Schmiedel S, Brüggemeyer H, Philipp J,Wendler J, Merzenich H, Schüz J. An evaluation of exposure metrics in an epidemiologic study on radio and television broadcast transmitters and the risk of childhood leukemia. Bioelectromagnetics Ahead of print November 21, 2008. DOI 10.1002/bem.20460
To properly assess the potential health effects of radiofrequency electromagnetic fields (RF-EMF), accurate exposure measurement is essential. Exposure is measured as RF-EMF signal strength and is determined by distance from the transmitter, source characteristics, propagation path, and signal frequency. RF-EMF signals include those emitted by radio (AM and FM) and television transmitters. Distance from the transmitter is commonly used to assess exposure in epidemiological studies. In a large case-control study from Germany investigating the effects of RF-EMF signals on childhood leukemia (herein referred to as the ‘main study’) RF-EMF exposure was instead calculated using wave-propagation models.
The objective of the study was to validate the exposure method used in the main study by comparing it to exposure values based solely on distance from the transmitter and by calculating the agreement between calculated and measured RF-EMF signal strength.
Exposure values in the main study were determined using retrospective information requested from transmitter operators on the operational states of all transmitters that could have contributed to exposure among cases and controls. Electrical field strength was calculated using modeling software for each study subject. The validation study consisted of measured and calculated RF-EMF signal strength values obtained from an independent survey conducted from 2001-2003 at 850 sites in five regions of Germany. For each measurement site, distance from the main transmitter was also recorded.
The validation study showed that the distance of the measurement site from the main transmitter was directly related to AM signal strength but did not influence FM and total RF-EMF signal strength to the same extent. The agreement between measured and calculated signal strength was better for AM than for FM frequencies. Of sites where calculated total RF-EMF exposures was classified in the top 10%, there was a 76.6% probability that they would also be classified in the top 10% of total RF-EMF exposures using measured values (sensitivity). Of sites where calculated total RF-EMF exposures was classified in the bottom 90%, there was a 97.4% chance that they would also be classified in the bottom 90% of measured total RF-EMF values (specificity). Using distance from the transmitter to measure signal strength had worse values of sensitivity and specificity than did the calculation method.
Implication and Limitations
Calculated total RF-EMF field strength is a better approximation of actual field strength than is distance from the transmitter, especially when FM transmitters or multiple transmitters emitting different frequencies are involved. The differences in exposure classification caused by using calculated versus measured values of field strength likely did not significantly alter the results of the main study. A limitation of this study is that the values of field strength measured in this study may not be representative of longer-term values due to variations caused by weather, ground circumstances, time, and topography. In addition, the validation study covered a large number of rural areas compared to the main study, which may have influenced the quality of exposure assessment.
Wave propagation modeling to calculate RF-EMF exposure is appropriate for large-scale epidemiological studies and is better than exposure values based on distance alone when more than one transmitter is included.