Autores
Bornkessel C, Blettner M, Breckenkamp J, Berg-Beckhoff G. Quality control for exposure assessment in epidemiological studies. Radiat Prot Dosimetry. Mar 21, 2010 Ahead of print.

Introduction and Objectives
Accurate determination of exposure is a challenge in epidemiological studies of possible health effects of exposure to radiofrequency fields (RF) in the vicinity of base stations. Estimates of exposure in epidemiological studies have been based on self-reports, distance between residence and base station, computation models, and measurements. Measurements are the most accurate way of assessing RF exposure. Various measurement methods exist, and one of them is the deployment of dosimeters. At present, dosimeters produced by Maschek and Antenessa are available; both have been used in epidemiological studies. In particular, 20 dosimeters from Antenessa were used in a German study investigating whether RF fields cause health complaints in persons living near mobile phone base stations. Measurement accuracy of these dosimeters has been investigated extensively. The question remains unanswered about stability of dosimeters’ performance during the measurement period while they are exposed to mechanical and electrical stress. Regular calibration by the manufacturer is one way to detect performance changes. However, calibrating 20 devices before, in the middle and after the 6-month study period is very expensive, and, during the calibration, the dosimeters would not be available for the study. To solve this problem, the authors have developed an easy and cost-effective method for dosimeter quality tests.

Methods
The checkups were conducted in a Gigahertz Transverse Electromagnetic (GTEM) cell. The cell generates a defined electromagnetic field and shields from external fields. It can generate fields of several volts per meter with relatively small input powers, which eliminates the need for expensive broadband amplifiers. The signal generator generates signals in 12 frequency bands of the dosimeter in a defined time sequence, and the dosimeter inside the GTEM cell records them. After the test, the dosimeter measurements are read via a computer. The reproducibility of the dosimeter was assessed in several consecutive measurements at unchanged position of the dosimeter and in two different dosimeter positions.  The variations over all 20 dosimeters were also taken into account.

Results
Nineteen of the 20 dosimeters were stable and reproducible. The deviations from their initial state were within 1 dB while the allowed value is ±3 dB. The deviations were smaller (±0.5 dB) in the mobile phone downlink bands. One dosimeter was found to be faulty. It was sent to Antennessa, where the findings were confirmed and the dosimeter was repaired.

Interpretation and Conclusion
This method for dosimeter testing is easy to perform, not as time-consuming and costly as a full calibration by the manufacturer, and at the same time reliable and effective.



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