Electromagnetic Fields 2015 Update
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
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
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
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.
Typical sources of electromagnetic fields
||Some examples of exposure sources
||video display units; MRI (medical imaging) and other diagnostic
or scientific instrumentation; industrial electrolysis; welding
|ELF [Extremely Low Frequencies]
||power lines; domestic distribution lines; domestic appliances;
electric engines in cars, trains and tramways; welding
|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;
|RF [Radio Frequencies]
||100 kHz - 300 GHz
||mobile telephones; broadcasting and TV; microwave ovens; radar
and radio transceivers; portable radios; MRI
||300 GHz – 20 THz
||applications are still in development, but currently mostly
telecommunication applications and body scanners are
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
kinetic energy) which then collide with each other, subsequently
causing tissue heating Even if the basic physical interaction is
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
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 distance to the
base station) 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
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
The terminal of a wireless computer network (Wireless Local Area
Network, WLAN) has a peak power of 200 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
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
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
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
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
Imaging (MRI), which in addition also uses very strong
fields (see question 8). MRI provides high-resolution
cross-sectional images of the body including the head without
shadowing by bony structures.