Directorate-General for Health and Food Safety
2. What are the sources of exposure to radio frequency fields?
- 2.1 How RF EMFs interact with the body?
- 2.2 How high is the exposure from mobile phones and wireless devices?
- 2.3 What is the level of exposure from mobile phone base stations and radio towers?
- 2.4 How are radio frequency (RF) fields used in medicine?
Local wireless computer networks generate radio fields
Credit: Ramzi Mashisho
Devices generating electromagnetic fields in the radio frequency (RF) range (from 100 kHz to 300 GHz) are in widespread use in our society. Key sources of RF fields include mobile phones, cordless phones, local wireless networks and broadcasting transmission antennas. They are also used in medical diagnosis and therapy, by radar systems and microwave ovens.
Information about the strength of radio frequency fields generated by a given source is readily available and useful in determining compliance with safety limits. Assessing everyday exposure of individuals to radio frequency fields is much more difficult, however, such data are crucial for epidemiologic studies of potential EMF health effects. Knowledge could be increased by improved methods such as using personal exposimeters, devices carried by individuals to measure their exposure to electromagnetic fields over time. Exposure assessment should not be restricted to single sources only, like mobile phone base stations, but should consider multi-source exposure.
The fact that there is a continuous change of technologies, e.g. from analogue to digital broadcasting, and an emergence of new solutions like ultra-wide band (UWB) technologies on the market, leads to changing exposure patterns of the population on a long-term scale. Sources of RF EMF operate in different frequency bands. The strength of electromagnetic fields falls rapidly with distance. Over time, a person may absorb more RF energy from a device near the body than from a powerful source that is farther away. Cordless phones, local wireless networks and anti-theft devices are sources for small distance communications. Long-range sources include radio transmission towers and mobile phone base stations.
In 2014 the International Telecommunication Union estimated that there are about 7 billion mobile phones in use worldwide. Most mobile communication in Europe uses either GSM or UMTS technology. The European Union has set safety limits on the energy absorbed by the body from exposure to a mobile phone. Mobile phones sold in Europe must undergo standardised tests to demonstrate compliance in accordance with the Specifications of the European Committee for Electrotechnical Standardization (CENELEC). Typical frequencies for devices generating radio frequency fields.
|Frequency range||Frequencies||Some examples of exposure sources|
|Static||0 Hz||video display units; MRI (medical imaging) and other diagnostic or scientific instrumentation; industrial electrolysis; welding devices|
|ELF [Extremely Low Frequencies]||0-300 Hz||power lines; domestic distribution lines; domestic appliances; electric engines in cars, trains and tramways; welding devices|
|IF [Intermediate Frequencies]||300 Hz - 100 kHz||video display units; anti-theft devices in shops; hands-free access control systems, card readers and metal detectors; MRI; welding devices|
|RF [Radio Frequencies]||100 kHz - 300 GHz||mobile and cordless telephones; broadcasting and TV; microwave ovens; radar and radio transceivers; portable radios; MRI|
|THz technologies||300 GHz – 20 THz||applications are still in development, but currently mostly telecommunication applications and body scanners are considered.|
2.1 How RF EMFs interact with the body?
The RF EMF interactions mechanisms are well established. Overall, it is energy absorption which is based on EMF’s mechanical forces’ that accelerate molecules (induce kinetic energy) which then collide with each other, subsequently causing tissue heating. Even if the basic physical interaction is non-thermal, the biochemical and physiological responses depend on temperature. These established mechanisms allow extrapolation of scientific results to the entire frequency range and wide-band health risk assessment. They have been used to help limit exposure to EMF and provide the same degree of protection over the entire frequency range.
A number of studies have suggested other hypothetical mechanisms, however, none has been firmly verified as operating in the human body at a level of exposure below existing limits.
2.2 How high is the exposure from mobile phones and wireless devices?
When exposed to radio frequency fields, the body absorbs energy over time. The rate at which energy is absorbed is named the Specific Absorption Rate (SAR). It varies throughout the body. The 1999/519/EC European Council Recommendation) defines the basic restrictions and reference levels for limiting EMF exposure of the general public, setting maximum SAR values which should not to be exceeded. Since many of the physical quantities used for setting the basic limits cannot be readily measured, reference levels are provided for practical exposure-assessment purposes to determine whether the basic restrictions are likely to be exceeded.
For mobile phone handsets, the exposure is largely confined to part of the head closest to the phone’s antenna. The Council Recommendation sets a radio frequency safety limit for a local Specific Absorption Rate (SAR) of 2 W (2000 mW) per kilogram, averaged over any 10g body tissue in human’s head and trunk.
Mobile phones are tested assuming worst-case conditions namely with mobile phones operating at maximum power. In practice, depending on the transmission quality, the power transmitted during a mobile phone conversation is generally much lower, frequently several orders of magnitudes lower than the device’s maximum power output. This is because the “power control” feature of a mobile phone continuously reduces the emitted power to the minimum needed for stable transmission. Moreover, output power depends on whether the user is talking or listening because transmission is considerably reduced during the listening period where no information is needed to be transferred (discontinued transmission mode). When a phone is in standby mode, the exposure is typically two orders of magnitudes lower than during conversation. No exposure occurs when a mobile phone is switched off.
GSM phones transmitting at 900 MHz, a frequency allocated to mobile communication, have a maximum time-averaged output power of 250 mW. The power is averaged as GSM phones transmit radio signals in short repetitive bursts of information rather than continuously.
Mobile phones do not make use of the entire permissible exposure range. Depending on the models the test-SAR values might be range between 10 and 80% of the limit, hence based on the labelled information allows consumers making informed decisions.
Wireless devices intended for indoor communication, like cordless phones and wireless networks (WLAN), also generate radio waves but with less output power than mobile phones. A cordless phone handset used by a typical household generates about 10 mW of time-averaged power. Cordless phone base stations are usually no more than a few tens of meters from the handsets and, there is also the field from the cordless phone base station to consider. Their maximum time-averaged power level is the same as for a mobile phone handset. But in contrast to mobile phone handsets the cordless phone base station is distant from the body, and hence because the field strength falls rapidly with distance, exposure is reduced by orders of magnitudes.
The terminal of a wireless computer network (Wireless Local Area Network, WLAN) has a peak power of 100 mW, but since the time-averaged power depends on data traffic the actual power is usually considerably lower. Even near a wireless network station used in homes and offices, the field intensity is typically below 0.5 mW/m2. Another system that is starting to be used in Europe is based on ultra-wide band (UWB) signals. The frequency range is centred around 500 MHz, applications are wireless microphones, health care applications and traffic control systems. With such systems, field levels are expected to be well below 0.1 mW/m2.
Some anti-theft devices expose people to electromagnetic fields of radio and intermediate frequency. Increasingly used, the devices are located at store exits to detect shoplifters. The radio frequency exposure varies depending on the type but is below safety limits. Radio frequency fields are also used in industry such as for inductive metal heating.
The discussion about exposure of workers to EFM is outside the scope of this summary.
2.3 What is the level of exposure from mobile phone base stations and radio towers?
In contrast to broadcasting transmission towers, which are designed for one-way communication, mobile phone base stations must allow two-way communication. Therefore they necessarily form a network to link the individual mobile phones with each other across the country. Consequently, in European countries, base stations are now almost ever-present, ensuring mobile communication over large areas.
At 900 MHz, an important frequency for mobile communication and for GSM mobile phone networks, the EU recommends that people should not be exposed to a field stronger than 4.5 W/m2 (power density). National measurement campaigns report that despite the increasing number of base stations and the deployment of additional mobile telecommunication technologies, the environmental electromagnetic radiation levels have essentially remained almost the same. The emitted power from indoor devices such as WiFi hotspots and DECT devices, even combined, still results in a very low exposure compared with reference levels of European and international guidelines.
For the newer UMTS networks, the use of Adaptive Power Control (APC), with which mobile phones reduce their output powers to allow for good signal quality, gives longer life to their batteries. The network continually monitors signal quality and may reduce the emitted power of a mobile phone by up to three orders of magnitudes for GSM and about nine orders of magnitudes for UMTS. Measurements of the exposure of the general population are limited as use of these mobile phones is low compared with GSMs. Where exposure has been measured, it was found to be at most a thousandth of a W/m2 and usually much less (SCENIHR, 2009).
The problem with exposure measurements is that, typically, these only encompasses either a short-term measurement of a maximum of 48 hours with personal monitoring, or a spot measurement providing only a snapshot of instantaneous exposure at a single location.
Furthermore, for epidemiologic studies on health risks from EMF, given the lack of clearly established biological or biophysical mechanism of action, several alternative measures of exposure are evaluated (for instance field strength, exposure frequency, cumulative exposure, time since first exposure etc.). The relevant time period for which exposure data would be needed is a period of perhaps several years.
Other important sources of radio waves are radio broadcasting systems (AM and FM). The maximum values measured in areas accessible to the public are typically below 0.01 W/m2. Close to the fence of very powerful transmitters, exposure of about 0.3 W/m2 were reported in some cases.
As for the new digital TV broadcasting technology (DVB-T), the highest mean exposure was registered in the FM frequency band in office environments and was 0.096 mW/m2. This is similar to the power densities of the older analogue TV broadcasting systems, but as digital systems require a denser network of, however, less powerful transmitters, higher exposure levels can be expected in some regions while there may be a reduction in others.
Other sources of long-range exposure to radio frequency fields are civil and military radar systems, private mobile radio systems, or new technologies like digital audio broadcasting systems and WiMAX.
Smart Meters are used to monitor energy consumption remotely and transmitting data to utility companies. There are a number of different types used, and one study concluded that smart meters “would make only minor contributions to the total background RF radiation level inside a home, which is in any event tiny in comparison to existing safety limits”.
2.4 How are radio frequency (RF) fields used in medicine?
Electromagnetic fields in the RF range are used in medicine for diathermy to heat body tissue, which can ease pain or, at higher temperatures, kill cancer cells. As the aim is a biological effect, the patient’s exposure to radio frequency fields is well above the recommended limits for the general public. Care must be taken to avoid exposure of medical staff exceeding limits for workers.
Another common application of RF fields in medicine is Magnetic Resonance Imaging (MRI), which in addition also uses very strong static magnetic fields (see question 8). MRI provides high-resolution cross-sectional images of the body including the head without shadowing by bony structures.