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Differential exposure to mixtures of environmental agents, including biological, chemical, physical, and psychosocial stressors, can contribute to increased vulnerability of human populations and ecologic systems. Cumulative risk assessment is a tool for organizing and analyzing information to evaluate the probability and seriousness of harmful effects caused by either simultaneous and/or sequential exposure to multiple environmental stressors. In this article we focus on elucidating key challenges that must be addressed to determine whether and to what degree differential exposure to environmental mixtures contributes to increased vulnerability of exposed populations. In particular, the emphasis is on examining three fundamental and interrelated questions that must be addressed as part of the process to assess cumulative risk: a) Which mixtures are most important from a public health perspective? and b) What is the nature (i.e., duration, frequency, timing) and magnitude (i.e., exposure concentration and dose) of relevant cumulative exposures for the population of interest? c) What is the mechanism (e.g., toxicokinetic or toxicodynamic) and consequence (e.g., additive, less than additive, more than additive) of the mixture's interactive effects on exposed populations? The focus is primarily on human health effects from chemical mixtures, and the goal is to reinforce the need for improved assessment of cumulative exposure and better understanding of the biological mechanisms that determine toxicologic interactions among mixture constituents.

Background: Airborne emissions from numerous point, area, and mobile sources, along with stagnant meteorologic conditions, contribute to frequent episodes of elevated air pollution in Houston, Texas. To address this problem, decision makers must set priorities among thousands of individual air pollutants as they formulate effective and efficient mitigation strategies. Objectives: Our aim was to compare and rank relative health risks of 179 air pollutants in Houston using an evidence-based approach supplemented by the expert judgment of a panel of academic scientists. Methods: Annual-average ambient concentrations by census tract were estimated from the U.S. Environmental Protection Agency's National-scale Air Toxics Assessment and augmented with measured levels from the Houston monitoring network. Each substance was assigned to one of five risk categories (definite, probable, possible, unlikely, uncertain) based on how measured or monitored concentrations translated into comparative risk estimates. We used established unit risk estimates for carcinogens and/or chronic reference values for noncarcinogens to set thresholds for each category. Assignment to an initial risk category was adjusted, as necessary, based on expert judgment about the quality and quantity of information available. Results: Of the 179 substances examined, 12 (6.7%) were deemed definite risks, 9 (5.0%) probable risks, 24 (13.4%) possible risks, 16 (8.9%) unlikely risks, and 118 (65.9%) uncertain risks. Conclusions: Risk-based priority setting is an important step in the development of cost-effective solutions to Houston's air pollution problem.

In the United States, the risk assessment – risk management paradigm that underpins federal decisions about environmental health risks was first established in 1983. In the beginning, the importance of public participation was not explicitly recognized within the paradigm. Over time, however, it has become evident that not only must risk-based decisions be founded on the best available scientific knowledge and understanding, but also that they must take account of the knowledge, values, and preferences of interested and affected parties, including community members, business people, and environmental advocates. This article examines the gradually expanding role of public participation in risk-based decision making in the United States, and traces its evolution from a peripheral issue labeled as an external pressure to an integral element of the 21 st century risk assessment – risk management paradigm. Today, and into the foreseeable future, public participation and stakeholder involvement are intrinsic features of the emerging American regulatory landscape, which emphasizes collaborative approaches for achieving cooperative and cost-effective solutions to complicated and often controversial environmental health problems.

Systematic evaluation of cumulative health risks from the combined effects of multiple environmental stressors is becoming a vital component of risk-based decisions aimed at protecting human populations and communities. This article briefly examines the historical development of cumulative risk assessment as an analytical tool, and discusses current approaches for evaluating cumulative health effects from exposure to both chemical mixtures and combinations of chemical and nonchemical stressors. A comparison of stressor-based and effects-based assessment methods is presented, and the potential value of focusing on viable risk management options to limit the scope of cumulative evaluations is discussed. The ultimate goal of cumulative risk assessment is to provide answers to decision-relevant questions based on organized scientific analysis; even if the answers, at least for the time being, are inexact and uncertain.

We measured volatile organic compound (VOC) exposures in multiple locations for a diverse population of children who attended two inner-city schools in Minneapolis, Minnesota. Fifteen common VOCs were measured at four locations: outdoors (O), indoors at school (S), indoors at home (H), and in personal samples (P). Concentrations of most VOCs followed the general pattern O ≈ S < P ≤ H across the measured microenvironments. The S and O environments had the smallest and H the largest influence on personal exposure to most compounds. A time-weighted model of P exposure using all measured microenvironments and time-activity data provided little additional explanatory power beyond that provided by using the H measurement alone. Although H and P concentrations of most VOCs measured in this study were similar to or lower than levels measured in recent personal monitoring studies of adults and children in the United States, p-dichlorobenzene was the notable exception to this pattern, with upper-bound exposures more than 100 times greater than those found in other studies of children. Median and upper-bound H and P exposures were well above health benchmarks for several compounds, so outdoor measurements likely underestimate long-term health risks from children's exposure to these compounds.

Although ambient concentrations have declined steadily over the past 30 years, Houston has recorded some of the highest levels of hazardous air pollutants in the United States. Nevertheless, federal and state regulatory efforts historically have emphasized compliance with the National Ambient Air Quality Standard for ozone, treating “air toxics” in Houston as a residual problem to be solved through application of technology-based standards. Between 2004 and 2009, Mayor Bill White and his administration challenged the well-established hierarchy of air quality management spelled out in the Clean Air Act, whereby federal and state authorities are assigned primacy over local municipalities for the purpose of designing and implementing air pollution control strategies. The White Administration believed that existing regulations were not sufficient to protect the health of Houstonians and took a diversity of both collaborative and combative policy actions to mitigate air toxic emissions from stationary sources. Opposition was substantial from a local coalition of entrenched interests satisfied with the status quo, which hindered the city's attempts to take unilateral policy actions. In the short term, the White Administration successfully raised the profile of the air toxics issue, pushed federal and state regulators to pay more attention, and induced a few polluting facilities to reduce emissions. But since White left office in 2010, air quality management in Houston has returned to the way it was before, and today there is scant evidence that his policies have had any lasting impact.

The US National Research Council recently released a report promoting sustainability assessment as the future of environmental regulation. Thirty years earlier, this organization (under the same senior author) had issued a similar report promoting risk assessment as a new method for improving the science behind regulatory decisions. Tools for risk assessment were subsequently developed and adopted in state and federal agencies throughout the US. Since then, limitations of the traditional forms of risk assessment have prompted some dramatic modifications toward cumulative assessments that combine multiple chemical and non-chemical stressors in community settings. At present, however, there is little momentum within the risk assessment community for abandoning this evolved system in favor of a new sustainability-based one. The key question is, how best to proceed? Should sustainability principles be incorporated into current risk assessment procedures, or vice versa? Widespread recognition of the importance of sustainability offers no clear guidance for the risk assessment community, especially in light of institutional commitments to sustainability tools and definitions that appear to have little in common with cumulative risk notions. The purpose of this paper is to reframe the sustainability challenge for risk assessors by offering analytical guidance to chart a way out. We adopt a decision analysis framework to overcome some conceptual barriers separating these two forms of assessment, and thereby, both escape the either/or choice and accept the inevitability of sustainability as a central regulatory concern in the U.S.

Objective. The objectives of the study were to detect high-risk areas and to examine how racial and ethnic status affect the geographic distribution of female breast cancer mortality in Texas. Analyses were based on county-level data for the years from 2000 to 2008. Materials and Methods. Breast cancer mortality data were obtained from the Texas Cancer Registry, and the Spatial Scan Statistics method was used to run Purely Spatial Analyses using the Discrete Poisson, Bernoulli, and Multinomial models. Results and Conclusions. Highest rates of female breast cancer mortality in Texas have shifted over time from southeastern areas towards northern and eastern areas, and breast cancer mortality at the county level is distributed heterogeneously based on racial/ethnic status. Non-Hispanic blacks were at highest risk in the northeastern region and lowest risk in the southern region, while Hispanics were at highest risk in the southern region along the border with Mexico and lowest risk in the northeastern region.

Cumulative risk assessment is a science policy tool for organizing and analyzing information to examine, characterize, and possibly quantify combined threats from multiple environmental stressors. We briefly survey the state of the art regarding cumulative risk assessment, emphasizing challenges and complexities of moving beyond the current focus on chemical mixtures to incorporate nonchemical stressors, such as poverty and discrimination, into the assessment paradigm. Theoretical frameworks for integrating nonchemical stressors into cumulative risk assessments are discussed, the impact of geospatial issues on interpreting results of statistical analyses is described, and four assessment methods are used to illustrate the diversity of current approaches. Prospects for future progress depend on adequate research support as well as development and verification of appropriate analytic frameworks.

We assessed concurrent exposure to a mixture of > 50 environmental chemicals by measuring the chemicals or their metabolites in the blood of 43 ethnically diverse children (3-6 years of age) from a socioeconomically disadvantaged neighborhood in Minneapolis. Over a 2-year period, additional samples were collected every 6-12 months from as many children as possible. We analyzed blood samples for 11 volatile organic compounds (VOCs), 2 heavy metals (lead and mercury, 11 organochlorine (OC) pesticides or related compounds, and 30 polychlorinated biphenyl (PCB) congeners. The evidence suggests that numerous VOCs originated from common sources, as did many PCBs. Longitudinal measurements indicate that between-child variance was greater than within-child variance for two VOCs (benzene, toluene), for both heavy metals (Pb, Hg), for all detectable OC pesticides, and for 15 of the measured PCB congeners (74, 99, 101, 118, 138-158, 146, 153, 156, 170, 178, 180, 187, 189, 194, 195). Despite the relatively small sample size, highest measured blood levels of 1,4-dichlorobenzene, styrene, m-/ p-xylene, Pb, Hg, heptachlor epoxide, oxychlordane, dichlorodiphenyldichloroethene (p,p'-DDE), trans-nonachlor, and PCB congeners 74, 99, 105, 118, 138, 146, 153, 156, 170, and 180 were comparable with or higher than 95th percentile measurements of older children and adults from national surveys. Results demonstrate that cumulative exposures to multiple environmental carcinogens and neurotoxins can be comparatively high for children from a poor inner-city neighborhood.