Twenty occupational settings and amateur radio operators were considered as potentially exposed to RF-EMF (table ). Exposure levels of all potential cohorts were rated by experts, e.g. occupational hygienists. Most of the experts' ratings were based on results of measurements, performed in different industries, mainly done in the frame of preventive actions to avoid work-related health hazards. At least two experts were asked for each of the occupational settings.
Occupational cohorts initially defined as exposed*
Eighteen of the twenty-one situations described in table were not further considered for a cohort study for one or more of the following reasons:
1. Exposure to RF-EMF rare or at very low level: e.g. for captains and boat personnel and persons working in a lock, exposure to RF emitted by radio communication does not play a major role. An exposure to microwave emitted by radar is probably very low if present at all, as the position of radar equipment is on the roof of the ship and rather far away from the working position.
2. Small number of exposed subjects: e.g. firms where gluing presses are used. Generally only very few persons are highly exposed in these firms. Assembling a cohort seems to be impracticable.
3. Automated and shielded working processes: e. g. blister packaging, is an automated and shielded process. Maintenance of the devices only takes place when the machine is switched off (see table ).
Comments and conclusions for exclusion are presented in table . Only three groups of persons were further considered after these considerations: (1) Personnel of medium/short wave broadcasting stations, (2) amateur radio operators, and (3) workers on dielectric heat sealers.
Selection of potential study population for an occupational RF-EMF cohort and the explanation for not being considered in the ongoing feasibility study
Personnel of medium/short wave broadcasting stations
At broadcasting stations about 200 to 250 employees (technicians) are potentially exposed to radiofrequencies from the antennas (medium wave: 526.5 kHz – 1.6065 MHz, bandwidth 9 kHz, or short wave: 3.4 MHz – 26.0 MHz, bandwidth 5 kHz). Personnel are employed only on stations with a transmitting power of ≥ 100 kW. In Germany 20 of 29 broadcasting stations operate at this power level. The employees in these workplaces (e.g. mechanical workshop) are continuously exposed and the duration of the exposure corresponds to the total working time.
The exact determination of the current individual exposure can be obtained by measurements. In addition, it is possible to obtain an estimation of past exposure from computer simulations basing on current exposure. This is important because the antennas were previously operated partly with a higher transmitting power and other modulation procedures. The operating conditions (transmitting power and field-strength) of the amplitude modulated medium wave antennas can be traced back quite well over approximately the last 20 years.
The strengths of this cohort would be the readiness for co-operation and support of the project, the measurable and/or valid estimation of exposure, the almost daily exposure over a long time of the working life, the relatively high constancy of the cohort and the good accessibility. Furthermore personnel data are retrospectively available for a period of at least 10 years.
The limitations of the study design include the rather low levels of exposure (mechanical workshop, Mühlacker broadcasting station, electrical field: 1.5 V/m, magnetic field: 0.2 A/m), the small cohort size (maximum 250 potentially exposed persons in Germany) and the fact that such a cohort consists exclusively of persons in technical occupations (highly selective group).
Amateur radio operators
Altogether, 80,000 amateur radio operators are registered in Germany. It is estimated that only two-thirds of them are active.
Radio equipment with a transmitting power of > 10 W is notifiable. The permissible frequencies for amateur radios lie between 2 MHz and about 300 GHz. However, less than 5% of amateur radio operators transmit in the frequency range 900 – 2.200 MHz and above. This is because most amateur radio operators do not possess the necessary technical equipment to transmit within this frequency range.
The average exposure time of an amateur radio operator will rarely exceed the value of 10 h/week, with large individual variation. A high exposure to RF-EMF arises as a result of the adjustment of the antenna or other work on the radio transmitters and radio traffic with an antenna which is installed in the house. Measured exposure values are not available. The whole body exposure, amongst others from antennas installed within the house, can however be comparably high.
A cohort with a large size can easily be ascertained and, due to the structure of the organization of amateur radio operators, is also retrospectively available for many years. Demographic data of the members are present, also for the past. Long-term exposure is common and varies widely among the individual members. The fluctuation of the membership is small due to the high expenditure involved (examination, costs for radio equipment).
Amateur radio operators are most likely not exposed to RF-EMF on a daily basis. This cohort was comprised of quite a specific study population (technicians and handicapped persons) which can not be easily compared with the general population.
Workers on dielectric heat sealers
High frequency dielectric heat sealers are mainly used for welding of plastic products, operate with the industrial frequency of 27.12 MHz and have been used in Germany since the 1960's. At dielectric heat sealers, the workers are mainly occupied with the introduction and removal of the product to be welded. Depending upon the shielding of the electrodes, exposure to different levels of RF-EMF can occur.
Measurements done at the request of the Lower Saxony Ministry of Social Affairs (Niedersächsisches Sozialministerium) and the Regional Office for Ecology (Niedersächsisches Landesamt für Ökologie) in 1996 showed that the majority of plants exceeded the licit exposure range. Discussions held with the Lower Saxony Regional Office for Ecology and the Trade Association for Precision Mechanics and Electro-Technology also confirmed that workers on dielectric heat sealers are highly exposed to RF-EMF.
The availability of information about the number of exposed workers per company was scarce. Interviewed experts of different professional associations and the Lower Saxony Regional Office for Ecology stated that in general these workers are employed for a long time. They also believed that protective clothing was rarely used. The workers are employed in small and medium sized companies.
The disadvantage of this cohort is the small number of exposed workers per company, as this would necessitate contacting a large number of companies. It was also unclear whether appropriate measurement of exposure could be obtained in all these firms. Additionally, several other occupational exposures (plastic vapours, low frequency fields, and noise) can arise. It was also not possible to obtain an estimation of the number of exposed employees due to an incomplete view of the company structures.
Measurements of the Lower Saxony Social Department and of the Trade Association of the Chemical Industry showed that workers on dielectric heat sealers are exposed to considerably higher levels of RF-EMF than the general population. The fact that workers are continuously employed at the same company for long periods of time and that they work on a regular daily basis on these machines support the classification of the long exposure duration as suitable (see additional file 1
The exposure of the cohort of workers on dielectric heat sealers is high; however, ascertainment of the cohort is rather difficult. The strength of the cohort of amateur radio operators is the large number of persons; however, exposure assessment seems to be difficult. The strength of the cohort of technicians is the quality of retrospective exposure assessment, but epidemiological research is hampered by the small number of persons working in this profession.