Bornkessel2011 Article AssessmentOfExposureToMobileTe

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Wien Med Wochenschr (2011) 161/9–10: 233–239
DOI 10.1007/s10354-011-0882-x
Ó Springer-Verlag 2011
Printed in Austria
Assessment of exposure to mobile telecommunication
electromagnetic fields
Christian Bornkessel
Test Centre, IMST GmbH, Kamp-Lintfort, Germany
Received November 28, 2010, accepted January 18, 2011
Erfassung und Bewertung der Exposition
durch elektromagnetische Felder
der Mobilkommunikation
Zusammenfassung. Die elektromagnetische Exposition der
Bevölkerung im Umfeld von Mobilfunk-Basisstationen schöpft
nur Bruchteile der empfohlenen Grenzwerte aus. Örtlich maximierte und auf maximale Anlagenauslastung extrapolierte
Immissionen liegen bezüglich des Leistungsflussdichte-Referenzwertes maximal im einstelligen Prozentbereich, in der Regel
werden nur Werte im oder unterhalb des Promillebereiches
erreicht. Der Abstand zur Basisstation ist bis zu einigen 100
Metern kein zuverlässiges Kriterium zur Abschätzung der Exposition; wichtiger sind die Orientierung zur Hauptstrahlrichtung
sowie die Sichtverhältnisse vom Messpunkt zur Basisstation.
Durch Mobiltelefone werden wesentlich höhere Expositionen
beim Nutzer verursacht als durch Basisstationen; sie erreichen
bei maximaler Sendeleistung bis zu 80 % des Basisgrenzwertes.
Maßnahmen zur Minimierung der Exposition durch Mobiltelefone, z. B. durch Verwendung von Headsets, haben deswegen ein
größeres Potenzial als Abschirmmaßnahmen gegenüber Emissionen von Basisstationen. Sowohl Basisstationen als auch Mobiltelefone verfügen über Leistungsregelungsmechanismen, die
die Sendeleistung und damit auch die Exposition je nach Gesprächsaufkommen erheblich reduzieren können. Derzeitige
wissenschaftliche Untersuchungen beschäftigen sich mit der
Frage, ob Kinder durch Mobilfunksysteme stärker exponiert
werden als Erwachsene.
Schlüsselwörter: Exposition, Basisstation, Mobiltelefon, GSM,
UMTS
Summary. Typical general public exposures around mobile
radio service base stations consume only tiny fractions of exposure levels. Maximal immissions at maximal transmit power of
base stations amount to several percent of power density reference levels; typical immission levels are about one tenth of a
Correspondence: Christian Bornkessel, M.D., Test Centre, IMST GmbH,
Carl-Friedrich-Gauss-Staße 2-4, 47475 Kamp-Lintfort, Germany.
Fax: þþ49-2842-981 299, E-mail: [email protected]
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percent or even less. The distance to base stations is no reliable
exposure classifier. More important are the orientation relative to
the main lobe of the station and sight conditions from measurement point to the base station. Mobile phones cause higher
exposures to the user than base stations. At maximal transmit
power up to 80 percent of the basic restrictions are consumed.
Therefore, actions to minimize exposure to mobile phones, e.g.
by using a headset, have a larger potential than shielding
against emissions from base stations. Both base stations and
mobile phones apply power control mechanisms, capable to
significantly reducing the transmit power and the associated
exposure depending on the communication traffic. Present research investigates, whether children are more exposed to
mobile telecommunication systems than adults.
Key words: Exposure, base station, mobile phone, GSM, UMTS
Introduction
Mobile telecommunication systems like GSM or
UMTS consist of mobile phones and a network of base
stations. For establishing good coverage, a sufficient
number of base stations are necessary. In Germany, e.g.
69,258 sites with one or more mobile radio service base
stations were registered at the German Federal Network
Agency as of November 2010 [1]). The voice and data
information between base stations and mobile phones
are transmitted by electromagnetic fields (EMF). While
the exposure to the mobile phone radiation is restricted
to the mobile phone user and his immediate surroundings, emissions from base stations are large-scale and
extend over the whole coverage area.
This paper deals with EMF emissions of mobile
radio service base stations and mobile phones, the
spatial distribution of the electromagnetic fields as well
as their maximal and average intensities. Occupational
exposures, e.g. for personnel installing or maintaining
base station antennas, are not addressed here. Before
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exposure levels are presented and discussed in the next
two sections, some basic aspects of exposure assessment methods are summarized.
Exposure limits in terms of basic restrictions are
based on intracorporal absorption of radio frequency
(RF) EMF. To allow quick assessment, reference levels
have been derived linking external field quantities such
as electric field strength or power density to exposure
limits for worst case exposure conditions. If reference
levels are met, compliance with exposure limits can be
assumed. If not, proof of compliance is necessary by
detailed dosimetric investigations based on measured
field quantities.
Base stations
Exposure assessment
EMF immissions around mobile radio service base
stations can be assessed by measurements or by numerical calculations. Determination of the individual
exposure (i.e. the absorbed power inside the human
body by dosimetric studies) is based on such immission
assessment.
Numerical methods for immission assessment are
widely used by radio service network operators in the
planning process to predict the coverage provided by
the stations. For prediction, different wave propagation
models are used. The calculated results depend very
much on the accuracy and the assumptions of the used
model. Large efforts have been undertaken to adapt
prediction models for immission forecast, e.g. to be
used in epidemiologic studies. However, in general
such models are suited only for a dichotomous exposure classification, but not for an accurate prediction of
individual exposure.
Measurements allow for determining the actual
immission level with good accuracy without the need of
detailed knowledge about position and transmitting
power of the base station. In contrast to calculations,
measurements can be performed only for a given point
or small volume, making large-scale immission assessment very time and cost consuming or even impossible.
Especially for epidemiologic studies, the usage of mobile exposure dosemeters is a new trend, which may be
worn all the day and result in personal exposure
profiles.
EMF immission assessments around mobile radio
service base stations were performed extensively during
the past years by different institutions, and most of
them were carried out by measurements. For measurements, spectrum analysers are used, which have a high
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Bornkessel – Exposure to mobile telecommunication EMF
sensitivity to provide reliable results also at large distances from the base station. In addition, they allow
identifying also other radio sources like broadcast or
TV transmitters and hence selectively measuring the
mobile radio service base station under investigation.
To determine the cumulative immission from all relevant sources the knowledge of the individual contribution of a specific radiation source is important. To
assess compliance with frequency-dependent reference levels, in a second step, emissions from base
stations must be extrapolated to their maximal output.
Measurements with a spectrum analyser assess
the electromagnetic immission, expressed either as
electric field strength (in V/m) or power density (in
W/m²). Both electric field strength and power density
may be used alternatively, because they contain identical information and, hence, can be converted into each
other, if the measurement point is at least some metres
away from the source (in the far field). The measured
values are compared with respective immission reference levels. In the framework of this paper, we use the
reference levels of ICNIRP 1998 [2], which have been
adopted by many countries. Reference levels for GSM
900, GSM 1800 and UMTS base stations, as defined at
the lower end of the respective frequency bands, are
41.7 V/m, 58.4 V/m and 61.0 V/m for electric field
strength and 4.6 W/m², 9.0 W/m² and 10.0 W/m² for
power density. Very often, measured values are expressed in per cent of ICNIRP’s reference levels. However, because of the quadratic relation between electric
field strength and power density it has to be taken into
account that percentages for both quantities are different: 1% of power density reference level equates to 10%
of the electric field strength!
Two special characteristics of mobile radio service
base station immissions require attention during the
measurements: Firstly, especially inside rooms RF EMF
distributions may be very inhomogeneous with local
variations of power densities up to 100 fold within few
centimeters. Therefore, measurement points only some
cm away from each other may exhibit quite different
results. Such variations are accounted for either by
measuring the immission at several points (e.g. 3 or
6) within the dimension of a human body with subsequent averaging, or by recording the maximal immission in a volume such as 2 m 2 m 2 m. Secondly, the
transmit power of a base station is traffic-dependent,
and so is the immission: During night, where data and
voice traffic are normally low, the immission is generated by permanently transmitted signalling by control
channels of a base station. During daytime emissions
typical peak at late afternoon when more channels are
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Results
One of the largest EMF measurement campaigns in the
surrounding of GSM and UMTS base stations has been
performed in the German federal state Bavaria during
the past years. In the framework of the “FEE”-project [4]
with a common measurement protocol (volume scanning) more than 300 single measurement projects were
carried out between 2001 and 2008. In 2009, measurement reports were evaluated and the results were fed
into a database, which in the meanwhile has increased
up to 1867 measurement data entries.
Figure 1 shows the distribution of the measured
values of the study within different percentages of the
reference levels. It has to be noted that the percentage
classes are not distributed equally between 0 and 100%.
For immissions below 10% of the electric field strength
limit (or 1% of the power density limit), ten different
classes are defined, whereas higher immissions are
assigned to only one class. The majority of the measurement values are very small with regard to reference
levels: nearly half of the values (845 out of 1867) are in
the lowest class (up to 1% with regard to the electric
field strength, or 0.01% to the power density, respectively). The median is 1.2% (electric field strength) or
0.014% (power density), respectively. In the largest class
are only 49 out of 1867 values. Obviously, immissions of
that order of magnitude are relatively seldom. The
highest immission was found to be 16.4 V/m (corre-
9–
10
900
25
18
13
0
8–
9
37
1000
>1
7–
8
64
6
5–
86
6–
7
4–
5
3–
4
2–
0–
1
1–
2
3
Electric field strength [% of limit]
0.08
2.83
80
0.07
70
2.65
0.06
60
2.45
0.05
50
2.24
0.04
40
2.00
0.03
30
1.73
0.02
20
1.41
0.01
10
1.00
0
0
50
100
150
200
250
300
350
400
Electric field strength [% of limit]
operated to carry the increased traffic. Because a base
station must comply with exposure limits also at maximal operation, techniques have been developed to
extrapolate the measured instantaneous immission to
the maximal operational state of the base station [3].
Power density [% of limit]
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00.00
450
Distance to base station [m]
Fig. 2: Exposure distance profile around a base station with antennas
installed at 30 m height, measured at ground level
sponding to 27.9%) or 715.2 mW/m² (corresponding to
7.8%), whereas the lowest immission was measured to
1 m V/m (corresponding to 0.003%) or 3 nW/m² (corresponding to 0.00000009%).
The spread between maximal and minimal immission is very large with a factor of about 86 million
concerning power density percentages. It is therefore
interesting to study the influence of several parameters
on the actual immission at a given measurement point.
The most obvious parameter is the distance to the base
station. To test the hypothesis that locations in larger
distance to the station exhibit a smaller immission
compared to points close to the station, several points
along a straight line were measured in [5]. Figure 2
shows the results of the line measurements around a
mast with antennas mounted in about 30 m height.
Contrary to the usual opinion it can be concluded that
the exposure is not continuously increasing when approaching the base station. The highest immissions
were not found close to the base station, but at a
distance of about 230 m.
The reason for this phenomenon is the vertical
radiation pattern of typical base station antennas. As
845
Number of points
800
700
600
500
351
400
300
230
149
200
100
49
.0
.0
>1
81
–1
0.
.8
1
4
64
–0
0.
–0
.6
49
36
0.
–0
.
49
36
–0
.
0.
.2
5
25
0.
0.
16
–0
.1
6
09
–0
0.
04
–0
.
09
04
0.
1–
0.
0.
0
0.
0–
0.
01
0
Power density [% of limit]
Fig. 1: Distribution of the measurement points of the study regarding base
station exposure [4] among different percentages of the exposure reference
levels
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Fig. 3: Vertical radiation pattern of a mobile phone base station
Bornkessel – Exposure to mobile telecommunication EMF
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Fig. 3 shows, a base station antenna does not radiate
the power uniformly in all directions (like e.g. a light
bulb), but concentrated in a so-called main lobe. Beside
the main lobe secondary radiation zones exist (side
lobes), in which transmission is again increasing however with reduced power with regard to the main lobe.
Between main lobe and side lobe as well as between
different side lobes there are areas, where the power
transmission is very low. Moving away from the base
station, one passes through several radiation maxima
and minima, unless the point is reached where the
main lobe touches ground. If the antennas are installed
high above ground level this may be at several 100 m
distance from the station. In the scenario shown in
Figure 2, this point is obviously about 230 m away from
the base station. Therefore, in the light of these findings
the demand for a “safety distance” from base stations of
about 100 m for “places of sensitive use” like kindergartens and schools as claimed by some groups, might
even be counterproductive.
In contrast to distance a parameter which is much
more important for exposure estimation around base
stations is the orientation to the main lobe: At heights
similar to that of the antenna, points are inside the
vertical main lobe and, therefore, higher immissions
may be measured than at places at other angular areas
in spite of comparable distance and sight condition.
When measuring inside houses vis-à-vis to the base
station, the tendency of immission to decrease toward
lower floors is typical [4]. Another important parameter
is the sight condition: In the Bavarian study it was
found, that at points where buildings or vegetation are
blocking direct sight, immission in terms of power
density percentage was on average 1/30 compared to
points with sight to the station. This finding is relevant
when comparing the immission inside rooms without
sight to the base station with measurement points
outside or on balconies having free sight conditions to
the station.
Whereas all immission data given above were
local maximal values, extrapolated to the maximal
operational state of the station, it might also be interesting to have information about typical exposures of a
person. This is important e.g. in the framework of
epidemiologic studies, where not the maximal, but the
typical immissions averaged over time and at different
(and not maximal exposed) locations are of interest.
Studies in the framework of the German Mobile Telecommunication Research Programme reported that
typical immissions are about a factor of 10 to 100 (with
regard to power density) smaller than the worst case
extrapolated immissions [6].
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Bornkessel – Exposure to mobile telecommunication EMF
Mobile phones
Exposure assessment
In contrast to base stations, where the whole body may
be exposed to emitted fields, the exposure to mobile
phones is very local. The phone exposure concentrates
on the parts of the body, where the mobile phone is
operated. If the mobile phone is held toward the head,
the exposure is concentrated within the ear region
(Fig. 4), whereas other regions (e.g. trunk) experience
almost no exposure.
Due to the close distance of the mobile phone to
the head (near field condition), electric field strength or
power density can no longer be used as dosimetric
values. The correct dosimetric quantity for describing
exposures of devices with body contact is the specific
absorption rate (SAR). The SAR is defined as the
amount of power (in W), which is absorbed in human
tissue (per unit mass). For localized exposure of a
mobile phone, for each 10 g contiguous tissue ICNIRP
[2] recommends a basic restriction of 2 W/kg for exposure of head and trunk, and 4 W/kg for exposure of
limbs. For intermittent exposure, the SAR has to be
averaged over 6 min intervals. While ICNIRP requires
the SAR to be spatially averaged over 10 g of contiguous
tissue of undefined shape the European standard EN
62209-1 [7] requires averaging over a 10 g cube.
Basically, the SAR of mobile phones can be determined by measurement in a head phantom and by
numerical calculations. Numerical calculations are performed mainly for research purposes to study the local
Fig. 4: Local exposure distribution of a mobile phone in a head model
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Fig. 5: Dosimetric assessment system for SAR measurement of mobile phones
exposure distribution inside the head or body dependent on the phone type, the frequency and the type of
the human model. For the phones and the heads/
bodies numerical models are necessary, the latter composed of different tissue types with spatial resolutions in
the mm or sub-mm range.
In the certification process, i.e. the process to
demonstrate conformity of mobile phones with European Directives before placing them on the market,
SAR assessment is necessary. Typically this is done by
measurement. For head exposure assessment, the mobile phone under test is mounted beneath phantoms of
a human head and torso, which are filled with a liquid
simulating homogeneous tissue (Fig. 5). The mobile
phone is operated at its maximal transmit power.
A robot-driven probe scans the electric field inside
the phantom for calculating the SAR value. The head
measurements are performed with the phone mounted
to the left and right ear at two typical use positions
and at all implemented communication options (e.g.
GSM 900, GSM 1800, and UMTS). The maximal SAR
value is recorded and compared with ICNIRP’s basic
restrictions.
Exposure results
SAR data of 70 currently available smartphones and 76
mobile phones show SAR values ranging from 0.168 to
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1.61 W/kg, corresponding to 8.4 and 80.5% of the SAR
basic restriction [8]. The median is 0.817 W/kg (40.8%).
A comparison of these values with the exposures to
EMF from mobile radio service base stations shows,
that despite the smaller maximal transmit powers of
mobile phones (125–250 mW time averaged) in contrast to base stations (typically 5 to 40 W per channel
and sector), the maximal personal exposure to mobile
phones is by several orders of magnitude closer to
exposure limits. The main reason for this difference is
the smaller distance of the phone to the human head or
body which over compensates its smaller transmit
power. On the other hand, the mobile phone exposure
is limited to the time periods of an active phone call,
whereas the base station transmits permanently. In the
present exposure guidelines, however, this is without
consequences, because the exposure limits are defined
as threshold levels and not as time-dependent dose
values [2].
Numerical simulations with inhomogeneous, anatomically correct head models show differences of the
peak spatial average SAR of more than a factor 2 for
different head models of adult persons [10], underlining
that inter-individual anatomic differences have a large
impact on SAR. Furthermore, it was shown that the
homogeneous phantom used for the mobile phone
certification measurements is conservative enough to
cope with the anatomic differences [9].
Bornkessel – Exposure to mobile telecommunication EMF
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Like base stations, mobile phones can regulate
their transmit power, depending on the quality of the
connection to the base station. Because for product
certification the SAR is measured under maximal transmit power conditions, in real life the exposure to the
mobile phones is smaller than that documented above.
Real life exposure to mobile phones in relation to
maximal exposure was investigated in a project of the
German Mobile Telecommunication Programme [11].
In the study, depending on the investigated scenario for
GSM operation the average transmit power was between 10 and 70% of the maximal transmit power. The
maximal transmit power was achieved only during 5 to
30% of the call time. For GSM it has to be noted, that
with every change of a base station cell (which can
occur frequently during a car or railway trip) for handover the transmit power is upregulated to the maximum
before being adjusted to the required level. This causes
an increase of the average transmit power. At UMTS,
after each cell change the power regulation mechanism
starts from low levels.
Discussion
Although the personal exposure to GSM and
UMTS mobile telecommunication systems is dominated by the mobile phones and not by the base stations,
very often the base stations are in the focus of the public
discussion. Sometimes the question arises, whether the
shielding of homes can reduce the exposure for people
living near to a base station. Besides the fact that even in
the direct vicinity to base stations shielding measures
are of questionable value in the light of the actually low
immissions, in practice their shielding effectiveness is
very limited: an RF effective shielding requires creation
of a complete metallic cage. However, care must be
taken since even small opening or even a slit may
considerably reduce the shielding effect. Moreover, a
shield can be counterproductive, because the electromagnetic fields produced by RF sources inside the
house, such as mobile phones or DECT cordless
phones, may generate resonances, which may cause
even higher local immissions than without shielding.
If the personal exposure is to be minimized it
makes more sense to start with the mobile phone
because of its higher exposure compared to base stations. For example, it is possible to choose a mobile
phone with a low SAR value, although conclusions of a
low SAR value at maximal transmit power to real life
transmit scenarios are somewhat limited. Another option is to use headsets while using the phone, which
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Bornkessel – Exposure to mobile telecommunication EMF
effectively reduces the head SAR in most practical
cases [12]. However, devices claimed to be mobile
phone radiation protectors are not recommended. Test
showed they are either useless or may even cause the
mobile phone to increase its transmit power to compensate for the impedance mismatch at the phone’s
antenna provoked by such devices. In the context of
exposure minimization, it should not be forgotten that a
good connection of the phone to the base station,
created by good base station coverage, effectively reduces the exposure to the mobile phone due to the
power control mechanisms installed in the phone.
A very complex, yet important topic is the question, whether children are more exposed to RF fields of
mobile telecommunication systems than adults. This
question should be discussed with regard to the higher
cumulative exposure of children to such technologies in
comparison to adults due to the rapid growth of mobile
communication systems during the past years. This
refers to both exposure to base stations and mobile
phones.
As far as base station exposure is concerned, some
studies have shown that whole body exposure of children and small persons (shorter than 1.3 m in height) to
reference levels under worst case conditions may lead
to an excess of basic restrictions (SAR values, averaged
over the whole body) at frequencies around 100 MHz
and 1–4 GHz [13–15]. A recent study [16] with anatomically correct children models and age dependent tissue
parameters quantified the excess of basic restrictions to
30% around 100 MHz more than 50% between 1.5 and
5 GHz for small children. As measurements show, far
field immissions of base station fields are generally well
below reference levels. However, these results indicate
there is a possible inconsistency of the recommended
system of basic restrictions and reference levels, which
requires further attention.
As far as mobile phone exposure is concerned,
many partly controversial numerical SAR studies were
published during the past years. They investigated
possible differences in the energy absorption depending on head size and anatomy, pinna thickness, and
dielectric head tissue parameters. Whereas some studies varying the head size reported significant increase of
spatial peak average SAR in children heads, other studies could not replicate such findings [9]. While the first
investigations used linearly downscaled adult head
models for children, models based on anatomically
correct MRI scans are presently used. A recent study
[17] also investigated possible absorption differences
due to different pinna thicknesses and head tissue
parameters. Characteristic differences of pinna
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thickness between adults and 6–8-year-old children,
which may influence SAR differences, could not be
observed; data for younger children were not available.
With the exemption of bone marrow, no systematic
influence of the age dependency of tissue parameters
on the local exposure was found. Concerning the peak
spatial average SAR, no characteristic differences between investigated children models (3, 6 and 11 years
old) and the adult model were found, taking into account the reported inter-individual differences between
different adult models. Concerning the local SAR distribution (i.e. without spatial 10 g averaging) differences
exist, leading to a higher exposure of some tissues and
organs (e.g. the eye) in children due to a closer distance
to the phone, where other regions of the children’s head
were found to be lower exposed than in adult heads.
These results should be taken into account in the
interpretation of epidemiologic studies and for research
on non-thermal effects.
As an outlook into the next few years it can be
stated, that the penetration of our life with mobile
communication services is further increasing: the new
generation of mobile communication technology LTE
(Long Term Evolution) is now being introduced with
new base stations being installed. The successor of LTE
(LTE advanced) is in its final standardization process
and ready to start within the next years. With the
“mobile Internet everywhere” LTE technology, especially mobile high data rate applications will increase,
eventually overwhelming the classical voice traffic. The
new mobile communication technologies will result in
new exposure and user scenarios which will have to be
actively monitored by radiation protection authorities.
Conflict of interest
The author declares that there is no conflict of
interest.
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der Basis relevanter biophysikalischer und biologischer Parameter.
Study on behalf of the German Federal Office for Radiation
Protection. http://www.emf-forschungsprogramm.de/forschung/
biologie/biologie_abges/bio_065.html, cited 17 Nov 2010.
[1] Radio equipment sites per Federal state, for which a site certification
was issued. http://emf2.bundesnetzagentur.de/en_statistik.html, cited 17 Nov 2010.
wmw
9–10/2011
Ó Springer-Verlag
Bornkessel – Exposure to mobile telecommunication EMF
239
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