What Are The Sources Of Exposure To Radio Frequency Fields?

Radio Frequency Fields, Radio Frequency Fields Exposure


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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).

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How High Is The Exposure From Mobile Phones And Wireless Devices?

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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).

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.

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