Champs électromagnétiques

2. What are the sources of exposure to radio frequency (RF) 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 waves used in medicine?

The source document for this Digest states:

3.3.1 Sources and distribution of exposure in the population

Nowadays 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. It has to be taken into account that information on the exposure of individual persons is scarce; such information is mainly needed for epidemiological studies, there is therefore a need to optimize methodology to assess individual exposure, e.g. by using and further developing existing dosimeters. The existing RF sources are operated in different frequency bands and can be subdivided in several categories.

Sources operated close to the human body

Many devices of this kind are mobile RF transmitters. One of the examples is mobile phones; more than 2 billion people are using mobile phones worldwide. The most common mobile communication technologies in Europe are the digital technologies GSM 900, GSM 1800 and UMTS, analogue technologies are nowadays almost not in use any longer in Europe. Mobile phone use is common in Europe and the proportion of users can reach values of 80 % or more. Before mobile phones can be brought 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.

Source & ©: SCENIHR  Possible Effects of Electromagnetic Fields (EMF) on Human Health (2007)
Section 3.3 Radio Frequency Fields, 3.3.1 Sources and distribution of exposure in the population, p.13-14

2.1 How high is the exposure from mobile phones and wireless devices?

The source document for this Digest 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, e.g., 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 gram of tissue range typically between 0.2 and 1.5 W/kg, depending on the type of mobile phone. It has to be taken into account that 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 for GSM and UMTS phones. The power control of a GSM phone automatically reduces the emitted power by up to a factor of 1,000 for GSM and about 100.000.000 for UMTS if the intensity is not needed for stable transmission. No exposure occurs from a mobile phone being switched off. Phones operated 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.

In addition to mobile phones, other wireless applications like cordless phones, e.g. DECT, or WLAN [Wireless Local Area network] systems are very common. Due to the fact that they are usually operated with lower output power compared to mobile phones 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), for a DECT handset 10mW. The peak value of a WLAN terminal is 200 mW, however the averaged power depends on the traffic and is usually considerable lower. The exposure from such systems is therefore typically 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 superior compared to the exposure from GSM or UMTS mobile phones. For example, close to a WLAN system exposure is typically below 0.5 mW/m². Anti-theft devices have become more and more common during recent years. They are typically operated at the exits of shops or similar areas to prevent theft of goods. Some of the existing systems are operated in the RF range; the exposure depends on the type of system and is, as long as the systems are operated according to the manufacturer’s requirements, below the exposure limits. Several industrial appliances are operated in the RF and microwave range, for example for heating (e.g. RF sealers) or maintenance of broadcasting stations. The exposure of the worker operating such systems can reach values close or even above the limits of the Directive 2004/40/EC.

Source & ©: SCENIHR  Possible Effects of Electromagnetic Fields (EMF) on Human Health (2007)
Section 3.3 Radio Frequency Fields, 3.3.1 Sources and distribution of exposure in the population, p.14

2.2 What is the level of exposure from mobile phone base stations and radio towers?

The source document for this Digest states:

Sources operated far away from the human body

Such sources are typically fixed installed RF transmitters. An example is base stations that are an essential part of mobile communication networks necessary to establish the link between the mobile telephone and the rest of the network. In most European countries, base stations have became ubiquitous to guarantee connectivity in large areas of the respective countries; e.g., 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 given in the European Council Recommendation 1999/519/EC is 4.5 W/m². This reference level is given as an example, it has to be taken into account that the reference levels are frequency dependent and that other limits have to be applied at 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 slightly 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. Close to the fences of very powerful transmitters, exposure of about 300 mW/m² can be expected in some cases. Looking at the new digital TV technology (DVB-T), exposures between around 40 mW/m² and 0.003 mW/m² were registered in an Austrian study. 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. In some countries digital audio broadcasting systems are already in operation. 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 WiMAX.

Source & ©: SCENIHR  Possible Effects of Electromagnetic Fields (EMF) on Human Health (2007)
Section 3.3 Radio Frequency Fields, 3.3.1 Sources and distribution of exposure in the population, p.14-15

2.3 How are radio waves used in medicine?

The source document for this Digest states:

Medical applications

Several medical applications use electromagnetic fields in the RF range. Therapeutic applications such as soft tissue healing appliances, hyperthermia for cancer treatment, or diathermy expose the patient well above the recommended limit values to achieve the intended biological effects. These include heating of tissue (analgetic applications) or burning cells (to kill cancer cells). In 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. Diagnostic applications, like magnetic resonance imaging (MRI), are allowed to exceed the basic restrictions of Council Recommendation 1999/519/EC as there is a benefit for the patient. Usual frequencies are those allowed for industrial, scientific, and medical applications similar to most industrial sources: 27 MHz, 433 MHz and 2.45 GHz. Magnetic resonance imaging devices in medical diagnostics use RF fields in addition to static and variable fields. Most actual clinical MRI devices work at 63 MHz.

Source & ©: SCENIHR  Possible Effects of Electromagnetic Fields (EMF) on Human Health (2007)
Section 3.3 Radio Frequency Fields, 3.3.1 Sources and distribution of exposure in the population, p.15