Authors
Frei P, Mohler E, Neubauer G, Theis G, Bürgi A, Fröhlich J, Braun-Fahrländer C, Bolte J, Egger M, Röösli M. Temporal and spatial variability of personal exposure to radio frequency electromagnetic fields. Environ Res. May 22, 2009. Ahead of print.

Background
Over the last 20 years, increased exposure to radio frequency electromagnetic fields (RF-EMF), especially related to mobile communication, has led to concerns regarding possible adverse health effects of this exposure.  In principle, there are two types of exposure sources: those applied close to the body usually causing high and periodic short-term exposures mainly to the head (e.g. mobile phones) and environmental sources which cause lower but relatively continuous whole-body exposures (e.g. mobile phone base stations).  Current methods for assessing exposure to environmental RF-EMF, such as measuring lateral distance between the home and mobile phone base stations, and spot measurements (usually in bedrooms) as a proxy for habitual exposure, are inadequate.  Furthermore, previous research usually only considered one source of RF-EMF.  Newly developed exposure meters (exposimeters) are useful to measure personal exposure to environmental RF-EMF in everyday life.

Objective
The objective of the study was to investigate the levels, sources, and the variability of exposure to environmental RF-EMF in a group of adult volunteers living in a Swiss city using personal exposimeters.

Methods
Participants were at least 18 years of age and residents of Basel, Switzerland and surrounding areas.  Between April 2007 and February 2008, 6 study participants were recruited each week, for a total of 166 participants.  Four different recruitment strategies were employed to maximize the range of exposure levels.  These strategies included recruiting subjects who resided close to mobile phone base stations or broadcast transmitters.  The remaining 129 participants were self-selected volunteers.  The exposimeter was programmed to take measurements every 90s during 1 week.  Participants were asked to wear the personal exposimeter on a belt or backpack when moving.  When stationary, they were asked to place the exposimeter in the vicinity but not exactly at the same place during the whole week.  Participants were also asked to document all locations at which they stayed for at least 10 min in a diary and to record all calls they made or received using a mobile phone or digital enhanced cordless telecommunications (DECT) phone.

Results
Measurements made when the participants were using a mobile phone handset were excluded from calculations of total weekly exposure.  The mean exposure to all measured RF-EMF sources over the whole week was 0.13 mW/m2 (range 0.014-0.881 mW/m2).  Exposure was mainly caused by mobile phone base stations (32.0%), mobile phone handsets (29.1%), and DECT cordless phones (22.7%).  Mean exposure was 0.21 mW/m2 among the 27 persons who were invited because they lived close to a mobile phone base station; 0.24 mW/m2 among the 8 participants invited because they lived close to a broadcast transmitter; and 0.11 mW/m2 in the remaining participants.  Mean total exposure was higher among those owning a mobile phone handset (0.14mW/m2) than among those not owning a handset (0.10 mW/m2).  Exposure on weekdays was higher during the daytime (mean 0.16 mW/m2) than at nighttime (mean 0.08 mW/m2).  The highest mean values were recorded for train journeys, stays at the airport, and rides on the tramway or bus.  Exposures at these locations were mainly due to mobile phone handsets.

Interpretation and Limitations
To the authors’ knowledge, this is the first study to assess RF-EMF exposure by comparing personal measurements with diary data, and it shows that this approach is feasible.  Exposimeters provide objective measurements, avoid recall bias, and allow the measurement of exposure to several RF-EMF sources separately.  Exposure measurements made while the participant was wearing the exposimeter may underestimate true exposure due to shielding effects of the body.

Conclusion
It is feasible to combine diary data with personal RF-EMF exposure measurements.  The type of information collected from personal exposimeters is helpful to reducing exposure misclassification in future epidemiological studies.


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