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?
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 radio transmission towers. They are also used by medical scanners, 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. But little is known about the exposure of individuals to radio frequency fields, data that are crucial for studies of health effects. Knowledge could be increased through better use of methods such as exposimeters, devices carried by individuals to measure their exposure to electromagnetic energy over time. 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, and an emergence of new technologies like ultra-wide band (UWB) on the market, leads to changing exposure patterns of the population on a long term scale. Sources of radio waves operate in different frequency bands, and the strength of the electromagnetic field falls rapidly with distance. Over time, a person may absorb more RF energy from a device that emits radio signals near the body than from a powerful source that is farther away. Mobile phones, cordless phones, local wireless networks and anti-theft devices are all sources used in close quarters. Long-range sources include radio transmission towers and mobile phone base stations.
More than 2.5 billion people use mobile phones 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 Electrical Standardization (CENELEC).
2.1 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 known as the Specific Absorption Rate (SAR), and it varies throughout the body.
For handheld mobile phones, the exposure is largely confined to part of the head closest to the phone’s antenna. The European Union has set a radio frequency safety limit for the human head at a Specific Absorption Rate (SAR) of 2 W (2000 mW) per kilogram of tissue.
Mobile phones are tested assuming worst-case conditions: the rate at which energy is transmitted by a mobile phone operating at maximum power. In practice, the power transmitted during a mobile phone conversation is generally hundreds or thousands of times lower than the maximum power assumed. Indeed, the “power control” feature of a mobile phone automatically reduces the emitted power if higher intensity is not needed for stable transmission. Moreover, output power depends on whether the user is talking or listening (discontinued transmission mode). No exposure occurs when a mobile phone is switched off. When a phone is in standby mode, the exposure is typically much lower than during operation at maximum power.
GSM phones transmitting at 900 MHz, an important frequency for mobile communication, have a maximum time-averaged power of 250 mW. In accordance with European regulations, the power is averaged over six minutes as GSM phones transmit radio signals in bursts of information rather than continuously.
On average, during a six-minute conversation under worst-case conditions – a mobile phone held to the head and operating at maximum power – the 10 grams of body tissue that absorb most energy would typically absorb between 200 and 1500 mW per kilogram depending on the type of phone.
Other wireless devices used in close quarters, like UMTS mobile phones, cordless phones and wireless networks, also generate radio waves but exposure from these sources is usually lower than from GSM phones.
One DECT cordless handset used by a typical household generates about 10 mW of time-averaged power, much less than a mobile phone operating at maximum power. Cordless handsets need less power than mobile phones because the signals do not have to travel as far to reach the base station – a few meters compared to up to a few kilometers. More power is required for radio communications over greater distances.
Cordless phone base stations are usually no more than a few tens of meters from the handsets. Mobile phone base stations can be kilometres from the mobile phone.
As communication is two-way, there is also the field from the cordless phone base station to consider. The maximum time-averaged power level for a DECT base station is the same as for a mobile phone handset – 250 mW. But the exposure is less because the cordless phone base station is not held to the head, and the field strength falls rapidly with distance.
Most people do not live or work close enough to the mobile phone base station for this field to be a concern. This is discussed further in question 2.2.
The terminal of a wireless computer network (Wireless Local Area Network, WLAN) has a peak power of 200 mW, but the time-averaged power depends on traffic and is usually a lot lower. Near a wireless network station used in homes and offices, the field intensity is typically below 0.5 mW per square meter.
The exposure from wireless systems is therefore typically below that of mobile phones. In certain circumstances, however, the exposure to radio frequency fields from wireless networks or cordless phones can exceed that from GSM or UMTS mobile phones.
Another system that is starting to be used in Europe is UWB, or ultra-wide band. It uses frequencies around 500 MHz, and has applications such as wireless microphones, medical applications and traffic control systems. With such systems, exposures are expected to be well below 0.1 mW per square meter.
Some anti-theft devices expose people to electromagnetic fields of radio and intermediate frequency (question 6). Increasingly popular, the devices are located at store exits to deter shoplifters. The radio frequency exposure varies depending on the type but is below safety limits if the device is used as directed by the manufacturer. Radio frequency fields are also used in industry, for example for heating or maintaining broadcasting stations. These systems can expose a worker to levels near or even above European safety limits (Directive 2004/40/EC). More...
2.2 What is the level of exposure from mobile phone base stations and radio towers?
Mobile phone base stations, like radio transmission towers, are structures designed to support antennas that transmit radio signals. They represent an essential part of the communication networks, linking the individual mobile phones with the rest of the network.
In most European countries, base stations are now ever-present, ensuring mobile communication over large areas.
The field is fairly even over the body and diminishes quickly with distance from the antenna. For such situations, to enable comparison with measured quantities, the European Union recommends maximum field strengths and power densities (reference levels), below which the energy absorbed would be considered safe.
At 900 MHz, an important frequency for mobile communication, the EU recommends that people are not exposed to a field stronger than 4.5 Watts per square meter (power density).
For GSM mobile phone networks, the exposure of the general population is typically much less – at least 100 times lower than the guidelines.
For the newer UMTS networks, 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 Watt per square meter and usually much less.
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 Watts per square meter. Close to the fence of very powerful transmitters, exposure of about 0.3 Watts per square meter can be expected in some cases.
As for the new digital TV broadcasting technology (DVB-T), an Austrian study registered power densities no higher than 0.04 Watts per square meter and as low as in the millionths of a Watt per square meter. This is similar to the power densities of the older analogue TV broadcasting systems, but as digital systems require more transmitters, higher exposure levels can be expected.
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.
2.3 How are radio frequency fields used in medicine?
Doctors use electromagnetic fields in the radio frequency range 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 often well above the recommended limits for the general public. Care must be taken to avoid medical staff exceeding exposure guidelines for workers.
Another common application of radio frequency fields in medicine is magnetic resonance imaging, or MRI, which also uses very strong static magnetic fields (see question 8). Magnetic Resonance Imaging (MRI) provides three-dimensional images of internal body structures like the brain. More...