In neither of these two studies were the associations found to be statistically significant when magnetic fields were measured in the home and used in the analysis. A 1993 Danish study, like the 1988 Denver study, found an association for incidence of all childhood cancers but not specifically leukemia. A Finnish study found an association with central nervous system tumors in boys. Eight studies have examined risk of cancer for adults living near power lines. Of these, two found significant studies involving cancer in people living near power lines.
Scientists call unusual occurrences of cancer in an area or in time a 'cancer cluster'.
In some cases, a cancer cluster has served as an early warning of a health hazard. For most reports of cancer clusters, however, the cause is never determined, or the perceived cluster is not really an unusual occurrence. Concerns have been raised about seemingly high numbers of cancers in some neighborhoods and schools close to electric power facilities. In recent years, three state health departments have studied apparent cancer clusters near electric power facilities. A Connecticut study involved five cases of brain and central nervous system cancers in people living near an electrical substation.