The electric and magnetic
fields produced in the body by a nearby electromagnetic source may cause
both thermal and non-thermal biological effects. The effects of magnetic
fields vary with frequency, and are probably greatest in biological tissue
containing small amounts of magnetite. Magnetite (Fe3O4)
is a naturally occurring oxide of iron. It is a ferromagnetic but behaves
similarly in magnetic fields to a Ferro magnet such as iron. Magnetite is
found in certain bacteria and in the cells of many
animals, including human beings. It is believed to be used by some species
of birds and fish to provide magnetic sensitivity, which they employ in
navigation. However, no other effects associated with the interactions of
electromagnetic fields with magnetite have been demonstrated in animals.
It has been calculated that the
interaction resulting from the largest RF magnetic fields generated by
mobile phones is extremely small (Adair, 1994), and that any other effects
of magnetic fields at these frequencies should be even smaller. Indeed, it
seems to be generally agreed that any biological effects from mobile phones
are much more likely to result from electric rather than from magnetic
fields.
Thermal effects are those caused by the rise in
temperature produced by the energy absorbed from oscillating
electric fields. The force produced by an electric field on charged objects,
such as the mobile ions present in the body, causes them to move, resulting
in electric currents, and the electrical resistance of the material in which
the currents are flowing results in heating. This heat input causes the
temperature to rise and it continues to do so until the heat input is
balanced by the rate at which it is removed, mostly by blood flowing to and
from other parts of the body. It is estimated that it takes several minutes
from the moment RF exposure occurs for the irradiated parts of the body to
reach their final equilibrium temperatures. In view of this slow response,
the equilibrium temperature arising from the pulsed fields of mobile
telecommunications will essentially be determined by the
average power absorbed. There will,
however, be small oscillations about that temperature at the pulse frequency
or frequencies.
Heating in the head
see computer
models
It has not yet proved
possible to measure these small changes in temperature directly, except
those at the outer skin (Adair et al,
1999) and, although temperature is a more direct determinant of thermally
induced tissue damage, the majority of theoretical studies up to the present
time have restricted themselves to the computation of SAR alone.
The relationship between
the SAR and the resulting temperature rise is complex, and significantly
dependent on antenna configuration, location and frequency. The most
problematic feature of a temperature calculation is modeling the effect of
blood flow on heat transfer. The traditional continuum heat-sink model
developed by Pennes (1948) has been found to give remarkably accurate
results in many circumstances, but numerous modifications have been
suggested more recently (Arkin et al,
1994).
In
a recently published study (Van Leeuwen et al,
1999) the heat deposition within the head was computed by coupling a finite
difference time domain model for SAR with a new thermal model.
The thermal model includes the convective effects of discrete
blood vessels, whose anatomy was determined using magnetic resonance
angiography of a healthy volunteer. For a 915 MHz dipole antenna with a
time-averaged power output of 0.25 W (equivalent to a typical mobile phone),
this study results in an SAR of about 1.6 W/kg and predicts a maximum brain
temperature rise of 0.11°C in the steady state. There is general agreement
between the brain temperatures calculated using the Pennes equation and that
using the new discrete vessel model, which suggests that the sensitivity of
the results to the exact blood flow model may not be critical. However,
further work should be done to apply this model to more realistic
simulations of mobile phone configuration, and to investigate the effect of
different antenna positions and frequencies (particularly in the 1800 MHz
band also used by mobile phones).
A
recent NRPB study (Wainwright, in press)
has applied the traditional Pennes thermal model to the SAR patterns
predicted by earlier work (Dimbylow and Mann, 1994). The radiation source
was modeled as a monopole antenna on a metal box, and both horizontal and
vertical orientations of the antenna were considered. Computations of the
final steady-state temperature rise were carried out for a 0.25 W antenna at
frequencies of 900 and 1800 MHz. The highest temperature rises found in the
brain were around 0.1°C.
