Mini-Monograph | Poulstrup and Hansen example, exhaust from vehicles. Fewer are present in well-defined geographical areas, and few are strong enough to have any signiﬁ- cant impact on health. These factors limit the opportunity to investigate environmental health relationships using spatial analytical methods, and inhibit the types of problems that can be addressed. The Address Project offers new and unique possibilities for performing studies of relation- ships between environmental exposures and health of the population in Denmark. These studies might be based on a range of different study designs (Aylin et al. 1999; Elliott et al. 1992). Because of the ability to track individu- als over time, retrospective, space–time studies are possible. In each case, the detailed address- based data now available and the ability to link data ﬁles are likely to enhance these studies. In this study we decided to use the knowledge of the migration of the popula- tion to apply two restrictions: to include only individuals who had actually stayed in the area and to include only the cancer cases that were diagnosed after the individual had moved into (or after their birth in) the area. Further restrictions could have been implemented, but this would have implicated the choice of a reference area with a popula- tion where the same restrictions could be made. The actual restrictions that were applied based on the individual migration data available are an improvement in dealing with this misclassification problem in epi- demiologic studies and indicate the vast opportunities in the system. There are, however, several important limitations on what can be expected even from a fully developed system, and several issues concerning choice of study design need to be carefully considered. First, it is important to base such studies on deﬁned and plausible hypotheses about the rela- tionships being examined. Possible exposure pathways also need to be identiﬁed. Without these preconditions, results are likely to be difﬁ- cult to interpret, at best, or are even misleading. Second, by considering environment, one is concerned with more than just the soil on which we walk, the water we drink, and the air we inhale. Environment is also what we eat and wear, what we smoke, where we work and relax, and from a biological point of view, it is more likely that the causes of death and diseases may be found here rather than geographically varying pollution in the ambient environment. In addition to these theoretical considera- tions, a number of other limitations must be recognized. For example, most geographically based studies assume that populations are sta- tic and that exposures occur in ﬁxed locations (usually the place of residence). Obviously, this is not true. People are highly mobile, both in terms of short-term activities (e.g., daily travel to work) and long-term migra- tion. Rates of migration in a population may be high. Therefore, knowledge about where people work or have worked is essential if all misclassiﬁcation is to be ruled out. The analysis of the stability of the popula- tion in the study area disclosed a high mobil- ity. Less than one-third lived at the same address after 13 years of observation, and only half were still residents in the study area. A high degree of mobility within the study area was also found. The chosen study area is an ordinary mixed residential and industrial sub- urb, and the observed mobility of the popula- tion is likely to be representative of similar areas in Denmark. In ecologic studies, infor- mation on exposure and the exposed individ- uals is of vital importance. Such studies will therefore be highly susceptible to the fact that only a relatively small proportion of the study population remains in the area during a pro- longed exposure in the local environment. A further improvement in exposure assessment would be to measure actual at-risk time for each individual. This was not done in this study although it is possible within the model and with the Danish data sets. We hope to perform such a study in the future. In an extensive analysis of geographic exposure modeling and its usefulness in envi- ronmental epidemiology (Beyea and Hatch 1999), the authors emphasized the importance of considering all uncertainty aspects when making the models: type and quantity of pol- lutants, their pathways into surroundings, exposed population, and time of pollution. The tested GIS with linkage of addresses and individual health information gives new opportunities for high-quality, small-area health studies in a wide range of situations. When fully developed and covering the whole of Denmark, it will create a useful tool both for administrators, planners, and public health ofﬁces as well as researchers. As with all such systems, however, it is crucial to recog- nize the limitations of the system and to apply it only where appropriate. The geographical stability of the study population is especially crucial to address, describe, and include in the exposure assessment. Otherwise, this misclas- siﬁcation may totally distort the true picture. REFERENCES Aylin P, Maheswaran R, Wakefield J, Cockings S, Järup L, Arnold R. 1999. A national facility for small area disease mapping and rapid initial assessment of apparent disease clusters around a point source: the UK Small Area Health Statistics Unit. J Public Health Med 21:289–298. Beyea J, Hatch M. 1999. Geographic exposure modeling: a valuable extension of geographic information systems for use in environmental epidemiology. Environ Health Perspect 107(suppl 1):181–190. 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