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Centre for Radiation, Chemical and Environmental Hazards Exposure Assessment for Epidemiological Studies International Scientific Conference Electromagnetic Fields & Public Health Simon Mann Session 1 11:20-11:50 16 November 2011

Exposure Assessment for Epidemiological Studies · Exposure Assessment for Epidemiological Studies International Scientific Conference Electromagnetic Fields & Public Health Simon

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Centre for Radiation, Chemical and Environmental Hazards

Exposure Assessment for Epidemiological Studies

International Scientific ConferenceElectromagnetic Fields & Public Health

Simon MannSession 1 11:20-11:5016 November 2011

Scope

How to assess exposure of individuals within an observationalstudy over a time-period of interest (hours, days, years)

• Measures of Exposure• Factors Affecting Exposure• Occupational Studies• Environmental Studies• Mobile Phones

Study Types•Retrospective•Prospective

Sources of Information for an Epidemiological Study

Questionnaire responses from participants • What, where, when, how?

Data gathered (prospectively) through electronic systems• Source emissions• Location of a person• Personal measurements

Databases for representative scenarios – lookup tables• Calculations• Measurements – spot or personal• Working/usage practices

Medical information (imaging etc) – tissues of interest

Centre for Radiation, Chemical and Environmental Hazards

Exposure Metrics

What is Exposure?Time & Space Aspects

Quantifying Exposure

Derive from measurements and/or calculations• A number with units• Relevant to the biological end-point

Physically meaningful quantity (internal?)• Electric field strength• Magnetic field strength• Induced current density (or electric field strength)• Energy absorption in the body

Sum (dose) quantity over time and space• In the tissues of interest

Example Metrics

Electricity Distribution•Whole-body exposure•Low frequenciesMetric•Induced current density averaged over body tissues of interestSurrogate•Magnetic flux density at body position

Mobile Phones•Localised exposure•High frequenciesMetric•Energy absorption averaged over tissues of interest (per kg)Surrogate•Cumulative call time

Centre for Radiation, Chemical and Environmental Hazards

Factors Affecting Exposure

General Considerations

Factors Affecting Exposure– Low Frequencies

Induced current density is proportional to•Imposed field strength•FrequencySpatial distribution•Non-uniform, even for a uniform exposure field (magnetic in this case)•Current flows preferentially through most conductive tissues

Current density Anatomy

Factors Affecting Exposure– High Frequencies

Energy absorption depends on the body size in relation to wavelength, posture, morphology, and direction and polarisation of wave incidence

Body resonance(Standing adult, arms by side, conducting ground, vertical E-field)

Factors Affecting Exposure– High Frequencies

As frequency increases

•Absorption becomes stronger•Fields penetrate less far into tissue•Energy absorption becomes nearer the surface

400 MHz 2450 MHz

Factors Affecting Exposure– Distance

• Wave spreads out around a source

• Power in the wave becomes less concentrated with increasing distance

• Power density describes theconcentration of power flowing through a point

Prad

d

Sphericalsurface(Area=4 d )π 2

Antenna

Feedcable

Power density at surface is S

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

1E+01

1E+02

1E+03

1E+04

0.1 1 10 100 1000 10000

Distance, m

Pow

er d

ensi

ty, W

m-2

1 W10 W100 W1 kW

Source Power

Inverse Square Law at Radio Frequencies

24),(

rGPS rad

πφθ

=

S Power density W m-2

Prad Radiated power WG Antenna gain (linear)r Distance m

Factors Affecting Exposure– Distance from source

Examples (power frequencies)•Net current in distribution circuit

•Load current in distribution circuit

•Single circuits or overhead power line with untransposed circuits

•Appliances or overhead power line with transposed circuits

•Emissions/Fields spread out around sources, and hence reduce with increasing distance•Fields reduce more rapidly with distance from small sources

Summary of Factors Affecting Exposure from a Source

• Source emissions (frequency, power, current, voltage)• Source configuration (size, shape)• Distribution of emissions from the source (angular)• Distance, location, orientation of source from the body• Body size, shape, morphology• Influence of the environment (obstacles, reflection)

Centre for Radiation, Chemical and Environmental Hazards

Occupational Studies

Working with EMF Sources

Occupational Sources

Spot Measurements– Limitations

• Published measurements are available for most sources, but what do they mean?• Most measurements are made as maximum exposures

reasonably foreseeable – regulatory requirements• Epidemiology usually needs typical average exposures,

which may be very different• Need to consider duty factors

• On/off cycle of equipment when working• Shift duration• Number of shifts per day, week etc

Personal Exposure Record– HF Broadcast Site

0

20

40

60

80

100

120

140

14:00 14:10 14:20 14:30 14:40 14:50 15:00 15:10

Time

Per

cent

age

of e

lect

ric fi

eld

refe

renc

e le

vel

Issues with Job Titles

• Can be too broad – misclassification• Working with radar?

• May have different meanings in different contexts• Assessments need to be specific and linked to actual

measurements• Experts making judgements should watch and talk to

people doing the work• Beware of other sources

• People climbing broadcast towers use walkie-talkies and mobile phones

Historical Exposures

Options• Make a measurement today• Use historical published measurements• Calculations based on expert judgement

Problems• Source decommissioned – power/frequency unknown• Exposure may depend critically on juxtaposition to source• Working practices unknown/undefined

Summary of Occupational Assessments

Solution•Develop job-exposure matrices based on representative measurements and dosimetry

Pitfalls•Misuse of compliance measurements•Omitting secondary sources of exposure•Not accounting for duty factors properly•Job titles not specific enough

Centre for Radiation, Chemical and Environmental Hazards

Environmental Studies

Communities Near Radio Transmitters or Power Lines

Environmental Studies– Exposimetry

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Wed 18 Thu 19 Fri 20 Sat 21 Sun 22 Mon 23 Tue 24

Mag

netic

Fie

ld (

uT)

Time

PERSONAL EXPOSURE TRACE

H:\ Goldxyz\ GoldData1\ Equinox\ PE\ GE004APE.MDX

Sep/ 17/ 199603:48:45 PM

Sep/ 24/ 199606:43:45 PM

Use of personal exposure meters (exposimeters) is established for assessments at power frequencies

Environmental Exposures– Contributing Sources

Span the frequency range: 80 MHz – 2.5 GHzVHF broadcast (radio) 88 - 108 MHzTETRA base stations 390 - 395 MHzUHF broadcast (TV) 470 - 854 MHzGSM900 phones 880 - 915 MHzGSM900 base stations 925 - 960 MHzGSM1800 phones 1710 - 1785 MHzGSM1800 base stations 1805 - 1880 MHzDECT cordless phones 1880 - 1900 MHz3G phones 1920 - 1980 MHz3G base stations 2110 - 2170 MHzWireless LANs (802.11g) 2400 - 2500 MHzWireless LANs (802.11a) 5150 - 5725 GHz

Typical field strengths: 1 mV m-1 – 1 V m-1

Typical power densities: 100 nW m-2 – 10 mW m-2

Spot Measurements from Base Stations

0

5

10

15

20

25

30

35

40

Total Band Exposure Quotient

Per

cent

age

10-1 10-1110-1010-910-810-710-610-510-410-310-2

0.2 μW m-2

8 mV m-1

3092 measurements made at 499 sites

5 mW m-2

1.4 V m-1

Environmental Studies– Antenna Beam Patterns

Typical antenna radiation patterns• 120 degree beam width in the plane of azimuth• 5-10 degree main beam (and sidelobes) in the plane of elevation

Inverse square law controls fall off of power density with distance within the main beamSidelobes cause oscillatory dependence on distance at at ground level below the main beam

MastBeam

Ground

Example of Power Density Variation at Ground Level

0

5

10

15

20

25

30

35

40

1 10 100 1000 10000Distance from mast, m

Pow

er d

ensi

ty, m

W m

-2

ICNIRP Public Reference Level= 45,000 mW m-2 Main Beam

Sidelobes

Frequency = 900 MHzHeight =15 mPower = 80 WGain =18 dB

Environmental Studies– Multipath Propagation

Field at a point is formed from• Direct wave (not always present)• Reflected wave contributions

Interference pattern formed• Spatial fading of fields over

distances comparable with the wavelength, i.e. 10s of centimetres

Other effects• Shadowing• Shielding• Diffraction

Measurementposition

Reflectionsfrom

buildings

Summary of Environmental Assessments

Solutions•Ask participants to wear personal exposure meters•Develop activity/location exposure matrices•Develop (and validate) calculation models

Pitfalls•Use of distance as a surrogate for exposure•Not calibrating/correcting personal dosimeters•Neglecting sources other than the one of concern

Centre for Radiation, Chemical and Environmental Hazards

Mobile Phone Studies

Transmitters Near the Body

Distributon of Specific Energy Absorption Rate (SAR)

• Phone emits radio waves near the head

• Waves penetrate the head for a few cm

• The radio energy is absorbed in the tissues

• SAR describes the local rate of energy absorption

• Unit of SAR is the watt per kg

Maximum Mobile Phone Emitted Powers

TDMA factors, e.g. GSM• Signals from 8 phones are

interleaved at base stations• Phone signal consists of

217 “bursts” per second• Time-averaged power is

1/8 of peak power One Frame = 4.615 ms

Time

Gen System FrequencyMHz

Peak powerW

TDMA factor

Maximum average power, W

11223

NMT450TACSGSMGSMUMTS

45090090018001950

0.90.6210.125

––1/81/8–

0.90.60.250.1250.125

Adaptive Power Control(GSM Phones)

Emitted power during mobile phone calls varies greatly and is generally less than the maximum

Vrijheid et al 2006

Average Mobile Phone Emitted Powers

Gen System Maximum powermW

APC Factor(no DTX)

Average powermW

12223

NMT450TACSGSM900GSM1800UMTSDECT

90060025012512510

––2225-500–

900600133 (52-172)65 (47-88)0.2-510

• APC reduces average powers much more when phones are in 3G mode than when they use GSM

• But 3G phones may switch to 2G mode when network coverage is poor

• APC has some dependence on call circumstances: urban/rural, moving/stationary, voice/data, network

Phone typeLaterality

Call env-ironment

Studysubject

Questionnaire

Weightedsummation

SAR distributiondatabase

Power distrib-ution database

MetricsSAR atlocalisation

Doseindices

Neuroradiologyreport

Tumour volum

e and origin localisation

Left Right

Measurements

SARmaxcompilation

Number and duration of calls

Phone outputpower database

Operators

Standards

Summary of Mobile Phone Assessments

Objective•Assess cumulative energy absorption in the tissues of interest

Challenges•Developing and selecting representative SAR distributions•Phones having a variety of transmission modes•Combining mobile phone exposures with other (environmental) exposures•Historical phones and networks

Overall Conclusions

A variety of tools are available for exposure assessment in epidemiological studies

• Modelling of electromagnetic fields produced by sources, including accounting for the effect of the environment

• Modelling of the electromagnetic field distribution produced inside the body by an externally imposed field

• Spot measurements of electromagnetic fields produced by sources at locations where people are exposed

• Personal measurements using body-worn instrumentsExposure models should be validated and simplifying assumptions should be testedIt is possible to spend as much time and effort on the dosimetryas on the epidemiology

This paper was produced for a meeting organized by Health & Consumers DG and represents the views of its author on thesubject. These views have not been adopted or in any way approved by the Commission and should not be relied upon as a statement of the Commission's or Health & Consumers DG's views. The European Commission does not guarantee the accuracy of the dataincluded in this paper, nor does it accept responsibility for any use made thereof.