animal studies suggested that exposure to ethylene oxide (EO) might induce hepatic lipid peroxidation and inflammatory lesions in various organs. However, the association between EO exposure and diabetes risk in humans is unknown. This study aimed to examine the association of EO exposure with the prevalence of diabetes mellitus in a general population of US adults. This study consisted of 3448 participants aged 20 years and older from the National Health and Nutrition Examination Survey (NHANES) 2013-2014 cycle and 2015-2016 cycle. Circulating levels of EO biomarker (hemoglobin adducts of EO (HbEO)) was measured by using high-performance liquid chromatography coupled with tandem mass spectrometry. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated using multivariate logistic regression. The weighted median level of HbEO was 29.9 (95% CI: 21.8, 56.0) pmol/g Hb. Elevated levels of HbEO were associated with higher HbA1c and lower high-density lipoprotein cholesterol (both P_trend <0.01). After multivariate adjustment including demographics, lifestyle factors, and body mass index (BMI), higher HbEO levels were significantly associated with an increased prevalence of diabetes mellitus. The OR (95% CI) of diabetes across increasing quartiles of HbEO was 1.00 (reference), 1.45 (1.08, 1.96), 1.76 (1.31, 2.36), and 1.77 (1.22, 2.57), respectively (P_trend <0.001). Similar results were observed when analyses were stratified by smoking status, age, sex, race/ethnicity, and BMI. In a nationally representative sample of US adults, higher levels of HbEO were significantly associated with an increased prevalence of diabetes mellitus. Further prospective studies are needed to confirm these findings.

Ethylene Oxide Exposure in U.S. Populations Residing Near Sterilization and Other Industrial Facilities: Context Based on Endogenous and Total Equivalent Concentration Exposures

Given ubiquitous human exposure to ethylene oxide (EO), regardless of occupation or geography, the current risk-specific concentrations (RSCs: 0.0001-0.01 ppb) from the U.S. Environmental Protection Agency (EPA) cancer risk assessment for EO are not useful metrics for managing EO exposures to the general U.S. population. The magnitude of the RSCs for EO are so low, relative to typical endogenous equivalent metabolic concentrations (1.1-5.5 ppb) that contribute ~93% of total exposure, that the RSCs provide little utility in identifying excess environmental exposures that might increase cancer risk. EO monitoring data collected in the vicinity of eight EO-emitting facilities and corresponding background locations were used to characterize potential excess exogenous concentrations. Both 50th and 90th percentile exogenous exposure concentrations were combined with the 50th percentile endogenous exposure concentration for the nonsmoking population, and then compared to percentiles of total equivalent concentration for this population. No potential total exposure concentration for these local populations exceeded the normal total equivalent concentration 95th percentile, indicating that excess facility-related exposures are unlikely to require additional management to protect public health.

Ethylene oxide review: characterization of total exposure via endogenous and exogenous pathways and their implications to risk assessment and risk management

This review is intended to provide risk assessors and risk managers with a better understanding of issues associated with total exposures of human populations to ethylene oxide from endogenous and exogenous pathways. Biomonitoring of human populations and lab animals exposed to ethylene oxide has relied upon the detection of hemoglobin adducts such as 2-hydroxyethylvaline (HEV), which provides a useful measure of total exposure to ethylene oxide from all pathways. Recent biomonitoring data from CDC provide an excellent characterization of total exposure to ethylene oxide to the general U.S. population by demographic factors such as age, gender, and race as well as smoking habit, which might be comparable to previous measurements reported for humans and lab animals. The biochemical pathways including gastrointestinal (production by bacteria) and systemic (enzymatic production) pathways by which endogenous ethylene is generated and converted to ethylene oxide are described. The relative importance of endogenous pathways and exogenous pathways via ambient air or tobacco smoke was quantified based upon available data to characterize their relative importance to total exposure. Considerable variation was noted for HEV measurements in human populations, and important sources of variation for all pathways are discussed. Issues related to risk assessment and risk management of human populations exposed to ethylene oxide are provided within the context of characterizing total exposure, and data needs for supporting future risk assessment identified.

Genotoxicity as a toxicologically relevant endpoint to inform risk assessment: A case study with ethylene oxide

The purpose of the present investigation is to analyze the in vivo genotoxicity dose-response data of ethylene oxide (EO) and the applicability of the derived point-of-departure (PoD) values when estimating permitted daily exposure (PDE) values. A total of 40 data sets were identified from the literature, and benchmark dose analyses were conducted using PROAST software to identify a PoD value. Studies employing the inhalation route of exposure and assessing gene or chromosomal mutations and chromosomal damage in various tissues were considered the most relevant for assessing risk from EO, since these effects are likely to contribute to adverse health consequences in exposed individuals. The PoD estimates were screened for precision and the values were divided by data-derived adjustment factors. For gene mutations, the lowest PDE was 285 parts per trillion (ppt) based on the induction of lacI mutations in the testes of mice following 48 weeks of exposure to EO. The corresponding lowest PDE value for chromosomal mutations was 1,175 ppt for heritable translocations in mice following 8.5 weeks of EO exposure. The lowest PDE for chromosomal aberrations was 238 ppt in the mouse peripheral blood lymphocytes following 48 weeks of inhalation exposure. The diverse dose-response data for EO-induced genotoxicity enabled the derivation of PoDs for various endpoints, tissues, and species and identified 238 ppt as the lowest PDE in this retrospective analysis.

Associations between observed concentrations of ethylene oxide in whole blood and smoking, exposure to environmental tobacco smoke, and cancers including breast cancer: data for US children, adolescents, and adults

For the first time, National Health and Nutrition Examination Survey released data on ethylene oxide (ETO) into public domain for US children aged 6-11 years, adolescents aged 12-19 years, and adults aged ≥ 20 years for 2013-2016. This study was undertaken to evaluate the associations between concentrations of ETO in whole blood and smoking, exposure to environmental tobacco smoke (ETS), and self-reported diagnosis of cancers including breast cancer. Both adolescent (29.6 vs. 49.6 pmol/g Hb, p < 0.01) and adult smokers (31.5 vs. 142.7 pmol/g Hb, p < 0.01) were found to have substantially higher adjusted levels of ethylene oxide than nonsmokers. Non-Hispanic blacks had higher levels of ethylene oxide than other race/ethnicities for children and adolescents. Non-Hispanic Asians had higher levels of ethylene oxide than other race/ethnicities for adults. Exposure to ETO measured by the number of smokers smoking inside the home (p < 0.01) and number of days smokers smoked inside the home (p = 0.03) during the prior week was found to be associated with elevated levels of ETO (p < 0.01) among US adults. Increased age was associated with elevated levels of ETO among adolescents (p = 0.02) and adults (p < 0.01) but the reverse was true for children (p = 0.04). For the general US population, levels of ETO were not found to be associated with cancers including breast cancer.

Considerations for the Use of Mutation as a Regulatory Endpoint in Risk Assessment

Assessment of a chemical's potential to cause permanent changes in the genetic code has been a common practice in the industry and regulatory settings for decades. Furthermore, the genetic toxicity battery of tests has typically been employed during the earliest stages of the research and development programs of new product development. A positive outcome from such battery has a major impact on the chemical's utility, industrial hygiene, product stewardship practices, and product life cycle analysis, among many other decisions that need to be taken by the industry, even before the registration of a chemical is undertaken. Under the prevailing regulatory paradigm, the dichotomous (yes/no) evaluation of the chemical's genotoxic potential leads to a conservative, linear no-threshold (LNT) risk assessment, unless compelling and undeniable data to the contrary can be provided to satisfy regulators, typically in a number of different global jurisdictions. With the current advent of predictive methods, new testing paradigms, mode-of-action/adverse outcome pathways, and quantitative risk assessment approaches, various stakeholders are starting to employ these state-of-the-science methodologies to further the conversation on decision making and advance the regulatory paradigm beyond the dominant LNT status quo. This commentary describes these novel methodologies, relevant biological responses, and how these can affect internal and regulatory risk assessment approaches. Environ. Mol. Mutagen. 61:84-93, 2020. © 2019 Wiley Periodicals, Inc.

Ethylene Oxide: Cancer Evidence Integration and Dose-Response Implications

The International Agency for Research on Cancer (IARC) and the United States Environmental Protection Agency (USEPA) classified ethylene oxide (EtO) as a known human carcinogen. Critically, both noted that the epidemiological evidence based on lymphoid and breast cancers was “limited,” but that the evidence in animal studies was “sufficient” and “extensive” (respectively) and that EtO is genotoxic. The USEPA derived one of the highest published inhalation unit risk (IUR) values (3 × 10⁻³ per [µg/m³ EtO]), based on results from 2 epidemiological studies. We performed focused reviews of the epidemiological and toxicological evidence on the carcinogenicity of EtO and considered the USEPA’s reliance on a genotoxic mode of action to establish EtO’s carcinogenicity and to determine likely dose–response patterns. Higher quality epidemiological studies demonstrated no increased risk of breast cancers or lymphohematopoietic malignancies (LHM). Similarly, toxicological studies and studies of early effect biomarkers in animals and humans provided no strong indication that EtO causes LHM or mammary cancers. Ultimately, animal data are inadequate to define the actual dose–response shape or predict tumor response at very low doses with any confidence. We conclude that the IARC and USEPA classification of EtO as a known human carcinogen overstates the underlying evidence and that the IUR derived by USEPA grossly overestimates risk.

Improving risk assessment approaches for chemicals with both endogenous and exogenous exposures

To conduct risk assessments of exogenous chemicals for which there are also endogenous exposures, knowledge of the chemistry and biology of both types of exposures needs to be integrated into problem formulation and carried through to risk characterization. This issue is framed in a risk assessment context, highlighting the importance of quantifying increments of dose from all sources of the same or similar chemicals interacting with biological targets; understanding the influence of endogenous chemical concentrations on disease risk; and assessing total dose to targets in evaluating risk from incremental environmental exposures. Examples of recent assessments illustrate the importance of addressing this issue. Evaluations of data on blood or organ concentrations of ammonia, methanol, formaldehyde, acetaldehyde, and three gaseous signaling molecules (hydrogen sulfide, carbon monoxide, and nitric oxide) provide examples where current data are already informing perspectives on relative exposures at the portal of entry and systemically. To facilitate quality risk assessments of exogenous chemicals with endogenous exposures, a series of specific questions are presented that need to be addressed in systematic review to enhance problem formulation, improve the development of holistic conceptual models, and to facilitate the identification of priority data needs for improving risk assessments.