Chemical mixtures research: significance and future perspectives

Applications of mixture methods in epidemiological studies investigating the health impact of persistent organic pollutants exposures: a scoping review

BACKGROUND

Persistent organic pollutants (POPs) are environmental chemicals characterized by long half-lives in nature and human bodies, posing significant health risks. The concept of the exposome, encompassing all lifetime environmental exposures, underscores the importance of studying POP as mixtures rather than in isolation. The increasing body of evidence on the health impacts of POP mixtures necessitates the proper application of statistical methods.

OBJECTIVES

We aimed to summarize studies on the overall effects of POP mixtures, identify patterns in applications of mixture methods-statistical methods for investigating the association of mixtures-and highlight current challenges in synthesizing epidemiologic evidence of POP mixtures on health effects as illustrated through a case study.

METHODS

We conducted a systematic literature search on PubMed and Embase for epidemiological studies published between January 2011 and April 2023.

RESULTS

We included 240 studies that met our eligibility criteria. 126 studies focused on per- and polyfluoroalkyl substances (PFAS) mixtures only, while 40 analyzed three or more classes of POPs in mixture analyses. We identified 23 unique mixture methods used to estimate the overall effects of POP mixtures, with Bayesian Kernel Machine Regression (BKMR), a type of response-surface modeling, being the most common. Additionally, 22.9% of studies used a combination of methods, including response-surface modeling, index modeling, dimension reduction, and latent variable models. The most extensively explored health outcome category was body weight and birth sizes (n = 43), and neurological outcomes (n = 41). In the case study of PFAS mixtures and birth weight, 12 studies showed negative associations, while 4 showed null results, and 2 showed positive associations.

IMPACT STATEMENT

This scoping review consolidates the existing literature on the overall effects of POP mixtures using statistical methods. By providing a comprehensive overview, our study illuminates the present landscape of knowledge in this field and underscores the methodological hurdles prevalent in epidemiological studies focused on POP mixtures. Through this analysis, we aim to steer future research directions, fostering a more nuanced comprehension of the intricate dynamics involved in assessing the health effects of POP mixtures. Our work stands as a significant contribution to the ongoing exploration of the chemical exposome.

Combined Exposures and Mixtures Research: An Enduring NIEHS Priority

BACKGROUND

The National Institute of Environmental Health Sciences (NIEHS) continues to prioritize research to better understand the health effects resulting from exposure to mixtures of chemical and nonchemical stressors. Mixtures research activities over the last decade were informed by expert input during the development and deliberations of the 2011 NIEHS Workshop "Advancing Research on Mixtures: New Perspectives and Approaches for Predicting Adverse Human Health Effects." NIEHS mixtures research efforts since then have focused on key themes including a) prioritizing mixtures for study, b) translating mixtures data from in vitro and in vivo studies, c) developing cross-disciplinary collaborations, d) informing component-based and whole-mixture assessment approaches, e) developing sufficient similarity methods to compare across complex mixtures, f) using systems-based approaches to evaluate mixtures, and g) focusing on management and integration of mixtures-related data.

OBJECTIVES

We aimed to describe NIEHS driven research on mixtures and combined exposures over the last decade and present areas for future attention.

RESULTS

Intramural and extramural mixtures research projects have incorporated a diverse array of chemicals (e.g., polycyclic aromatic hydrocarbons, botanicals, personal care products, wildfire emissions) and nonchemical stressors (e.g., socioeconomic factors, social adversity) and have focused on many diseases (e.g., breast cancer, atherosclerosis, immune disruption). We have made significant progress in certain areas, such as developing statistical methods for evaluating multiple chemical associations in epidemiology and building translational mixtures projects that include both in vitro and in vivo models.

DISCUSSION

Moving forward, additional work is needed to improve mixtures data integration, elucidate interactions between chemical and nonchemical stressors, and resolve the geospatial and temporal nature of mixture exposures. Continued mixtures research will be critical to informing cumulative impact assessments and addressing complex challenges, such as environmental justice and climate change. https://doi.org/10.1289/EHP14340.

Exposure to metal mixtures and adverse pregnancy and birth outcomes: A systematic review

BACKGROUND

Prenatal exposure to metal mixtures is associated with adverse pregnancy and birth outcomes like low birth weight, preterm birth, and small for gestational age. However, prior studies have used individual metal analysis, lacking real-life exposure scenarios.

OBJECTIVES

This systematic review aims to evaluate the strength and consistency of the association between metal mixtures and pregnancy and birth outcomes, identify research gaps, and inform future studies and policies in this area.

METHODS

The review adhered to the updated Preferred Reporting Items for Systematic Review and Meta-analysis (PRISMA) checklist, along with the guidelines for conducting systematic reviews and meta-analyses of observational studies of etiology (COSMOS-E). Our data collection involved searching the PubMed, MEDLINE, and SCOPUS databases. We utilized inclusion criteria to identify relevant studies. These chosen studies underwent thorough screening and data extraction procedures. Methodological quality evaluations were conducted using the NOS framework for cohort and case-control studies, and the AXIS tool for cross-sectional studies.

RESULTS

The review included 34 epidemiological studies, half of which focused on birth weight, and the others investigated neonate size, preterm birth, small for gestational age, miscarriage, and placental characteristics. The findings revealed significant associations between metal mixtures (including mercury (Hg), nickel (Ni), arsenic (As), cadmium (Cd), manganese (Mn), cobalt (Co), lead (Pb), zinc (Zn), barium (Ba), cesium (Cs), copper (Cu), selenium (Se), and chromium (Cr)) and adverse pregnancy and birth outcomes, demonstrating diverse effects and potential interactions.

CONCLUSION

In conclusion, this review consistently establishes connections between metal exposure during pregnancy and adverse consequences for birth weight, gestational age, and other vital birth-related metrics. This review further demonstrates the need to apply mixture methods with caution but also shows that they can be superior to traditional approaches. Further research is warranted to deeper understand the underlying mechanisms and to develop effective strategies for mitigating the potential risks associated with metal mixture exposure during pregnancy.

Associations between repeated measurements of urinary polycyclic aromatic hydrocarbon metabolites and thyroid hormones among reproductive-aged men

BACKGROUND

Polycyclic aromatic hydrocarbons (PAHs) have been shown to disrupt thyroid function in toxicological studies, but epidemiological evidence is inconsistent. Furthermore, little is known on potential effects of mixtures of PAHs.

OBJECTIVE

This study aimed to examine the associations of exposure to PAHs as individual chemicals and mixtures with thyroid hormones.

METHODS

We included 378 men from a Reproductive Medicine Center in Wuhan, China. Ten monohydroxylated PAH (OH-PAH) metabolites in repeated urine specimens collected at two-time points and three thyroid hormones [thyroid-stimulating hormone (TSH), free thyroxine (FT4), and free triiodothyronine (FT3)] in one serum sample were measured. Multivariable linear regression models were applied to assess the associations between individual OH-PAH metabolites and thyroid hormones, and the associations with mixtures of OH-PAH metabolites were assessed by Bayesian kernel machine regression (BKMR) models.

RESULTS

Multivariable linear regression models showed inverse associations between urinary 1-OHNa and TSH, between urinary 1-OHPh and 9-OHPh and FT3, as well as between urinary 2-OHPh, 3-OHPh, 9-OHPh and ∑OHPh and FT4, regardless of these individual OH-PAH metabolites modeled as continuous or tertile variables (e.g., -21.57 % in TSH; 95 % CI: -35.33 %, -4.88 % for the third vs first tertiles of 1-OHNa; p for trend = 0.014). BKMR models showed negative overall effects of all urinary OH-PAH metabolite mixtures on TSH, FT3, and FT4, and 1-OHNa, 9-OHPh, and 2-OHPh as the most important contributors, respectively, with linear inverse exposure-response associations when holding other OH-PAH metabolites at their median concentrations.

CONCLUSION

Urinary OH-PAH metabolites as individual chemicals and mixtures were adversely associated with thyroid hormones among reproductive-aged men.

Pesticides and Pesticide Degradates in Groundwater Used for Public Supply across the United States: Occurrence and Human-Health Context

This is the first assessment of groundwater from public-supply wells across the United States to analyze for >100 pesticide degradates and to provide human-health context for degradates without benchmarks. Samples from 1204 wells in aquifers representing 70% of the volume pumped for drinking supply were analyzed for 109 pesticides (active ingredients) and 116 degradates. Among the 41% of wells where pesticide compounds were detected, nearly two-thirds contained compound mixtures and three-quarters contained degradates. Atrazine, hexazinone, prometon, tebuthiuron, four atrazine degradates, and one metolachlor degradate were each detected in >5% of wells. Detection frequencies were largest for aquifers with more shallow, unconfined wells producing modern-age groundwater. To screen for potential human-health concerns, benchmark quotients (BQs) were calculated by dividing concentrations by the human-health benchmark, when available. For degradates without benchmarks, estimated values (estimated benchmark quotients (BQE)) were first calculated by assuming equimolar toxicity to the most toxic parent; final analysis excluded degradates with likely overestimated toxicity. Six pesticide compounds and 1.6% of wells had concentrations approaching levels of potential concern (individual or summed BQ or BQE values >0.1), and none exceeded these levels (values >1). Therefore, although pesticide compounds occurred frequently, concentrations were low, even accounting for mixtures and degradates without benchmarks.

Combined effects of di (2-ethylhexyl) phthalate and bisphenol A on thyroid hormone homeostasis in adolescent female rats

Phthalates and bisphenols are two typical classes of endocrine-disrupting chemicals (EDCs) which cause endocrine disorder in humans and animals. Phthalates and bisphenols are suggested to be associated with thyroid dysfunction. However, the effects of combined exposure and the detailed mechanisms are yet poorly understood. We investigated the combined effects of di (2-ethylhexyl) phthalate (DEHP) and bisphenol A (BPA) on thyroid function during puberty. Female Sprague Dawley rats were gavaged from postnatal 28 to 70 days with a single or combined exposure of DEHP (0, 150, and 750 mg/kg/day) and BPA (0, 20, and 100 mg/kg/day) according to a 3 × 3 factorial design. The thyroid weights reduced after combined exposure to the highest dose of DEHP and BPA, which noted their adverse effects on thyroid. Additionally, DEHP could increase the number of follicular epithelial cells in thyroid. Both DEHP and in combination with BPA could disturb the levels of thyroid hormones in serum, such as TT3 and TT4. Meanwhile, the possible mechanism was also discussed in the present study. DEHP treatment induced a significant increase of phosphorylation of cAMP-response element binding protein (Creb) via estrogen receptor α (Esr1), while the upregulation was nullified by the concomitant presence of BPA. In conclusion, the complex action of DEHP/BPA mixture may disturb the thyroid hormone homeostasis, which ultimately would affect the development of thyroid during puberty.

The effects of postnatal exposure of endocrine disruptors on testicular function: a systematic review and a meta-analysis

BACKGROUND

Despite many epidemiological studies having been conducted, the impact of postnatal exposure of endocrine disruptors (EDs) on testicular function remains a controversial issue.

AIM

To systematically review the literature and perform a quantitative synthesis to evaluate the effect of EDs on testicular function.

MATERIALS AND METHODS

A comprehensive search was conducted in the MEDLINE, Scopus, and CENTRAL databases. Eligible for the systematic review were observational (cross-sectional and cohort) studies with (i) adult men who had a high probability of postnatal exposure to EDs ("exposed"), (ii) adult men who had a low probability of postnatal exposure to EDs ("non-exposed"), and (iii) an outcome of interest [seminal parameters and reproductive hormone concentrations]. The continuous outcomes in each of the studies were synthesized by the random effects model and expressed as standardized mean difference (SMD) with 95% confidence interval (CI).

RESULTS

Thirteen studies, including 959 exposed and 907 non-exposed men, fulfilled the inclusion criteria. Exposure to EDs was associated with decreased LH [SMD - 0.17, 95% CI - 0.33 to - 0.02, 10 studies (616 exposed, 563 non-exposed), I² 40%, p = 0.09], progressive motility [SMD - 0.45, 95% CI - 0.77 to - 0.13, three studies (133 cases, 153 controls), I² 38%, p = 0.20], and normal morphology [SMD - 0.50, 95% CI - 0.85 to - 0.14, eight studies (562 cases, 540 controls), I² 87%, p < 0.01] compared with non-exposure. No difference was observed between the other study groups.

CONCLUSIONS

Postnatal exposure to EDs is associated with decreased semen quality. Nevertheless, there is no evidence that a disruption of testicular function mediates the deterioration in semen quality.

Heavy Metal Mixture Exposure and Effects in Developing Nations: An Update

The drive for development and modernization has come at great cost. Various human activities in developed and developing countries, particularly in sub-Saharan Africa (SSA) have given rise to environmental safety concerns. Increased artisanal mining activities, illegal refining, use of leaded petrol, airborne dust, arbitrary discarding and burning of toxic waste, absorption of production industries in inhabited areas, inadequate environmental legislation, and weak implementation of policies, have given rise to the incomparable contamination and pollution associated with heavy metals in recent decades. This review evaluates the public health effects of heavy metals and their mixtures in SSA. This shows the extent and size of the problem posed by exposure to heavy metal mixtures in regard to public health.

Phytotoxicity, bioaccumulation and potential risks of plant irrigations using cyanobloom-loading freshwater

The toxicity of cyanotoxins on plant has been reported. However, in eutrophic waters harmful cyanobacteria are associated with other environmental pollutants, such as persistent organic pollutants (POPs) and metals. Information on the phytotoxicity and bioaccumulation of coexisted cyanotoxins and these environmental pollutants is still lacking. In this study, the combined phytotoxicities of three types of cyanobacteria-associated pollutants, i.e., microcystin-LR (MC-LR), cadmium (Cd), 2, 4, 4'-Trichlorobiphenyl (PCB-28) were systematically investigated. After 7-days exposure, strong synergistic effects can be detected when Arabidopsis thaliana seeds and seedlings exposed to binary mixtures of MC-LR+PCB-28 and PCB-28+Cd. The strongest inhibition occurred when A. thaliana exposed to their ternary mixture under both glasshouse and semi-field conditions. Moreover, bioaccumulation of MC-LR, Cd and PCB-28 was enhanced when seedlings exposed to their binary/ternary mixtures, especially when seedlings were treated with higher concentrations of toxicants (MC-LR, 1mgL^-1; Cd, 10mgL^-1; PCB-28, 1μgL^-1). Additionally, pronounced toxic effects could be determined under 7-days after seedlings were irrigated with raw cyanobloom-containing water (collected from Lake Taihu in China)and its dilutions. Seeds production decreased significantly after the continuous irrigation with cyanoblooms-containing water. Collectively, this work will be an informative implication for risks of cyanoblooms and adequate utilization of freshwater containing cyanoblooms for crop irrigation.

An open-sourced statistical application for identifying complex toxicological interactions of environmental pollutants

The rising number of chemicals that humans are exposed to on a daily basis, as well as advances in biomonitoring and detection technologies have highlighted the diversity of individual exposure profiles (complex body burdens). To address this, the toxicological sciences have begun to shift away from examining toxic agents or stressors individually to focusing on more complex models with multiple agents or stressors present. Literature on interactions between chemicals is fairly limited in comparison with dose-response studies on individual toxicants, which is largely due to experimental and statistical challenges. Experimental designs capable of identifying these complex interactions are often avoided or not evaluated to their fullest potential because of the difficulty associated with appropriate analysis as well as logistical factors. To assist with statistical analysis of these types of experiments, an online, open-sourced statistical application was created for investigators to use to analyze and interpret potential toxicant interactions in laboratory experimental data using a full-factorial three-way analysis of variance (ANOVA). This model utilizes backward selection on interaction terms to model main effects and interactions.

Arsenic, cadmium, lead and mercury levels in blood of Finnish adults and their relation to diet, lifestyle habits and sociodemographic variables

The Northern Finland Birth Cohort program (NFBC) is the epidemiological and longitudinal prospective general population research program, which was established to promote health and wellbeing of the population in northern Finland. The aim of present study, as a part of the NFBC program, was to analyze the blood levels of arsenic (B-As), cadmium (B-Cd), lead (B-Pb), total mercury (B-Hg) and selenium (B-Se); to compare these levels with threshold limits; to study sociodemographic factors; and to correlate these levels with calcium and haemoglobin. The study was comprised of 249 NFBC subjects, of which 123 were female and 126 were male (ages 31.1 ± 0.3 and 31.1 ± 0.4, respectively). All participants were asked to complete a questionnaire regarding diet and living habits. The geometric means (± SD) of B-As were 0.49 ± 2.80 μg/l and 0.44 ± 2.72 μg/l; B-Cd were 0.18 ± 4.02 μg/l and 0.12 ± 3.21 μg/l; B-Pb were 17.0 ± 1.8 μg/l and 9.06 ± 2.20 μg/l; B-Hg were 2.18 ± 2.02 μg/l and 1.85 ± 1.78 μg/l; and B-Se were 106.0 ± 1.3 and 94.3 ± 1.3 μg/l in males and females, respectively. Among the subjects in the present analysis, 23 % of males and 17.1 % of females had B-As levels above the ATSDR normal human levels of B-As in unexposed individuals (1.0 μg/l). The B-Pb geometric mean (12.44 μg/l) was approximately one eighth the CDC toxicological cut-off point of 100 μg/l. Twenty-one individuals (8.4 %) exceeded a B-Hg level of 5.8 μg/l. Fifty-eight females (47 %) had a B-Hg higher than 2.0 μg/l, the German Federal Environmental Agency cut-off point for women (18-69 years) who consume fish at least three times/month; therefore, their babies could be at risk of adverse effects during development.

Neurological Toxicity of Individual and Mixtures of Low Dose Arsenic, Mono and Di (n-butyl) Phthalates on Sub-Chronic Exposure to Mice

The objective of this study was to evaluate the toxicity of individual and mixtures of di(n-butyl) phthalates (DBP) and their active metabolite monobutyl phthalate (MBP) and arsenic (As) on spatial cognition associated with hippocampal apoptosis in mice. Mice were exposed, individually or in combination, to DBP (50 mg/kg body weight, intragastrically), MBP (50 mg/kg body weight, intragastrically), and As (10 mg/L, per os) for 8 weeks. The Morris water maze test showed that mice exposed to DBP/MBP combined with As exhibited longer escape latencies and the lower average number of crossing the platform. The As content in the hippocampus after As exposure increased as compared to those without As exposure. In mice exposed to DBP/MBP combined with As, pathological alterations and oxidative damage to the hippocampus were found. Expression of apoptosis-related protein: Bax and caspase-3 were significantly increased in the hippocampus, while there was no significant change in expression of Bcl-2. The results suggested that DBP and MBP combined with As can induce spatial cognitive deficits through altering the expression of apoptosis-related protein and As played a critical role in cognition impairments. And the joint exposure has antagonistic effect.

Application of the combination index integrated with confidence intervals to study the toxicological interactions of antibiotics and pesticides in Vibrio qinghaiensis sp.-Q67

It is necessary to explore the effect of confidence intervals on the combination index (CI) so that rationally evaluate the toxicological interaction (synergism or antagonism) which is dependent on the concentration ratio, the mixture concentration and the exposure time. To effectively detect the toxicological interaction taking place in mixtures, we combined the CI with the observation-based confidence intervals (OCI) which can characterize the uncertainty in toxicity test and in data fitting. In time scale, the short-term (15min) and long-term (12h) toxicities of three chemicals (imidacloprid (IMI), pirimicarb (PIR) and streptomycin sulfate (STR)) and their binary mixtures on Vibrio qinghaiensis sp.-Q67 were determined by the microplate toxicity analysis (MTA). The mixtures of IMI, PIR and STR have additive actions all but four IMI-PIR rays (R2-R5) at the effect levels above about 30-40% whose long-term toxicological interaction are synergism.

Two-stage prediction of the effects of imidazolium and pyridinium ionic liquid mixtures on luciferase

The predicted toxicity of mixtures of imidazolium and pyridinium ionic liquids (ILs) in the ratios of their EC₅₀, EC₁₀, and NOEC (no observed effect concentration) were compared to the observed toxicity of these mixtures on luciferase. The toxicities of EC₅₀ ratio mixture can be effectively predicted by two-stage prediction (TSP) method, but were overestimated by the concentration addition (CA) model and underestimated by the independent action (IA) model. The toxicities of EC₁₀ ratio mixtures can be basically predicted by TSP and CA, but were underestimated by IA. The toxicities of NOEC ratio mixtures can be predicted by TSP and CA in a certain concentration range, but were underestimated by IA. Our results support the use of TSP as a default approach for predicting the combined effect of different types of ILs at the molecular level. In addition, mixtures of ILs mixed at NOEC and EC₁₀ could cause significant effects of 64.1% and 97.7%, respectively. Therefore, we should pay high attention to the combined effects in mixture risk assessment.

Mixtures research at NIEHS: an evolving program

The National Institute of Environmental Health Sciences (NIEHS) has a rich history in evaluating the toxicity of mixtures. The types of mixtures assessed by the Division of the National Toxicology Program (DNTP) and the extramural community (through the Division of Extramural Research and Training, DERT) have included a broad range of chemicals and toxicants, with each study having a unique set of questions and design considerations. Some examples of the types of mixtures studied include: groundwater contaminants, pesticides/fertilizers, dioxin-like chemicals (assessing the toxic equivalency approach), drug combinations, air pollution, metals, polycyclic aromatic hydrocarbons, technical mixtures (e.g., pentachlorophenol, flame retardants), and mixed entities (e.g., herbals, asbestos). These endeavors have provided excellent data on the toxicity of specific mixtures and have been informative to the human health risk assessment process in general (e.g., providing data on low dose exposures to environmental chemicals). However, the mixtures research effort at NIEHS, to date, has been driven by test article nominations to the DNTP or by investigator-initiated research through DERT. Recently, the NIEHS has embarked upon an effort to coordinate mixtures research across both intramural and extramural divisions in order to maximize mixtures research results. A path forward for NIEHS mixtures research will be based on feedback from a Request for Information (RFI) designed to gather up-to-date views on the knowledge gaps and roadblocks to evaluating mixtures and performing cumulative risk assessment, and a workshop organized to bring together mixtures experts from risk assessment, exposure science, biology, epidemiology, and statistics. The future of mixtures research at NIEHS will include projects from nominations to DNTP, studies by extramural investigators, and collaborations across government agencies that address high-priority questions in the field of mixtures research.

Chemical mixtures in untreated water from public-supply wells in the U.S.--occurrence, composition, and potential toxicity

Chemical mixtures are prevalent in groundwater used for public water supply, but little is known about their potential health effects. As part of a large-scale ambient groundwater study, we evaluated chemical mixtures across multiple chemical classes, and included more chemical contaminants than in previous studies of mixtures in public-supply wells. We (1) assessed the occurrence of chemical mixtures in untreated source-water samples from public-supply wells, (2) determined the composition of the most frequently occurring mixtures, and (3) characterized the potential toxicity of mixtures using a new screening approach. The U.S. Geological Survey collected one untreated water sample from each of 383 public wells distributed across 35 states, and analyzed the samples for as many as 91 chemical contaminants. Concentrations of mixture components were compared to individual human-health benchmarks; the potential toxicity of mixtures was characterized by addition of benchmark-normalized component concentrations. Most samples (84%) contained mixtures of two or more contaminants, each at concentrations greater than one-tenth of individual benchmarks. The chemical mixtures that most frequently occurred and had the greatest potential toxicity primarily were composed of trace elements (including arsenic, strontium, or uranium), radon, or nitrate. Herbicides, disinfection by-products, and solvents were the most common organic contaminants in mixtures. The sum of benchmark-normalized concentrations was greater than 1 for 58% of samples, suggesting that there could be potential for mixtures toxicity in more than half of the public-well samples. Our findings can be used to help set priorities for groundwater monitoring and suggest future research directions for drinking-water treatment studies and for toxicity assessments of chemical mixtures in water resources.

Environmental determinants of chronic disease and medical approaches: recognition, avoidance, supportive therapy, and detoxification

The World Health Organization warns that chronic, noncommunicable diseases are rapidly becoming epidemic worldwide. Escalating rates of neurocognitive, metabolic, autoimmune and cardiovascular diseases cannot be ascribed only to genetics, lifestyle, and nutrition; early life and ongoing exposures, and bioaccumulated toxicants may also cause chronic disease. Contributors to ill health are summarized from multiple perspectives--biological effects of classes of toxicants, mechanisms of toxicity, and a synthesis of toxic contributors to major diseases. Healthcare practitioners have wide-ranging roles in addressing environmental factors in policy and public health and clinical practice. Public health initiatives include risk recognition and chemical assessment then exposure reduction, remediation, monitoring, and avoidance. The complex web of disease and environmental contributors is amenable to some straightforward clinical approaches addressing multiple toxicants. Widely applicable strategies include nutrition and supplements to counter toxic effects and to support metabolism; as well as exercise and sweating, and possibly medication to enhance excretion. Addressing environmental health and contributors to chronic disease has broad implications for society, with large potential benefits from improved health and productivity.

Categorizing biomarkers of the human exposome and developing metrics for assessing environmental sustainability

The concept of maintaining environmental sustainability broadly encompasses all human activities that impact the global environment, including the production of energy, use and management of finite resources such as petrochemicals, metals, food production (farmland, fresh and ocean waters), and potable water sources (rivers, lakes, aquifers), as well as preserving the diversity of the surrounding ecosystems. The ultimate concern is how one can manage Spaceship Earth in the long term to sustain the life, health, and welfare of the human species and the planet's flora and fauna. On a more intimate scale, one needs to consider the human interaction with the environment as expressed in the form of the exposome, which is defined as all exogenous and endogenous exposures from conception onward, including exposures from diet, lifestyle, and internal biology, as a quantity of critical interest to disease etiology. Current status and subsequent changes in the measurable components of the exposome, the human biomarkers, could thus conceivably be used to assess the sustainability of the environmental conditions with respect to human health. The basic theory is that a shift away from sustainability will be reflected in outlier measurements of human biomarkers. In this review, the philosophy of long-term environmental sustainability is explored in the context of human biomarker measurements and how empirical data can be collected and interpreted to assess if solutions to existing environmental problems might have unintended consequences. The first part discusses four conventions in the literature for categorizing environmental biomarkers and how different types of biomarker measurements might fit into the various grouping schemes. The second part lays out a sequence of data management strategies to establish statistics and patterns within the exposome that reflect human homeostasis and how changes or perturbations might be interpreted in light of external environmental stressors. The underlying concept is to identify probative outliers from the "unremarkable exposome" in individuals or subpopulations that could be used for discerning deviations from the healthy environment, much like current diagnostic medicine uses batteries of blood and urine tests to screen for preclinical disease conditions. Such empirically derived human in vivo data could subsequently be integrated into high-throughput in vitro and in silico testing of environmental and manufactured chemicals to support real-world toxicity evaluations.

Heat map visualization of complex environmental and biomarker measurements

Over the past decade, the assessment of human systems interactions with the environment has permeated all phases of environmental and public health research. We are invoking lessons learned from the broad discipline of Systems Biology research that focuses primarily on molecular and cellular networks and adapting these concepts to Systems Exposure Science which focuses on interpreting the linkage from environmental measurements and biomonitoring to the expression of biological parameters. A primary tool of systems biology is the visualization of complex genomic and proteomic data using "heat maps" which are rectangular color coded arrays indicating the intensity (or amount) of the dependent variable. Heat maps are flexible in that both the x-axis and y-axis can be arranged to explore a particular hypothesis and allow a fast overview of data with a third quantitative dimension captured as different colors. We are now adapting these tools for interpreting cumulative and aggregate environmental exposure measurements as well as the results from human biomonitoring of biological media including blood, breath and urine. This article uses existing EPA measurements of environmental and biomarker concentrations of polycyclic aromatic hydrocarbons (PAHs) to demonstrate the value of the heat map approach for hypothesis development and to link back to stochastic and mixed effects models that were originally used to assess study results.

Effects of environmental pollutants on the reproduction and welfare of ruminants

Anthropogenic pollutants comprise a wide range of synthetic organic compounds and heavy metals, which are dispersed throughout the environment, usually at low concentrations. Exposure of ruminants, as for all other animals, is unavoidable and while the levels of exposure to most chemicals are usually too low to induce any physiological effects, combinations of pollutants can act additively or synergistically to perturb multiple physiological systems at all ages but particularly in the developing foetus. In sheep, organs affected by pollutant exposure include the ovary, testis, hypothalamus and pituitary gland and bone. Reported effects of exposure include changes in organ weight and gross structure, histology and gene and protein expression but these changes are not reflected in changes in reproductive performance under the conditions tested. These results illustrate the complexity of the effects of endocrine disrupting compounds on the reproductive axis, which make it difficult to extrapolate between, or even within, species. Effects of pollutant exposure on the thyroid gland, immune, cardiovascular and obesogenic systems have not been shown explicitly, in ruminants, but work on other species suggests that these systems can also be perturbed. It is concluded that exposure to a mixture of anthropogenic pollutants has significant effects on a wide variety of physiological systems, including the reproductive system. Although this physiological insult has not yet been shown to lead to a reduction in ruminant gross performance, there are already reports indicating that anthropogenic pollutant exposure can compromise several physiological systems and may pose a significant threat to both reproductive performance and welfare in the longer term. At present, many potential mechanisms of action for individual chemicals have been identified but knowledge of factors affecting the rate of tissue exposure and of the effects of combinations of chemicals on physiological systems is poor. Nevertheless, both are vital for the identification of risks to animal productivity and welfare.

Towards the development of a multidisciplinary understanding of the effects of toxic chemical mixtures on health

Mixtures can be divided into simple (chemicals with comparable properties--health risk assessments on the chemicals) and complex, which can be further subdivided into defined (a reasonably distinct composition, created at a specific time and place despite dissimilar components--risk assessments on the common source) and coincidental (chemicals without similar properties or constant composition in time or space-risk assessments on the receptor). Interactions recognized are: independent action, dose addition (additivity), and potentiation (synergy and antagonism). Unpredicted outcomes need recognition. New approaches in higher education and multidisciplinary investigations are essential. The community of the Society for Environmental Geochemistry and Health should help clarify points such as when transformations in mixtures may become important enough to alter the classification and the risk assessment. The multidisciplinary community is also well placed to support the integration of non-chemical influences into mixture analysis and to contribute to the investigation of cumulative and multiple exposures.

Endocrine disruptors and other food-contaminating environmental pollutants as risk factors in animal reproduction

Pollutants of many chemical classes, derived primarily from anthropogenic activities, are ubiquitous in the environment, persistent, biologically available and can exert adverse effects on the reproductive and other, indirectly related, physiological systems. Food is generally considered to be the major route of animal exposure in vertebrate species but the relative contributions of other routes of exposure such as through lungs, gills or skin are not well studied and may be of importance for certain animal groups, depending on their immediate environment. Animals are particularly sensitive to exposure during developmental stages but the pattern of exposure to chemicals is likely to be different to that of adults. Quantification of the risk posed by the ingestion of pollutants in food is complex and depends on many factors including species, diet composition, duration of exposure to the food, efficiency of pollutant absorption, subsequent metabolism, sensitivity of target organs and stage of development. While the effects of high doses of single chemicals are proven, dietary exposure to pollutants generally involves prolonged, low-level exposure to a large number of compounds, each of which has different chemical characteristics, exerts different biological effects and is present at varying concentrations. Thus, while exposure to pollutants through feed is undoubtedly a significant risk factor for many species and may be the most important one for many terrestrial vertebrates, other routes of exposure may be more important in other groups.

DNA microarrays detect effects of soil contamination on Arabidopsis thaliana gene expression

Soil contamination, such as heavy metals and benzene compounds, is a widespread problem on military installations. It is important to be able to determine the effects of soil contamination before any adverse effects appear in organisms in surrounding areas. We examined gene expression in Arabidopsis thaliana grown in soil from three sites at the Radford Army Ammunition Plant in Radford, Virginia, USA, using DNA microarrays. We analyzed soil, germination, and growth rate to compare with the microarray data. Soil contamination affected both external phenotype and gene expression. Plants grown in soil with high levels of contaminants were chloritic and were smaller than control plants grown in potting soil. Plants grown in soil with the highest copper concentration had the lowest growth rates and had genes up-regulated across several functional groups. Plants grown in soils with elevated lead had many genes down-regulated that were related to photosystem II, metabolism, cellular transport, and protein synthesis. Genes consistently up-regulated across most microarrays were genes related to photosystem I, genes related to water deprivation and oxidative stress response, heat shock proteins, and toxin catabolism genes such as glutathiones. DNA microarrays, in concert with a model genetic organism such as A. thaliana, were an effective assessment tool to determine the presence of toxic substances in soil at a site used for the production of military explosives.

If cumulative risk assessment is the answer, what is the question?

Cumulative risk refers to the combined threats from exposure via all relevant routes to multiple stressors including biological, chemical, physical, and psychosocial entities. Cumulative risk assessment is a tool for organizing and analyzing information to examine, characterize, and possibly quantify the combined adverse effects on human health or ecologic resources from multiple environmental stressors. The U.S. Environmental Protection Agency (EPA) has initiated a long-term effort to develop future guidelines for cumulative risk assessment, including publication in 2003 of a framework that describes important features of the process and discusses theoretical issues, technical matters, and key definitions. The framework divides the process of cumulative risk assessment into three interrelated phases: a) planning, scoping, and problem formulation; b) analysis; and c) interpretation and risk characterization. It also discusses the additional complexities introduced by attempts to analyze cumulative risks from multiple stressors and describes some of the theoretical approaches that can be used. The development of guidelines for cumulative risk assessment is an essential element in the transition of the U.S. EPA risk assessment methodology from a narrow focus on a single stressor, end point, source, pathway, and exposure route to a broader, more holistic approach involving analysis of combined effects of cumulative exposure to multiple stressors via all relevant sources, pathways, and routes.

A two-generation chronic mixture toxicity study of Clophen A60 and diethyl phthalate on histology of adrenal cortex and thyroid of rats

This study was undertaken to observe the type of interaction that exists between polychlorinated biphenyls (Clophen A60) and diethyl phthalate (DEP) on the adrenal and thyroid glands of male and female Wistar rats. Animals were divided into four groups of six animals each, group I male and female rats were fed on a normal diet and water ad libitum. Groups II, III and IV male and female rats were given Clophen A60, DEP, or mixture of Clophen A60 and DEP, respectively, each dissolved in corn oil mixed with the diet at 50mg/kg of the diet/day. One hundred days after treatment, females were mated with males for 10 days. Exposure to the pollutants was continued throughout mating, gestation (21 days) until termination at weaning (21 days), which was 150 days of total treatment period of the parental generation. When the F1-generation pups (six males and six females of each group) were 75-100g in weight, they were treated in a similar manner to the parental generation, again for a period of 150 days, with the dose reduced to 25mg/kg of the diet/day in all treated groups. After 150 days of treatment, animals were sacrificed and histology of the adrenal and thyroid glands was asessed. An antagonistic interactive effect of treatment was seen in male parental and F1-generation rats, while an inhibitory type of interactive effect was observed in female rats. In the zona fasciculata region of the adrenal cortex of treated rats of both generations, vacuolations and degeneration were seen in samples from male animals and intracellular vacuolations in samples from females. A synergistic interactive toxic effect to the thyroid gland was observed in treated parental generation male rats, and mild changes in F1-generation-treated male rats, showing follicular shrinkage, loss of thyroglobulin and fibrosis of the interfollicular epithelium. In females, an antagonistic effect to the thyroid gland was observed in both parental and F1-generation-treated rats, showing similar effects as observed in males. From this study, we can conclude that combined administration of Clophen A60 and DEP shows an enhanced toxic effect on adrenal glands of F1-generation male and female rats, but the effect is much more marked in the thyroid gland of F1-generation male rats, and seen to a lesser extent in F1-generation female rats.

Employing dynamical and chemical processes for contaminant mixtures outdoors to the indoor environment: the implications for total human exposure analysis and prevention

There are many physical and chemical processes that affect the accumulation of outdoor pollutants. In recent years some of the information and concepts previously ascribed to outdoor pollution has been found to be useful in examining indoor dynamic and chemical processes. Further, becau se of the confining nature of the indoor environment, processes such as the "grasshopper effect" can lead to sustained higher levels of semivolatile chemicals indoors and affect multiroute (inhalation, dermal, incidental dietary, and nondietary ingestion) exposures. Such processes can also lead to a complex mixture of both semivolatile and volatile compounds in indoor air and on surfaces or within objects. This article specifically examines the above in combination with another indoor issue, indoor chemistry, and places the results into a context that can be used to evaluate (1) multipollutant cumulative or aggregate exposures and risks indoors, (2) exposure reduction strategies that can create healthy indoor environments. It is not a review of the entire field of the indoor environment or indoor air or the indoor environment, which has been covered in numerous volumes and reports. The complexities of the scientific issues are discussed by also placing them into our traditional approaches outdoor and indoor to pollution management, to indicate the difficulty in establishing the exposures that require mitigation or prevention. Further, some emerging issues are discussed as well as how to specifically address long-term single or multiroute exposures to semivolatile compounds within the "Total Indoor Environment."

In vivo-in vitro toxicogenomic comparison of TCDD-elicited gene expression in Hepa1c1c7 mouse hepatoma cells and C57BL/6 hepatic tissue

Background

In vitro systems have inherent limitations in their ability to model whole organism gene responses, which must be identified and appropriately considered when developing predictive biomarkers of in vivo toxicity. Systematic comparison of in vitro and in vivo temporal gene expression profiles were conducted to assess the ability of Hepa1c1c7 mouse hepatoma cells to model hepatic responses in C57BL/6 mice following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).

Results

Gene expression analysis and functional gene annotation indicate that Hepa1c1c7 cells appropriately modeled the induction of xenobiotic metabolism genes in vivo. However, responses associated with cell cycle progression and proliferation were unique to Hepa1c1c7 cells, consistent with the cell cycle arrest effects of TCDD on rapidly dividing cells. In contrast, lipid metabolism and immune responses, representative of whole organism effects in vivo, were not replicated in Hepa1c1c7 cells.

Conclusion

These results identified inherent differences in TCDD-mediated gene expression responses between these models and highlighted the limitations of in vitro systems in modeling whole organism responses, and additionally identified potential predictive biomarkers of toxicity.

Toxicogenomics in Risk Assessment: Applications and Needs

Since its inception, there have been high expectations for the science of toxicogenomics to decrease the uncertainties associated with the risk assessment process by providing valuable insights into toxic mechanisms of action. However, the application of these data into risk assessment practices is still in the early stages of development, and proof of principle experiments have yet to emerge. The following discusses some potential applications as well as impediments that warrant a concerted investigation from all stakeholders in order to facilitate the acceptance and subsequent incorporation of toxicogenomics into regulatory decision making.

Exposure to metal mixtures and human health impacts in a contaminated area in Nanning, China

Cadmium and lead have been identified as very toxic metals, which are widely present in the environment due to natural and anthropogenic emissions. Many studies have shown that the food chain is the main pathway of cadmium and lead transfer from the environment to humans. It is well documented that many factors will affect their transfer through food chains. Previous investigations on heavy metals were mostly concentrated on one contaminant in isolation. However, in real environments, exposure to mixtures of metals is ubiquitous such that cadmium pollution is invariably being associated with lead and zinc, etc. This study focuses on the contamination and health effects of the metal mixtures. For this purpose, a dietary survey was taken for 3 groups in Nanning in October 2002. Samples of soils, plants (vegetables), urine and blood of humans were measured for Cd, Fe, Cu, Zn, Ca and Pb, in addition, the urinary indicators of renal dysfunction Albumin (ALB), N-acetyl-beta-D-glucosaminidase (NAG), Beta-2-microglobulin (beta2-MG) and Retinol-binding protein (RBP) in urine were also measured. Results showed that soil contamination with metal mixtures had caused significant renal dysfunction of the local residents living in the contaminated area, and the dose-response curve was somewhat altered by the mixed contamination of Cd and Pb as well as the intake of other minerals. The importance of mixtures of metal contamination and human health are also discussed in this paper.

Evidence of interaction between polychlorinated biphenyls and phthalates in relation to human sperm motility

Previously, we reported evidence of inverse associations between exposure to some polychlorinated biphenyls (PCBs) and some phthalate monoesters in relation to semen parameters, specifically sperm motility. Because humans are exposed to both phthalates and PCBs and because experimental studies suggest that PCBs may interact with glucuronidative enzymes that are responsible for phthalate metabolism, we explored the potential interaction between phthalates and PCBs in relation to human semen quality. We studied 303 men who were partners in subfertile couples seeking infertility diagnosis from the andrology laboratory at Massachusetts General Hospital. Semen parameters were dichotomized based on World Health Organization reference values, and phthalate and PCB levels were dichotomized at their respective medians. After adjusting for age and abstinence time, for below reference sperm motility there was a greater than additive interaction between monobenzyl phthalate and PCB-153 [relative excess risk due to interaction (RERI) = 1.40; 95% confidence interval (CI), 0.41-3.22], sum of PCBs (RERI = 1.24; 95% CI, 0.15-2.94), and cytochrome P450 (CYP450)-inducing PCBs (RERI = 1.30; 95% CI, 0.21-3.06). For below-reference sperm motility, there was also a greater than additive interaction between monobutyl phthalate (MBP) and PCB-153 (RERI = 1.42; 95% CI, 0.09-3.76) and CYP450-inducing PCBs (RERI = 1.87; 95% CI, 0.56-4.52) and a suggestive interaction between MBP and sum of PCBs (RERI = 1.35; 95% CI, -0.11 to 3.48). In conclusion, because there are important risk assessment and public health implications of interactions between these two ubiquitous classes of compounds, further studies need to be conducted to confirm these results and identify potential mechanisms of interactions.

Alkyl phenols and diethylhexyl phthalate in tissues of sheep grazing pastures fertilized with sewage sludge or inorganic fertilizer

We studied selected tissues from ewes and their lambs that were grazing pastures fertilized with either sewage sludge (treated) or inorganic fertilizer (control) and determined concentrations of alkylphenols and phthalates in these tissues. Mean tissue concentrations of alkylphenols were relatively low (< 10-400 microg/kg) in all animals and tissues. Phthalates were detected in tissues of both control and treated animals at relatively high concentrations (> 20,000 microg/kg in many tissue samples). The use of sludge as a fertilizer was not associated with consistently increased concentrations of either alkylphenols or phthalates in the tissues of animals grazing treated pastures relative to levels in control animal tissues. Concentrations of the two classes of chemicals differed but were of a similar order of magnitude in liver and muscle as well as in fat. Concentrations of each class of compound were broadly similar in tissues derived from ewes and lambs. Although there were significant differences (p < 0.01 or p < 0.001) between years (cohorts) in mean tissue concentrations of both nonylphenol (NP) and phthalate in each of the tissues from both ewes and lambs, the differences were not attributable to either the age (6 months or 5 years) of the animal or the duration of exposure to treatments. Octylphenol concentrations were generally undetectable. There was no consistent cumulative outcome of prolonged exposure on the tissue concentrations of either class of pollutant in any ewe tissue. Mean tissue concentrations of phthalate were higher (p < 0.001) in the liver and kidney fat of male compared with female lambs. We suggest that the addition of sewage sludge to pasture is unlikely to cause large increases in tissue concentrations of NP and phthalates in sheep and other animals with broadly similar diets and digestive systems (i.e., domestic ruminants) grazing such pasture.

Molecular circuits, biological switches, and nonlinear dose-response relationships

Signaling motifs (nuclear transcriptional receptors, kinase/phosphatase cascades, G-coupled protein receptors, etc.) have composite dose-response behaviors in relation to concentrations of protein receptors and endogenous signaling molecules. "Molecular circuits" include the biological components and their interactions that comprise the workings of these signaling motifs. Many of these molecular circuits have nonlinear dose-response behaviors for endogenous ligands and for exogenous toxicants, acting as switches with "all-or-none" responses over a narrow range of concentration. In turn, these biological switches regulate large-scale cellular processes, e.g., commitment to cell division, cell differentiation, and phenotypic alterations. Biologically based dose-response (BBDR) models accounting for these biological switches would improve risk assessment for many nonlinear processes in toxicology. These BBDR models must account for normal control of the signaling motifs and for perturbations by toxic compounds. We describe several of these biological switches, current tools available for constructing BBDR models of these processes, and the potential value of these models in risk assessment.