The exposure limits come in two parts.
Firstly, there is the “basic
restriction”. That is the limit on the
quantity of direct biological relevance, the power deposited in tissue by the
radiofrequencies, known as the Specific Absorption Rate (SAR). That is measured in watts per kilogram
(W/kg). The limit is:
- 0.08 W/kg for the average over the whole body
- 2 W/kg for localised exposure to any 10 g of tissue in the head and trunk
- 4 W/kg for localised exposure to any 10 g of tissue in the limbs
Each of these is averaged over 6 minutes.
Secondly, there is the “reference
level”. This is measured in watts per
square metre (W/m2) and gives the power in the radiofrequency wave
that would be needed in order to produce the Specific Absorption Rate specified
in the basic restriction. So, if you
know that the power in the radiofrequency transmission is below the reference
level, you know that you are also below the basic restriction. The reference level varies with frequency,
but for the two commonest radiofrequencies, it is:
- 4.5 W/m2 at 900 MHz
- 9 W/m2 at 1800 MHz
(The reference levels can also be expressed
as an electric field in V/m, but for simplicity, we stick to W/m2 here.)
The significance of the reference level is
that it is often a lot easier to measure than the basic restriction, so we
often assess compliance using the reference level. If we are very close to a source of exposure,
the power density (as given in the reference level) is not very helpful, so we
would probably have to use the SAR and the basic restriction. But once we are a reasonable distance away
from the source, we would usually use the reference level to assess compliance.
It is important to note that these exposure
limits are set so as to include a large safety margin. And these are the limits
for the general public. There are higher
limits for workers – the limits for the public are lower partly to take account
of the greater variability in health status among the general population.