evaluation of compliance with sar limits on the basis of external … · 2014. 4. 24. · sar...
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Evaluation of compliance with SAR limits on the basis of external RFEM- field and induced current
measurements
Current trends in health and safety risk assessment of work-related exposure to EMFsMilan, February 14-16, 2007
Kari JokelaNon-Ionizing Radiation Laboratory
STUK, Radiation and Nuclear Safety Authority (Finland)
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Content
• Exposure limit values and action values
• Exposure assessment for non-homogeneous RF-fields in broadcast tower
• Exposure assessment for non-homogeneous RF fields in the vicinity of microwave antennas
• Assessment of SAR and induced body current in a reactive nearfield of a dielectric heater
• Localized exposure from a small source near the body
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Exposure limit values for RF fields(Directive 2004/40/EC)
50----10 - 300 GHz
-20100.4-0.01 - 10 GHz
-20100.4f/100( f in kHz)
0.1 - 10 MHz
Power density
(W/m2)
Localised10 g average
SAR forlimbs
(W/kg)
Localised10 g average
SAR forhead and
trunk(W/kg)
Whole body
average SAR
(W/kg)
Current density for head and
trunk
(mA/m2,rms)
Frequency range
SAR values are 6 min time averages
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
SAR in uniform plane wave
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Action values for RF-fields(Directive 2004/40/EC)
--500.361372 - 300 GHz
--f/400,008f½3f½400 - 2000 MHz
--100.1661110 - 400 MHz
10040100.166110 – 110 MHz
-40-1.6/f610/f1 - 10 MHz
-40-1.6/f6100.1 - 1 MHz
Limb induced current
IL(mA)
Contact current
IC(mA)
Equivalent plane wave power density Seq(W/m2)
Magnetic field strength H(A/m)
Electric field strength E(V/m)
Frequency range
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
The reference levels (action values) are spatiallyaveraged values over the body with the importantproviso that the basic restrictions (exposure limitvalues) on localized exposure are not exceeded
ICNIRP Guidelines 1998
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Source characteristics (frequency / waveform,
power, etc.)
Low power?
Assess fields / currents at user position
Below action values
Meets other product -specific compliance criterion?
Further assessment exposure limit values
Below exposure limit values
Compliant Non-compliant
Yes
Yes
Yes
Yes
No
No
No
No
Occupational exposure limits for
RF Fields
Action values
• External electric and magnetic field strength
• Induced body current
Exposure limit values
• Peak SAR (10 g)• Whole body average SAR• Induced current density (< 10
MHz)
Flow chart for exposure assessment
(EN 50392)
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
http://www.elmagn.chalmers.se/~elfrk/work/dosimetry/dosimetry.html
measured in homogeneousliquid phantom
computed in realistic head model
SAR assessment for a localized exposurefrom a mobile phone
Tim Toivo and Tommi Toivonen, STUK
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
RF exposure sources in a FM/TV mast
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
0
10
20
30
40
50
-2 0 2 4 6 8 10 12 14 16 18 20
Distance from the bottom of the antenna [m]
Eq
uiv
ale
nt
pla
ne
-wa
ve
po
we
r d
en
sit
y [
W/m
2]
Measurement results with the average transmission power of 19.5 kW
Averaged over the whole body with the average transmission power of 19.5 kW
Reference level for the occupational exposure
Measurement results scaled to normal average transmission power of 50 kW
Top of the
antenna
E-field measurement in FM-radio antenna
100 MHz
E field
STUK measurements by Jokela, Puranen and Toivo 2004
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
0
50
100
150
200
250
300
-2 -1 0 1 2 3 4 5 6 7Distance from the bottom of the antenna [m]
Eq
uiv
ale
nt
pla
ne
-wa
ve
po
we
r d
en
sit
y [
W/m
2]
Measurement results with the average transmission power of 5 kW
Averaged over the whole body with the average transmission power of 5 kW
Measurement results scaled to normal average transmission power of 15 kW
Reference level for the occupational exposure
Top of the
antenna
E-field measurement in Digi TV antenna
DVB-T antenna
E field
STUK measurements by Jokela, Puranen and Toivo 2004
572 MHz
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
RF EXPOSURE ASSESSMENT IN THE FM/TV
MAST
• Routine SAR assessment impossible in so complexexposure environment
• Technical problems with body current measurements
• Electric and magnetic field measurements with isotropicbroadband probes at distances not less than 20 cm fromthe metallic structures
• Body average of E2, or SE is a good exposure indicator.
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Near field of a base station antenna
Hansen et al., Univ. of Wuppertal
900 MHz1 W
vertical
horizontal
1 m
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Computed SAR near a base station
Cooper et. al. Bioelectrom.23:429-443, 2002
935 MHz
• For distance exceeding 30 cmexposure is limited by the wholebody average (WBA) SAR.
• At GHz range WBA-SAR iscorrelated with body averagepower density
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Planar layer model for calculation of SAR in surfaceabsorption range
k
Einc
Hinc
ICNIRP guidelines 1998
Power density limit from 10 to 300 GHz
50 W/m2 averaged over 20 cm2
1000 W/m2 averaged over 1 cm2for 10 g tissue cubed=2.2 cm
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Concentration of SAR in the surface of the bodyat microwave frequencies
0 5 10 15 200
0.1
0.2
0.3
0.4
0.5
3 GHz, 10 W/m2
(b)
Distance from the surface [mm]
SA
R [
W/k
g]
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Effect of averaging of local peakSAR in the body surface
Frequency (GHz)
Loca
lpea
kS
AR
(W
/kg)
10 W/m2
planar layer model
simple formula
.
Jokela K. EMC & SAFETY OF MULTIMEDIA COMMUNICATION TERMINALSStandard Evaluation Report, report 2000
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Relaxation of the reference level for spatial peakpower density above 1 GHz.
Peak power density may exceed at least 6 dB the body average power density, if the distanceto the antenna exceeds 30 cm
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Non-uniform exposure near an industrialdielectric heater
Electric field induces longitudinalRF-currents in the body
27 MHz
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Research problem
Estimate the whole body average andlocal maximal SAR from the measuredRF current
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Measurement of induced current
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
SAR simulation
Kännälä et al. 2006
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Electric field
Distance 0.3 m
measured
computed
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Measured and computed body current
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Current density in pelvic cross-section
(heterogeneous model)
vertical (longitudinal) horizontaldistance 0.3 m
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Relation of SAR to current and electric field
5.3 (in ankle)0.062931.0
5.2 (in ankle)0.0792120.5
7.3 (in arm)0.1303780.3
SAR10g
[W/kg]
(20)
SARwba
[W/kg]
(0.4)
Etot,max
[V/m]
(60)
Distance [m]
Current in the ankle = 100 mA
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
For non-uniform exposure to near
electric field from a dielectric heater
• induced body currents are longitudinal and distributed evenly along the high conducting tissues below the skin,
• currents depend on the body average electric field E,
• hot spots in the ankles and wrists are most critical,
• whole body and local peak SAR can be estimated from the measuredcurrent for electrode distance > 0.3 m,
• for distance <0.3 m, critical hot spot may arise in the tissue most adjacentto the electrode,
• SAR limits are not exceeded when current < 100mA and distance >0.3 m.
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
CONCLUSIONS
• Below 30 MHz the measurement of induced body currents and bodyaverage electric field is a good practice for assessing exposure toelectric field (high impedance sources).
• From 100 to 1000 MHz the equivalent electric field averaged over the whole body is probably sufficient exposure indicator in the FM/TV mastwhere the fields are relatively evenly distributed over the whole body.
• Above 1 GHz the local power density may exceed 6 dB the whole bodyaverage.
• At distance less than 20 cm determination of SAR is necessary for realisticexposure assessment.
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SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY
Ackowledgments
I wish to thank my colleagues
Lauri PuranenTim ToivoTommi ToivonenHeidi NybergAri-Pekka SihvonenSami Kännälä
for the data provided for thispresentation