Alanko T, Hietanen M. A practical method to evaluate radiofrequency exposure of mast workers. Radiat Prot Dosimetry Dec 2 2008 Ahead of print.
People who work on telecommunication transmitter masts are at risk of occupational exposure to radiofrequency (RF) fields. To minimize this risk, it is important to know where on the height of the antenna elevated exposure to RF fields occurs. Two methods are used to measure RF exposure among mast workers. The first is a manually-operated instrument carried by the worker which is difficult to use when climbing up a mast. The second is a recording dosimeter worn by the worker. However the accuracy of this method is easily influenced by body positioning. In both methods, the worker’s position on the mast at the time of measurement is difficult to determine.
To describe a novel method to simultaneously measure the RF exposure and the height of the worker’s position on the mast.
The method was studied using a worker on a radio and TV broadcasting tower in Finland. The worker stayed in the antenna elevator throughout nearly the entire experiment. The personal dosimeter was attached to the front of the climber’s harness. Exposure was expressed as a percent of the exposure guidelines, with a maximum value of 200%. The dosimeter took 32 measurements per second, which were stored in the instrument memory. Two barometric pressure meters, one in the worker’s pocket, the other at the base of the tower, were used to measure the altitude of the worker on the antenna tower. The pressure difference between the two barometers was directly proportional to the height difference. Values recorded by the dosimeter and barometers were uploaded to a computer and were linked to each other with an accuracy of within 1 second.
A graph of time versus altitude, and another of time versus exposure level were produced. Overlapping these two graphs gave a figure of altitude versus exposure. The highest RF exposure values were recorded when the worker was in the elevator at the same height as the radio transmitter antennas.
Implications and Limitations
The elevator cage, which supposedly acts as a shield against RF fields, was insufficient in protecting the worker. True exposure values could not be assessed since they were beyond the dosimeter’s range. Altitude readings using the barometers were easily influenced by changes in atmospheric temperature, wind, and changes in atmospheric pressure, with greater errors in measurement values associated with greater antenna heights.
This measurement method is ideal for use on towers where transmitters are in groups several meters from each other. In this situation, only crude estimates of altitude are required to distinguish which antenna group is contributing to a worker’s exposure.