In a study by Wertheimer and Leeper in 1979, a possible association between residential exposure to extremely low frequency magnetic fields of power supply, especially of power lines, and the occurrence of childhood leukemia was observed for the first time. Since then a lot more epidemiological studies have been conducted. The results of these studies and of pooled analyses showed an increased risk of leukemia for children exposed to magnetic flux density greater than 0.3–0.4 µT.
Based on these epidemiological studies, the International Agency for Research on Cancer (IARC) of the World Health Organization (WHO) classified extremely low frequency magnetic fields into group 2B possibly carcinogenic to humans (IARC Monographs Vol. 80). The evaluation by WHO in 2007 came to the conclusion that the findings of more recent epidemiological, animal and in vitro studies did not change the classification.
Leukemia is a malignant disease of early blood forming cells found in the bone marrow. Leukemia is characterized by an uninhibited proliferation of early white blood cells (leukocytes). According to the cell type, leukemia is divided into lymphoblastic leukemia and myelogenous leukemia which can be both acute or chronic (Kinderkrebsinfo).
The most common leukemia type in children is the acute lymphoblastic leukemia. It is characterized by an uncontrolled proliferation of immature precursor cells of lymphocytes which play a specific role in the immune reaction of the body. Advances in treatment have been achieved in past decades. About 90% of the patients live disease free at least 5 years after diagnosis (Kinderkrebsinfo).
Causes for acute lymphoblastic leukemia are largely unknown. A few known risk factors are inherited disorders (e.g., trisomy 21), ionizing radiation, chemical substances (e.g., benzene) and chemotherapy drugs. According to the present knowledge different factors have to interact to trigger acute lymphoblastic leukemia.
Overall 1,800 children in Germany are diagnosed with cancer before their 15th birthday. The most common cancer type is childhood leukemia accounting for about one-third of childhood cancers (German Childhood Cancer Registry - Report 2013/14).
Childhood leukemia is a relatively rare disease, the incidence is about 49,000 children per year worldwide. In Germany there are approximately 600 new cases of childhood leukemia per year. Accounting for the number of children in Germany, this means that about 1 out of 1,200 children falls ill with leukemia per year (see figure).
The magnetic flux density levels which were evaluated for an increased risk of childhood leukemia were greater than 0.3–0.4 µT (see Why is childhood leukemia investigated?). Residential exposure to magnetic flux densities greater than 0.3 µT is found for about 1–4% of the children in Germany. One-third is contributed to sources outside the home (e.g. power lines) and two-thirds to sources in the home such as household appliances.
If the association between exposure to magnetic fields and childhood leukemia is causal, then about 100 to 2,400 cases per year could be attributed to magnetic fields worldwide. That is to say that in Germany, 6 out of 600 cases of childhood leukemia per year could be attributable to exposure to magnetic fields, two cases due to exposure to outdoor sources and 4 cases due to exposure to indoor sources (Schüz, 2011.) The numbers are illustrated in the figure below.
The association which has been observed in epidemiological studies between residential exposure to extremely low frequency magnetic fields and the incidence of childhood leukemia, is not necessarily causal, it could be by chance, by systematic errors such as selection bias or by a not yet identified confounder (WHO, 2007).
In general, in vitro and animal studies are additionally necessary to epidemiological studies in order to confirm observed possible effects, reveal underlying mechanisms of actions and to test hypotheses. Animal and in vitro studies often provide more reliable results than epidemiological studies, especially in cases of weak and difficult detectable effects due to standardized procedure protocols and larger sample sizes. However, their disadvantages are the difficulties in transferring the results to humans.
So far no mechanisms of action were identified in animal and in vitro studies that can explain the development of leukemia in weak extremely low frequency magnetic fields. There is currently no adequate animal model for acute lymphoblastic leukemia. The results of epidemiological studies on childhood leukemia and exposure to extremely low frequency magnetic fields could not be verified by animal-experimental studies (WHO, 2007).
Several authors have comprehensively summarized and evaluated the available publications on exposure to extremely low frequency magnetic fields and childhood leukemia in following reviews and meta-analyses:
WHO has recommended the following topics with high priority: the conduction of new pooled analysis including recent epidemiological studies on childhood leukemia and extremely low frequency magnetic fields, the development of transgenic rodent models of childhood leukemia and the evaluation of possible co-carcinogenic effects using in vitro and animal studies (WHO Research Agenda, 2007).
Current research is going on for example in the European project ARIMMORA (Advanced Research on Interaction Mechanisms of electroMagnetic exposures with Organisms for Risk Assessment), funded by the European Commission within the 7th Framework Program. Aim of the project is to scrutinize the underlying biophysical mechanisms and to clarify a possible causal relationship between exposure to extremely low frequency magnetic fields and cancer, especially childhood leukemia by applying novel experimental and computational techniques as well as advanced in vitro and animal studies under well-defined exposure conditions. Research groups from Switzerland, Israel, Germany, France, Italy and Spain are participating in this project (begin in October 2011).