its methodological rigor or its conclusions (Brown, 1995; Szasz and Meuser, 1997). Beyond these beliefs, however, the chain of causality scientifically demonstrating a lack of environmental justice grows weaker.

One limitation is that with a few exceptions, an adequate body of knowledge directly establishing the link between those things researchers believe to be environmental hazards and specific adverse health outcomes typically does not exist. A good start on this research has been made in the occupational health field (Frumkin and Walker, 1997), but much more research is needed to document the nature, scope, and severity of the toxic effects of environmental stressors, particularly when multiple sources or types of potentially harmful substances exist. When causal relationships between environmental hazards and adverse health outcomes are established, risk assessments are needed to evaluate the relative contributions of the various hazards, both in terms of the numbers of people affected and the severity of illness. Judgments can then be made about research and intervention priorities.

A second limitation is that policymakers need better evidence about the precise roles of environmental hazards in creating adverse health outcomes and the burden borne by racial or ethnic minorities or demographically defined populations (Brown, 1995). Although evidence of disproportionate exposure to potentially harmful substances strongly suggests a disappropriate health burden, a variety of other factors might be involved. Clarifying these relationships will require detailed epidemiological studies, often on small, targeted populations, examining the relationships among disease incidence, environmental hazards, and other behaviors or exposures that may cause or exacerbate health problems. Properly disseminated, the research arising from such a broad-based public health perspective will help increase awareness that a given health outcome can be caused by multiple environmental hazards (with cumulative or interactive effects) and that a particular hazard can contribute to multiple adverse outcomes. Therefore, efforts are needed to evaluate total exposures and then to identify the most severe and remediable hazards as a guide to the most effective courses of action.

As discussed in Chapter 3, uncertainties in the scientific analysis of environmental justice issues have several sources, some reflecting limitations of current methodologies, others reflecting the lack of funding for implementing available methodologies. One large gap in current research data is the lack of tools for measuring important elements of environmental health and environmental justice issues, including health status indicators, sociodemographic characteristics, and other lifestyle factors that influence a population's health. Even where demographic and environmental databases do exist, it may be hard to link them. Small sample sizes and the inability to disaggregate data by such factors as race, ethnicity, income, and education further impede rigorous analysis and robust conclusions. In addition, it is very difficult to identify specific environmental hazards and to document their role in specific adverse health outcomes. These problems are even more difficult when there are complex interactions among multiple environmental hazards and long latency periods. Finally, little is

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