General public exposure to electromagnetic fields generated by mobile phone base stations:

A simple model

Hugo Lehmann1, Ulrich Herrmann1, Bernhard Eicher1, Alain Trostel1, Peter Fritschi1, Mirjana Moser2

1Environment & Electromagnetic Compatibility, Swisscom Innovations, CH-3050 Bern

2Federal Office of Public Health, Radiation Protection Division, CH-3003 Bern, Switzerland

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Agenda:

Motivation

Model properties

Results: – field distributions – probability density distributions – medians / parts of populations exposed over 1 V/m

Extrapolation to different average powers per cell

Conclusions

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Motivation:

Assessment of electromagnetic fields (EMF) generated by mobile phone networks: by measurements?

– huge spatial variations (interference) – temporal changes (traffic, uncontrolled parameters) of the signal

strength – quite important uncertainty – results depend on the chosen measurement procedure / equipment

measurements give only extremely restricted information on the average exposure

Time demanding > high costs

It is almost impossible to get the necessary data for exposure classification for epidemiological studies solely by measurements

Personal dosimeters have yet to prove validity and liability

exposure assessment by calculations

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Simple free space propagation model:

24 dEIRPS

⋅=

πI equals II

0

2

µcEHES =⋅=

I

γδ ⋅=

ERPd

E 7

II

III

Attenuation factors δ and γ : − δ: attenuation of the construction materials: set to 1− γ: attenuation characteristics of the antenna as provided by

the antenna manufacturer Eq. III is used in Swiss regulation for the estimation of the electrical fields generated by a BS before its construction

Average E-field strength

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Region of interest (i), Bern north, suburban area:

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Further assumptions of the model/restrictions:

Only base stations of the Swisscom mobile network have been included in the calculations

Neglecting topographical data, a flat surface was adopted.

Geometrical properties: relative distance of antennas, height and main beam directions have been taken into account

No absorption due to construction materials and no shadowing effect have been considered

Calculation of the electrical field at 1.5 m over ground

The contributions of the different base stations have been summed up according to (n=# of base stations):

∑=

=n

iiEE

1

2

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Region of interest (ii), Zurich City, urban aera:

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Properties of the chosen areas:

1218# of base stations

GSM & UMTS:march 04

GSM: summer 03UMTS: march 04

Status of network

urbansuburbanDemographical type

5528 persons/km21734 persons/km2Population density

445041’615Inhabitants

0.805 km224 km2area

Zurich CityBern NorthRegion of interest

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Time averaged emitted power:

BCCH

TCH

100%: maximal power

Time of day24:000:00 12:00

Emitt

ed p

ower

BCCH

TCH 1

TCH 2 TCH 2

35

4

43

3

60

2

25

6

2528Average ERP in % of the maximal ERP

UMTS5Number of carriers of the cell GSM

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Results (i): field distributions, Bern north

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Region of interest (i), Bern north, suburban area:

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Results (i): field distributions, Zurich City

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Results (ii): probability density distributions: GSM

0

1

2

3

4

5

6

7

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

Electrical field strength [V/m]

% o

f p

op

ula

tio

n in

th

e b

in

Bern North Zurich City Fit with a lognormal distribution

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Results (ii): probability density distributions: UMTS

0

2

4

6

8

10

12

14

0.00 0.10 0.20 0.30 0.40

Electrical field strength [V/m]

% o

f p

op

ula

tio

n in

th

e b

in

Bern North

Zurich City

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Results (iii): characteristic numbers

Suburban Bern North

Urban Zurich City

Network GSM UMTS GSM UMTS

Proportion of the population exposed to more than 1 V/m [%]

0.12 0 0.80 0

Median exposure [V/m] 0.18 0.10 0.58 0.18

How does the system behave for different average powers of the cells?

Assuming that the population density over the field strength shows a lognormal behaviour (as for the GSM-system), an extrapolation to higher average powers per cell was done.

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Extrapolation to different average powers per cell (i): probability distributions

0

1

2

3

4

5

6

7

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4

Electrical field strength [V/m]

% o

f p

op

ula

tio

n i

n t

he

bin

Bern North

Fit with a lognormal distribution; P=P0

P/P0=2

P/P0=4

P/P0=10

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Extrapolation to different average powers per cell (ii): cumulated distributions

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00

Electrical field strength [V/m]

% o

f p

op

ula

tio

n e

xpo

sed

to

less

th

anth

e g

iven

fie

ld s

tren

gth

actual power P0

P/P0=2

P/P0=4

P/P0=10

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Extrapolation to different average powers per cell (iii) : Results

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

0.1 0.5 1.5 2.5 3.5 4.5 6 8 10

P/P0

% o

f p

op

ula

tio

n e

xpo

sed

to

mo

re t

han

1 V

/m

0

0.1

0.2

0.3

0.4

0.5

0.6

Med

ian

exp

osu

re [V

/m]

Characteristic numbers change rapidly with changing power of thecellsThe extrapolation is only valid if the network structure remainsidentical.

P/P0=4:- % over 1 V/m: 2%- median exposure: 0.36 V/m

H. Lehmann, cost281 workshop on exposure assessment, Paris 20/9/04

Conclusions:

The proposed model based on data used for regulation purposes allows for an estimation of the general public exposure to EMF generated by mobile phone networks.

Time averaged exposition is generally much lower than 1 V/m.

Characteristic numbers can easily be calculated: – median field strength – proportions of the population exposed to a given field

strength

These characteristic numbers change rapidly with changing radiated power per cell.

Improvements by integration of the topographical information andthe average attenuation of construction materials