Mini-Monograph | GIS and exposure modeling excluded because they comprise a consider- able number of rather harmless cancers. The reference population chosen was the Danish population in the same period, 1986–1998. It was desirable but not finan- cially possible to obtain a reference area similar to the study area. Study Population At the start of follow-up, 1 January 1986, 15,404 individuals resided in the study area. During the next 13 years, between 2,069 and 3,470 individuals moved into the area each year; a total of 46,392 different individuals resided in the area during the 13-year follow-up Figure 1. Map of Kolding Town with dioxin source in red and address points in green . All health registers in Denmark use the CPR number as entry key, which makes it easy to merge health data into a GIS where CPR has been incorporated. In this study all malignant cancer (except skin cancer) was used as the health outcome indicator. By merging the Danish Cancer Register with CPR data, the necessary cancer incidence information was retrieved. Details on Danish health registers have been described elsewhere (Briggs et al. 2002), particularly in the Danish Cancer Register (Storm 1991). In this study, 1986–1998 was the time span chosen for analyses. The ﬁrst year, 1986, was chosen as a compromise between the arduous work of geocoding historical addresses and the cost of this operation versus having a sufﬁcient number of years with cancer data for analyses. The main dioxin producer, the alu- minum recycling plant, became operational in 1970, which was sufﬁcient time from start of production (and pollution) to account for the induction and latency time of developing (eventual) cancer in the surrounding popula- tion. The end of study year, 1998, was chosen because that was the last year with obtainable cancer data at the time a request was sent to the Danish Cancer Register. In this study the exposure simulation model, OML, was used to demarcate three zones relevant for studying cancer develop- ment related to the dioxin exposure: • Zone 1 encompassed the whole residential area identiﬁed to be exposed to dioxin. • Zone 2 included the area identified to be exposed to 3.5 ng dioxin/m3/hr or higher. Zone 2 is part of zone 1. • Zone 3 included the inner and highest exposed area with estimated dioxin immis- sions of 4.5 ng dioxin/m3/hr or higher. Zone 3 is part of zones 2 and 3. These zones and the selection are illustrated in Figure 2, where the immission concentration band borders are used to demar- cate the three zones. In Figure 3 the malignant cancers have been linked and overlayered, appearing as yellow dots. For confidentiality reasons, any single, outstanding cancer case has been obscured so that only clusters of cancers (aggregated over 13 years) are visible. The following criteria were used to select the study population: • Individuals were included if they lived in the area between 1986 and 1998. Individuals were included in a calendar year if they moved into the area on or after 1 January in the same year. • Cancer cases were included if the year of diagnosis was in or later than the year the individual moved into the area (i.e., cancer cases with year of diagnosis before migration into the area were excluded). • Skin cancer diagnoses (ICD-7 code 191; National Board of Health 2003) were Figure 2. Computer-simulated exposure of dioxin from three sources (red) are layered onto the electronic map (GIS) and seen as different colored bands, with highest dioxin immissions in bright red and lowest in faint green. The immission concentration band borders (blue) are used to demarcate the three zones used for analyses of cancer development. Environmental Health Perspectives • VOLUME 112 | NUMBER 9 | June 2004 1033