Extremly low frequency magnetic fields and breast cancer

Abstract: Extremely low frequency magnetic fields (ELF EMF) arise around all electrical leaders and equipment that is ran by electricity. In our modem society basically everyone is therefore to some extent exposed to magnetic fields. The possibility that long-term exposure to relatively weak magnetic fields could increase the risk for breast cancer has been discussed during the past decade and is a concern to many people. The underlying hypothesis for a biological mechanism is based on the assumption that magnetic field exposure affects the melatonin production and that melatonin is protective against breast cancer, possibly by affecting the level of estrogen. The purpose of this thesis was to test the hypothesis that exposure to extremely low frequency magnetic fields increase the risk for breast cancer. The intention was also to describe the exposure to magnetic fields in homes situated in the vicinity of power lines and the exposure in occupations common among women by creating a job-exposure matrix. To do this three epidemiological studies and two measurement studies were performed. The information about how women are exposed to magnetic fields in the occupation has been very limited and exposure assessment in epidemiological studies to a far extent based on men's exposure. In the measurement study of occupational exposure we found several occupations common among women that have not previously been identified as highly exposed. Both measurement studies also showed that the average level of exposure is diluted when consideration is taken to exposure during 24 hours since high exposure during a large part of the 24 hours is rare. All three epidemiological studies that were performed to evaluate the effect of magnetic fields on breast cancer risk were designed as case-control studies. Residential exposure was estimated through calculations of the magnetic fields generated by the power lines. Occupational codes were obtained from the censuses and linked to a job-exposure matrix. The results of the epidemiological studies gave no support for the hypothesis of an increased risk for breast cancer. In the first two studies there was a tendency for elevated risks for young women and particularly for ER-positive breast cancer. However, these studies had several weaknesses; they were based on small numbers of subjects and the job-exposure matrix that was used was based on measurements performed on men in their occupations. In the third epidemiological study the number of subjects was substantially higher and the exposure assessment was improved by the new job-exposure matrix for women. No effects of magnetic field exposure were found in any of the subgroups. If the effect found in the first two studies were real one would expect to find a stronger association in the third study considering the improvement of the exposure assessment. This suggests that the observed risk increase in the first two studies was due to chance. The conclusion of this thesis is that exposure to extremely low frequency magnetic fields does not seem to be a major risk factor for breast cancer.