PROCEEDINGS OF THE 7 INTERNATIONAL ... - Fizika

N. Vaiciunaite / Medical Physics in the Baltic States 7 (2009) 106 - 108
Table 2. Permissible levels of electromagnetic
irradiation from industrial electromagnetic fields
Country Inhabita Worker Inhabita Worker
or nts s nts s
organiz Electric Electric Magneti Magneti
ation field, field, c field, c field,
kV/m kV/m mT mT
ICNIRP
(1998)
4,16 6,33 0,0833 0,4166
ACGIH
(1996)
25,0 1,0
CENEL 8,33 25,0 0,53 1,33
EC
(1995)
3
3 3
NRPB 10,0 10,0 1,33 1,33
(1993)
3 3
NH&M
RC
(1989)
5,0 10,0 0,1 0,5
German
y
(1989)
20,6 20,6 5,0 5,0
Russia
(1985)
5,0 1,76
Poland
(1980)
15,0
Lithuani
a
(2001)
25-5,3* 0,7-4,0*
* - depends on working hours
ICNIRP – International Commission on Non – Ionizing
Radiation Protection;
ACGIH – Amerikan Conference of Governintel
Industrial Hygienist;
CENELEC – Comite Europeen de Normalization
ELECtrotechnique;
NRPB – National Radiological Protection Board;
NH&MRC – National Health & Medical Research
Council (Sante nationale et Conseil de recherches
medicales).
Permissible electric and magnetic field intensities were
investigated using the results of experimental
investigation, mathematical modelling and
extrapolation.
It was assumed that a human body has homogeneous
and isotropic electrical conductivity. The sine equation
of time and Faraday law was used for the evaluation of
the induced current intensity:
J = πRfσB
,
Where B is magnetic flux density; R is a radius of loop
of the induced current..
Elliptic model was used for the estimation of the
current density in the body. Assuming, that the
homogeneous conductivity of the body is 0,2 S/m and
the magnetic flux density is 100 μ T, the resulting
current density of 0,2-2mA/m 2 in a human body was
calculated [CRP 1997]. The highest values of current
density were found in the head–neck region. If an
electric field vector is parallel to the axis of human
107
body, the 5kV/m field induces 2mA/m 2 current at in the
head and neck area of human’s body [ILO 1994].
2. Measurement of electromagnetic radiation
Measurements of electromagnetic radiation were
performed using equipment and methodology of the
Computer equipment testing laboratory (CETL) at
Kaunas University of Technology (KTU). CETL is
accredited at the Lithuanian National Accreditation
Office as the electromagnetic radiation of computer
displays exploring institution (www.kompirsveikata.lt).
Electromagnetic radiation limits for video display units
(VDUs) are set in „TN01:1998. Displays. Permissible
levels of electromagnetic radiation“, which corresponds
to international standard MPR II..These limits are
presented in Table 3.
Table 3. Permissible levels of electric field strengths
and magnetic flux densities [4].
Field Frequence
range
Electric
Magnetic
MPR II,
TN01:1998
TCO
0 Hz 500 V 500 V
5 – 2 kHz 25 V/m 10 V/m
2 – 400
kHz
2.5 V/m 1.0 V/m
5 – 2 kHz 250 nT 200 nT
2 – 400
kHz
25 nT 25 nT
*according to MPR II, measuring device should be
located in 50 cm in front of the VDU; according to
TCO - in 30 cm in front of the VDU and at 50 cm
around the VDU.
Hygiene requirements for the computerized working
place environment and equipment are regulated by
Lithuanian Hygiene Norm HN-32:2004 „Working at
video terminals. Safety and health requirements“.
However the levels of electromagnetic radiation
emitted by modern VDUs are lower than those from
natural sources, and are not are not considered to be
harmful to the human’s health [5].
3. Results
Measurements of VLF electric and magnetic fields at
124 workplaces in 42 rooms of Kaunas University of
Technology have been performed. The results of
measurements are presented in the Table 4.
It was found that Hygiene Norm (HN) levels of electric
field strength were exceeded at 13 places and were
twice as high as recommended at 3 workplaces, when
measurements were performed for the electric field
frequency of 2 to 5Hz. Also, the magnetic field levels
were higher than recommended at 19 workplaces and
were twice as high as recommended at 7 places for the
frequency range 2 - 5Hz.