Mini-Monograph | Poulstrup and Hansen period. During the study period the population gradually increased to 20,217 individuals by the end of 1998. Among the 46,392 persons who lived in the study area from 1986 to 1998, 3,205 individuals were newborns who had their ﬁrst-ever address in the area. Among the 15,404 individuals residing in the study area 1 January 1986, 7,758 (50.4%) were still living in the study area at the end of 1998, and 4,799 (31.2%) had not changed their address. Figure 4 illustrates this develop- ment. Among those who were < 10 years of age on 1 January 1986, 57% were still residing in the area 13 years later, whereas only 30% of those 10–20 years of age still lived in the area at the end of 1998. This ﬁgure was 43% for the group 21–30 years of age; 65% for the group 31–40 years of age, and increased to 71% for the group 41–50 years of age. The same ﬁgure gradually decreased for the group 51–60 years of age to 65% for those who remained in the area until the end of the study period, and rapidly decreased for older age groups. The 46,392 individuals who lived in the study area had a total of 75,437 periods of residence. Among the same 46,392 individu- als, 61.5% had one address, 21.5% had two different addresses, and 17% had three or more addresses within the study area during 1986–1998. On 1 January 1986, 50.3% of the females and 55.8% of the male residents in the study area were < 40 years of age. The correspond- ing figures for the whole population of Denmark in 1986 were 54.0% for male residents and 58.3% for females. Statistical Methods For each year from 1986 through 1998, infor- mation on the number of eligible residents and cancers from the study area (zones 1–3) were retrieved through the GIS model and cumulated into nine 10-year age bands (0–9, 10–19, 20–29 . . . ≥ 80) stratiﬁed by sex. The Danish population during the same period was used as the reference population. Figure 3. Demarcation of zone 3 and the addresses (and individuals) in blue within the polygon. Cancers diagnosed among the individuals in zone 3 during 1986–1998 are marked in yellow (overlayed on the blue dots). 18,000 16,000 ts14,000 en12,000 is10,000 d fr e 8,000 .o 6,000 N4,000 o 2,000 0 1986 1034 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 Year Figure 4. Migration of residents living in the study area in January 1986 from the area from 1986 to the end of 1998. Number of cancers was derived from the Danish Cancer Register and population data were from the Bureau of Statistics (Danmarks Statistik). These data were similarly grouped into cumulated 10-year age bands stratiﬁed by sex. Years of risk were calculated using the number of residents in each calendar year in the study area (“living in and not moved out”) and in the general population, respectively. Each resident in a given year was counted as 1 year of exposure. The number of expected cases of cancer was calculated based on the total number of person-years for each 10-year age category multiplied by the cancer rate of the Danish men and women, respectively, dur- ing the same period. The standardized inci- dence ratio (SIR)—the ratio between observed and expected numbers—was calculated with 95% conﬁdence limits (95% CL) assuming a Poisson distribution of the cancer cases. Normal distribution for observed cancers was assumed when ﬁgures were above 100. Results The method of using an air pollution simulation model to identify exposure and exposed population was operational, and the subsequent incorporation into a GIS environ- ment integrating individual statistics of address, vital statistics, and cancer created no severe technical problems. Results of the statistical analyses are presented in Table 1. Only a single age band in zone 1 had conﬁdence limits above 1.0. No excess of cancer in the study area during 1986–1998 could be demonstrated. The study population was anticipated to be geo- graphically stable, but this appeared not to be true, with only one-third of the original resi- dents still living in the area at the end of the study period. Discussion The OML, a commercial product (Danish Environmental Protection Agency 1997), is used widely by environmental regulatory bod- ies in Denmark to assess immission values of airborne pollutants. This product proved use- ful to visualize exposure in a GIS milieu to outline the research area. The incorporation of the model in GIS presented no serious technical problems. However, the OML output, like with most models, is no better than the quality of the input data, and in this case only two dioxin measurements from the chimney smoke were available. In 2000, when the environmental authorities discovered grossly excessive emissions (180 ng dioxin/m3/hr) with a legal threshold of 1 ng dioxin/m3/hr, the aluminum recycling plant immediately started injecting active carbon and chalk into the smoke-cooling process, hence reducing the content of dioxin to far below thresholds. VOLUME 112 | NUMBER 9 | June 2004 • Environmental Health Perspectives