An interdisciplinary framework for derivation of occupational exposure limits.

Protecting the health and safety of workers in industrial operations is a top priority. One of the resources used in industry to ensure worker safety is the occupational exposure limit (OEL). OELs are derived from the assessment and interpretation of empirical data from animal and/or human studies. There are various guidelines for the derivation and implementation of OELs globally, with a range of stakeholders (including regulatory bodies, governmental agencies, expert groups and others). The purpose of this manuscript is to supplement existing guidance with learnings from a multidisciplinary team approach within an industry setting. The framework we present is similar in construct to other risk assessment frameworks and includes: (1) problem formulation, (2) literature review, (3) weight of evidence considerations, (4) point of departure selection/derivation, (5) application of assessment factors, and the final step, (6) derivation of the OEL. Within each step are descriptions and examples to consider when incorporating data from various disciplines such as toxicology, epidemiology, and exposure science. This manuscript describes a technical framework by which available data relevant for occupational exposures is compiled, analyzed, and utilized to inform safety threshold derivation applicable to OELs.

Lung physiology and controlled exposure study design.

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

A framework for cumulative risk assessment in the 21st century.

The ILSI Health and Environmental Sciences Institute (HESI) has developed a framework to support a transition in the way in which information for chemical risk assessment is obtained and used (RISK21). The approach is based on detailed problem formulation, where exposure drives the data acquisition process in order to enable informed decision-making on human health safety as soon as sufficient evidence is available. Information is evaluated in a transparent and consistent way with the aim of optimizing available resources. In the context of risk assessment, cumulative risk assessment (CRA) poses additional problems and questions that can be addressed using the RISK21 approach. The focus in CRA to date has generally been on chemicals that have common mechanisms of action. Recently, concern has also been expressed about chemicals acting on multiple pathways that lead to a common health outcome, and non-chemical other conditions (non-chemical stressors) that can lead to or modify a common outcome. Acknowledging that CRAs, as described above, are more conceptually, methodologically and computationally complex than traditional single-stressor risk assessments, RISK21 further developed the framework for implementation of workable processes and procedures for conducting assessments of combined effects from exposure to multiple chemicals and non-chemical stressors. As part of the problem formulation process, this evidence-based framework allows the identification of the circumstances in which it is appropriate to conduct a CRA for a group of compounds. A tiered approach is then proposed, where additional chemical stressors and/or non-chemical modulating factors (ModFs) are considered sequentially. Criteria are provided to facilitate the decision on whether or not to include ModFs in the formal quantitative assessment, with the intention to help focus the use of available resources to have the greatest potential to protect public health.

Problem formulation for risk assessment of combined exposures to chemicals and other stressors in humans.

When the human health risk assessment/risk management paradigm was developed, it did not explicitly include a "problem formulation" phase. The concept of problem formulation was first introduced in the context of ecological risk assessment (ERA) for the pragmatic reason to constrain and focus ERAs on the key questions. However, this need also exists for human health risk assessment, particularly for cumulative risk assessment (CRA), because of its complexity. CRA encompasses the combined threats to health from exposure via all relevant routes to multiple stressors, including biological, chemical, physical and psychosocial stressors. As part of the HESI Risk Assessment in the 21st Century (RISK21) Project, a framework for CRA was developed in which problem formulation plays a critical role. The focus of this effort is primarily on a chemical CRA (i.e., two or more chemicals) with subsequent consideration of non-chemical stressors, defined as "modulating factors" (ModFs). Problem formulation is a systematic approach that identifies all factors critical to a specific risk assessment and considers the purpose of the assessment, scope and depth of the necessary analysis, analytical approach, available resources and outcomes, and overall risk management goal. There are numerous considerations that are specific to multiple stressors, and proper problem formulation can help to focus a CRA to the key factors in order to optimize resources. As part of the problem formulation, conceptual models for exposures and responses can be developed that address these factors, such as temporal relationships between stressors and consideration of the appropriate ModFs.

Illustrative case using the RISK21 roadmap and matrix: prioritization for evaluation of chemicals found in drinking water.

The HESI-led RISK21 effort has developed a framework supporting the use of twenty-first century technology in obtaining and using information for chemical risk assessment. This framework represents a problem formulation-based, exposure-driven, tiered data acquisition approach that leads to an informed decision on human health safety to be made when sufficient evidence is available. It provides a transparent and consistent approach to evaluate information in order to maximize the ability of assessments to inform decisions and to optimize the use of resources. To demonstrate the application of the framework's roadmap and matrix, this case study evaluates a large number of chemicals that could be present in drinking water. The focus is to prioritize which of these should be considered for human health risk as individual contaminants. The example evaluates 20 potential drinking water contaminants, using the tiered RISK21 approach in combination with graphical representation of information at each step, using the RISK21 matrix. Utilizing the framework, 11 of the 20 chemicals were assigned low priority based on available exposure data alone, which demonstrated that exposure was extremely low. The remaining nine chemicals were further evaluated, using refined estimates of toxicity based on readily available data, with three deemed high priority for further evaluation. In the present case study, it was determined that the greatest value of additional information would be from improved exposure models and not from additional hazard characterization.

A 21st century roadmap for human health risk assessment.

The Health and Environmental Sciences Institute (HESI)-coordinated Risk Assessment in the 21st Century (RISK21) project was initiated to develop a scientific, transparent, and efficient approach to the evolving world of human health risk assessment, and involved over 120 participants from 12 countries, 15 government institutions, 20 universities, 2 non-governmental organizations, and 12 corporations. This paper provides a brief overview of the tiered RISK21 framework called the roadmap and risk visualization matrix, and articulates the core principles derived by RISK21 participants that guided its development. Subsequent papers describe the roadmap and matrix in greater detail. RISK21 principles include focusing on problem formulation, utilizing existing information, starting with exposure assessment (rather than toxicity), and using a tiered process for data development. Bringing estimates of exposure and toxicity together on a two-dimensional matrix provides a clear rendition of human safety and risk. The value of the roadmap is its capacity to chronicle the stepwise acquisition of scientific information and display it in a clear and concise fashion. Furthermore, the tiered approach and transparent display of information will contribute to greater efficiencies by calling for data only as needed (enough precision to make a decision), thus conserving animals and other resources.

Risk assessment in the 21st century: roadmap and matrix.

Abstract The RISK21 integrated evaluation strategy is a problem formulation-based exposure-driven risk assessment roadmap that takes advantage of existing information to graphically represent the intersection of exposure and toxicity data on a highly visual matrix. This paper describes in detail the process for using the roadmap and matrix. The purpose of this methodology is to optimize the use of prior information and testing resources (animals, time, facilities, and personnel) to efficiently and transparently reach a risk and/or safety determination. Based on the particular problem, exposure and toxicity data should have sufficient precision to make such a decision. Estimates of exposure and toxicity, bounded by variability and/or uncertainty, are plotted on the X- and Y-axes of the RISK21 matrix, respectively. The resulting intersection is a highly visual representation of estimated risk. Decisions can then be made to increase precision in the exposure or toxicity estimates or declare that the available information is sufficient. RISK21 represents a step forward in the goal to introduce new methodologies into 21st century risk assessment. Indeed, because of its transparent and visual process, RISK21 has the potential to widen the scope of risk communication beyond those with technical expertise.

Mode of action human relevance (species concordance) framework: Evolution of the Bradford Hill considerations and comparative analysis of weight of evidence.

The mode of action human relevance (MOA/HR) framework increases transparency in systematically considering data on MOA for end (adverse) effects and their relevance to humans. This framework continues to evolve as experience increases in its application. Though the MOA/HR framework is not designed to address the question of "how much information is enough" to support a hypothesized MOA in animals or its relevance to humans, its organizing construct has potential value in considering relative weight of evidence (WOE) among different cases and hypothesized MOA(s). This context is explored based on MOA analyses in published assessments to illustrate the relative extent of supporting data and their implications for dose-response analysis and involved comparisons for chemical assessments on trichloropropane, and carbon tetrachloride with several hypothesized MOA(s) for cancer. The WOE for each hypothesized MOA was summarized in narrative tables based on comparison and contrast of the extent and nature of the supporting database versus potentially inconsistent or missing information. The comparison was based on evolved Bradford Hill considerations rank ordered to reflect their relative contribution to WOE determinations of MOA taking into account increasing experience in their application internationally. This clarification of considerations for WOE determinations as a basis for comparative analysis is anticipated to contribute to increasing consistency in the application of MOA/HR analysis and potentially, transparency in separating science judgment from public policy considerations in regulatory risk assessment.

An organizational approach for the assessment of DNA adduct data in risk assessment: case studies for aflatoxin B1, tamoxifen and vinyl chloride.

The framework analysis previously presented for using DNA adduct information in the risk assessment of chemical carcinogens was applied in a series of case studies which place the adduct information into context with the key events in carcinogenesis to determine whether they could be used to support a mutagenic mode of action (MOA) for the examined chemicals. Three data-rich chemicals, aflatoxin B1 (AFB1), tamoxifen (Tam) and vinyl chloride (VCl) were selected for this exercise. These chemicals were selected because they are known human carcinogens and have different characteristics: AFB1 forms a unique adduct and human exposure is through contaminated foods; Tam is a pharmaceutical given to women so that the dose and duration of exposure are known, forms unique adducts in rodents, and has both estrogenic and genotoxic properties; and VCl, to which there is industrial exposure, forms a number of adducts that are identical to endogenous adducts found in unexposed people. All three chemicals produce liver tumors in rats. AFB1 and VCl also produce liver tumors in humans, but Tam induces human uterine tumors, only. To support a mutagenic MOA, the chemical-induced adducts must be characterized, shown to be pro-mutagenic, be present in the tumor target tissue, and produce mutations of the class found in the tumor. The adducts formed by AFB1 and VCl support a mutagenic MOA for their carcinogenicity. However, the data available for Tam shows a mutagenic MOA for liver tumors in rats, but its carcinogenicity in humans is most likely via a different MOA.

Estimates of daily di-isodecyl phthalate (DIDP) intake calculated from urinary biomonitoring data.

Biomonitoring of chemical species in biologic media can be a valuable tool in risk assessment since it informs directly on aggregate exposures. While advances in the analytical techniques required for biomonitoring have resulted in increased sensitivity for the detection of chemical species, the detection of a chemical substance in a biological medium (such as blood, urine or breast milk) does not mean that the chemical causes or is associated with an adverse health outcome. In this paper, intake estimates for di-isodecyl phthalate (DIDP), a high molecular weight general purpose plasticizer, were calculated from urinary biomonitoring data representing various population segments and geographic locales. From these data, intake estimates converge on a mean that is <1 µg/kg/day (95th% < 5 µg/kg/day). This intake estimate is at least two orders of magnitude lower than the health-based exposure guidance values (38-150 µg/kg/day) which have been proposed by regulatory authorities and other authoritative bodies as safe exposure levels.

A comprehensive review of intake estimates of di-isononyl phthalate (DINP) based on indirect exposure models and urinary biomonitoring data.

Di-isononyl phthalate (DINP) is a high molecular weight general purpose plasticizer used principally in the manufacture of flexible polyvinyl chloride (PVC) articles. DINP metabolites can be measured in biological media such as blood and urine. However, measurement of a substance in the blood or urine does not by itself mean that the chemical causes or is associated with adverse health outcomes. This is particularly pertinent given the advances in modern analytical techniques whereby ever diminishing trace amounts of substances can be detected. Therefore, it is a scientific necessity that risk assessors understand the relationship of biomonitoring data to estimation of exposure so that appropriate comparisons can be made to the no observed adverse effects levels (NOAELs) or other points of departure from toxicological studies in animals. In this paper, estimates of daily DINP intake are calculated for various population segments based on urinary biomonitoring data and are compared to estimates of exposure based on indirect methods and to health-based exposure guidance values. In general, intake estimates converge on a mean of 1-2µg/kg/day regardless of source of exposure or population cluster; a value 2-orders of magnitude lower than health-based exposure guidance values, ranging from 120 to 290µg/kg/day, which have been established by regulatory authorities and other authoritative bodies as representing acceptable levels.

Altered methylation in gene-specific and GC-rich regions of DNA is progressive and nonrandom during promotion of skin tumorigenesis.

Altered DNA methylation, an epigenetic mechanism, likely contributes to tumorigenesis, with an inverse relationship existing between methylation in a promoter region and transcription. Using the SENCAR two-stage mouse skin tumorigenesis model, altered methylation was characterized in precancerous tissue and in tumor tissue. Mouse skin was initiated with 7,12-dimethylbenz[a]anthracene and promoted three times a week with 3, 9, 18, or 27 mg cigarette smoke condensate (CSC) for 4, 8, or 29 weeks; tumors were collected at 29 weeks. In addition, reversibility of changes in methylation was assessed following cessation of the promoting stimulus. DNA was isolated, and GC-rich methylation was assessed quantitatively via methylation-sensitive restriction digestion, arbitrarily primed PCR, and electrophoretic separation of PCR products. Analysis focused on regions of altered methylation (RAMs), which persisted from 4 to 8 weeks and from 8 weeks to tumor tissue. Persistent RAMs (i.e., seen in precancerous tissue and carried forward to tumors) are likely to play a key role in tumorigenesis. Twenty-two CpG sites in the upstream region of the Ha-ras promoter were unmethylated in control skin, 27 mg CSC, and tumor tissue. At two CpG sites closer to the transcriptional start site the incidence of hypomethylation increased with the dose of CSC. Hypomethylation was detected in all tumor samples. Expression of Ha-ras increased with 18 and 27 mg CSC promotion and more so in tumor tissue. These data support our hypothesis that tumor promotion involves instability of the epigenome, providing an environment where changes in the methylation status of specific regions of the genome accumulate progressively and contribute to the clonal expansion of initiated cells that leads to tumor formation.

Phenobarbital induces progressive patterns of GC-rich and gene-specific altered DNA methylation in the liver of tumor-prone B6C3F1 mice.

Altered DNA methylation contributes to tumorigenesis by affecting gene expression in a heritable fashion. Phenobarbital (PB) is a nongenotoxic rodent carcinogen which induces global hypomethylation and regions of hypermethylation in mouse liver. Liver tumor-sensitive (B6C3F1) and -resistant (C57BL/6) male mice were administered 0.05% (wt/wt) PB in drinking water for 2 or 4 weeks, and a 2-week recovery was included following each dosing period. DNA was isolated from liver (target) and kidney (nontarget) tissues. The methylation status of GC-rich regions of DNA was assessed via methylation-sensitive restriction digestion, arbitrarily primedpolymerase chain reaction, and capillary electrophoretic separation of products. PB-induced regions of altered methylation (RAMs) which carry forward from an early to a later time point are more likely to be mechanistically relevant as compared to those that do not. Twelve of 69 RAMs (17%) present in B6C3F1 liver at 2 weeks were also seen at 4 weeks, while only 1 of the 123 RAMs (< 1%) present in C57BL/6 liver was seen at 4 weeks. In the B6C3F1 mice, 57 unique (as compared to the C57BL/6) regions of altered hepatic methylation (RAMs), predominantly hypomethylation, were observed at 2 weeks, increasing to 86 at 4 weeks. Changes in methylation were largely reversible. Altered methylation in liver was highly dissimilar to that of kidney. Following 4 weeks PB, bisulfite sequencing revealed hypomethylation of Ha-ras in B6C3F1, but not C57BL/6, which correlated with increased gene expression. These data indicate that (1) progressive, nonrandom changes in methylation provide an epigenetic mechanism underlying the ability of PB to cause mouse liver tumorigenesis and (2) susceptibility to tumorigenesis is related inversely to the capacity to maintain normal patterns of methylation.

Diethanolamine and phenobarbital produce an altered pattern of methylation in GC-rich regions of DNA in B6C3F1 mouse hepatocytes similar to that resulting from choline deficiency.

DNA methylation is an epigenetic mechanism regulating transcription, which when disrupted, can alter gene expression and contribute to carcinogenesis. Diethanolamine (DEA), a non-genotoxic alkanolamine, produces liver tumors in mice. Studies suggest DEA inhibits choline uptake and causes biochemical changes consistent with choline deficiency (CD). Rodents fed methyl-deficient diets exhibit altered methylation of hepatic DNA and an increase in liver tumors, e.g., CD causes liver tumors in B6C3F1 mice. We hypothesize that DEA-induced CD leads to altered methylation patterns which facilitates tumorigenesis. B6C3F1 hepatocytes in primary culture were grown in the presence of either 4.5 mM DEA, 3 mM Phenobarbital (PB), or CD media for 48 h. These concentrations induced comparable increases in DNA synthesis. PB, a nongenotoxic rodent liver carcinogen known to alter methylation in mouse liver, was included as a positive control. Global, average, DNA methylation status was not affected. The methylation status of GC-rich regions of DNA, which are often associated with promoter regions, were assessed via methylation-sensitive restriction digestion and arbitrarily primed PCR with capillary electrophoretic separation and detection of PCR products. DEA, PB, and CD treatments resulted in 54, 63, and 54 regions of altered methylation (RAMs), respectively, and the majority were hypomethylations. A high proportion of RAMs (72%) were identical when DEA was compared to CD. Similarly, 70% were identical between PB and CD. Altered patterns of methylation in GC-rich regions induced by DEA and PB resemble that of CD and indicate that altered DNA methylation is an epigenetic mechanism involved in the facilitation of mouse liver tumorigenesis.