Extremely low frequency magnetic fields in residences in Germany. Distribution of measurements, comparison of two methods for assessing exposure, andpredictors for the occurrence of magnetic fields above background level

We examined the results of 1,835 magnetic field measurements in German resi dences conducted between November 1997 and September 1999. The measurements were part of an epidemiological study on the relationship between magnetic fields and childhood leukemia. We performed a fixed-location measurement o f the magnetic field at 50 Hz and 16 2/3 Hz (frequency of the German railwa y system) over 24 h in the child's bedroom in the residence of each study p articipant. In addition, we conducted a second 24 h-measurement in the livi ng room at 50 Hz, and spot measurements while walking through all rooms of the respective dwelling. Median 50 Hz magnetic fields above 0.2 muT were fo und to be infrequent in Germany (only 1.4% of all residences). Fields produ ced by high-voltage power lines (123-420 kV) were lower than expected: the median magnetic field was above 0.2 muT in only 8 (32.0%) of 25 residences located 50 m or closer to a high-voltage power line indicating that power l ines in Germany are usually run well below the maximum power load. We found that magnetic fields were correlated with the type of residence and higher magnetic fields were measured in apartment buildings. There was also some evidence for a positive correlation between magnetic fields and traffic den sity and an inverse association between magnetic fields and family net inco me. The 24 h-magnetic field measurements correlated well with the spot meas urements (r>0.7). However, when dichotomized with a cut-off point of 0.2 mu T, there was only a poor agreement between the two measurement methods. A l oss of the strength of the association after categorization was also observ ed when comparing the arithmetic mean and median of the same 24 h-measureme nt. In summary, these analyses give a valuable overview of magnetic field d istributions in German residences.