Age of exposure to infections and risk of childhood leukaemia.

Between January and August 1992 in Greece, researchers conducted telephone interviews with parents of children with leukemia (136 cases, most in Attica [Athens and its environs] and the others on the island of Crete) and with parents of children not afflicted with leukemia (187 controls) to determine whether childhood leukemia may be a result of a subclinical infection at an earlier age. They controlled for place of residence when they conducted the multiple logistic regression analyses. They used attendance at a day care facility as a proxy for earlier infection because children come in close contact with each other at day cares, thereby exposing them to many infectious agents. Children who attended a day care had a lower relative risk of developing leukemia than those who did not attend day care (.67), but attendance did not have a significant protective effect. It did appear to have a significant protective effect for children who attended day care during infancy (for at least 3 months during the first 2 years of life), however, (relative risk = .28; p = .03). Significance remained even when the researchers considered different operational definitions of early attendance. Exposure to magnetic fields appeared to have a protective effect also against the development of childhood leukemia, but this effect did not reach significance (p = .07). The relative risk for 100 m from an electricity substation was 0.35. There was a slight, but insignificant increase in the relative risk for children living close to power lines, however (1.19 for 5 m; p = .63). Significant risk factors included young age at diagnosis and mothers with less than high school education.