2. What are the sources of exposure to radio frequency fields?
- 2.1 How high is the exposure from mobile phones and wireless devices?
- 2.2 What is the level of exposure from mobile phone base stations and radio towers?
- 2.3 How are radio frequency fields used in medicine?
The SCENIHR opinion states:
3.3. Radio Frequency Fields (RF fields)
3.3.1. Sources and distribution of exposure in the population
The use of RF sources is widespread in our society. Prominent examples are mobile communication, broadcasting or medical and industrial applications. Information on emissions arising from RF sources is often available and can be used for compliance assessment or similar applications such as in-situ measurements. However, information on the exposure of individual persons is still scarce. Such information is mainly needed for epidemiological studies, and there is a need to optimise methodology to assess individual exposure, e.g. by using and further developing existing exposimeters. It is furthermore important to consider multi-source exposure and not to focus on single sources, e.g. mobile phone base stations. The fact that there is a continuous change of technologies, e.g. from analogue to digital TV, or appearance of new technologies like ultra-wide band (UWB) on the market, leads to changing exposure patterns of the population on a long term scale. This requires identification of exposure patterns at adequate intervals. The existing RF sources are operated in different frequency bands and can be subdivided into several categories:
Sources operated close to the human body
Many devices of this kind are mobile RF transmitters such as mobile phones. Worldwide, there are more than 2 billion people using mobile phones worldwide, a number which has been continuously increasing during recent years. The most common mobile communication technologies in Europe are the digital technologies GSM 900, GSM 1800 and UMTS. Analogue technologies are hardly used any more in Europe.
Before mobile phones can be introduced into the European market they have to show compliance with the requirements of European directives, i.e., it has to be shown that the limits for the amount of power absorbed in the human body are not exceeded. Standardized methods specified by the European Committee for Electrical Standardisation (CENELEC) are used to test mobile phones in Europe.
2.1 How high is the exposure from mobile phones and wireless devices?
The SCENIHR opinion states:
The limit for mobile phone use is the specific absorption rate (SAR) of 2 W/kg for the human head. Mobile phones are tested under worst case conditions, i.e. at the highest power level. As an example, a 2 W peak power corresponding to 250 mW maximum time averaged transmitted power for GSM at 900 MHz. Maximum local SAR values averaged over 10 g of tissue range typically between 0.2 and 1.5 W/kg, depending on the type of mobile phone. The emitted power is often orders of magnitude lower than the maximum power leading to much lower exposure due to power control and discontinuous transmission mode (output power is different when the user is talking or listening) for GSM and UMTS phones. The power control of a mobile phone automatically reduces the emitted power by up to a factor of 1,000 for GSM and about 100,000,000 for UMTS if higher intensity is not needed for stable transmission. The exposure arising from a UMTS mobile phone can typically be expected to be lower than the one from a GSM phone. The actual transmitted power depends in both cases on several factors, e.g. the cell size of the respective base station and the type of the mobile phone. A comparison showed that the exposure due to a UMTS phone was about 1,000 times lower compared to the exposure due to a GSM phone (Baumann et al. 2006). No exposure occurs from a mobile phone being switched off. Phones in the standby mode cause typically much lower exposure compared to mobile phones operated with maximum power, but an accurate figure for this lower exposure depends on the exact details of the transmission path to base stations and on the traffic requested by the communication protocol and by incoming/outgoing SMS and the position of the phone.
In addition to mobile phones, other wireless applications like cordless phones, e.g. DECT or WLAN systems are very common. They are usually operated with lower output power compared to mobile phones and the exposure is typically below the level of mobile phones. The maximum time averaged power level of a DECT base station is 250 mW (worst case for a professional application handling communication with 25 handsets in parallel, a typical household application communicating with one handset has a time averaged power of 10 mW), and for a DECT handset 10 mW. The peak value of a WLAN terminal is 200 mW; however the averaged power depends on the traffic and is usually considerably lower. Therefore, the exposure from such systems is usually below that of mobile phones. However, under certain circumstances, e.g. closeness to WLAN access points, exposure due to WLAN or DECT systems can become higher compared to exposure from GSM or UMTS mobile phones.
Another system starting to be used in Europe is UWB, where exposure can be expected to be well below 0.1 mW/m2. Applications include communication as well as detection and identification. Wireless microphone systems are already available and a number of office, home entertainment, and medical applications are likely to enter the market in the near future. Ground and wall penetrating systems can be used to detect buried persons, and UWB radar systems can be used to avoid traffic collisions and to transmit traffic data (Schmid et al. 2008; article in German).
Anti-theft devices that are typically operated at the exits of shops or similar areas have become more and more common during recent years. Some of the existing systems are operating in the RF range. The exposure depends on the type of system and is below the exposure limits. Finally, several industrial appliances are operated in the RF and microwave range, for heating (e.g. RF sealers) or maintenance of broadcasting stations. The exposure of workers operating such systems can reach values close to or even above the limits of the Directive 2004/40/EC.
2.2 What is the level of exposure from mobile phone base stations and radio towers?
The SCENIHR opinion states:
Sources operated far away from the human body
Such sources are typically fixed installed RF transmitters like mobile phone base stations and broadcast transmitters. In most European countries, base stations have become ubiquitous to guarantee connectivity in large areas; e.g. around 18,000 base stations are operated in Austria. The so called reference level for the exposure of the general population at 900 MHz (an important frequency for mobile communication) set in the European Council Recommendation 1999/519/EC is 4.5 W/m2. The reference levels are frequency dependent and other limits have to be applied for other frequencies. The range of exposure of the general population due to GSM signals is typically between some hundred nW/m2 and some tens of mW/m2. The reasons for this large variation are both technical and environmental factors including distance. For UMTS, the available measurements are limited and so far the traffic is rather low compared to GSM. Values just over 1 mW/m2 have been measured in a few cases, while minimum levels are a few hundred nW/m2. Other important RF sources are broadcasting systems (AM and FM). The maximum values measured in areas accessible for the public are typically below 10 mW/m2. Exposure levels of ca 300 mW/m2 have been noted close to the fences of very powerful transmitters. Regarding the new digital TV technology (DVB-T), exposures between 0.003 and 40 mW/m2 were registered in an Austrian study (Giczi 2004; article in German). The range of exposure is similar compared to analogue TV systems. However, the digital systems require more transmitters than the older analogue systems; therefore, somewhat higher average exposure levels can be expected. Other examples of sources relevant for far field exposure of the general population are civil and military radar systems, private mobile radio systems, or new technologies like digital audio broadcasting systems and WiMAX.
2.3 How are radio frequency fields used in medicine?
The SCENIHR opinion states:
The usual frequencies that are allowed for industrial, scientific, and medical applications are similar to most industrial sources: 27 MHz, 433 MHz and 2.45 GHz. Several medical applications use electromagnetic fields in the RF range. Therapeutic applications include soft tissue healing appliances, hyperthermia for cancer treatment, and diathermy. These expose the patient to field strengths well above the recommended limit values to achieve the beneficial intended biological effects, which include tissue heating (analgetic applications) or burning cells (to kill cancer cells). MRI devices commonly use 63 MHz RF fields in addition to static and time-variable gradient fields. In all these cases, exposure of therapists or other medical personnel needs to be controlled to avoid that their exposure exceeds the exposure limit values foreseen by Directive 2004/40/EC for occupational exposure.