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Section 4A: Safety 148
How is radiofrequency radiation measured?
RF waves and RF fields have both electrical and magnetic components. It is often
convenient to express the strength of the RF field in terms of each component. For
example, the unit “volts per meter”(V/m) is used to measure the electric field
strength, and the unit “amperes per meter”(A/m) is used to express the magnetic
field strength. Another common way to characterize an RF field is by means of the
power density. Power density is defined as power per unit area. For example, power
density can be expressed in terms of milliwatts (one thousandth of a watt) per
square centimeter (mW/cm2 or microwatts (one millionth of a watt) per square
centimeter (μW/cm2).
The quantity used to measure how much RF energy is actually absorbed by the body
is called the Specific Absorption Rate or SAR. The SAR is a measure of the rate of
absorption of RF energy. It is usually expressed in units of watts per kilogram (W/kg)
or milliwatts per gram (mW/g).
What biological effects can be caused by RF energy?
The biological effects of radiofrequency energy should not be confused with the
effects from other types of electromagnetic energy.
Very high levels of electromagnetic energy, such as is found in X-rays and gamma
rays can ionize biological tissues. Ionization is a process where electrons are stripped
away from their normal locations in atoms and molecules. It can permanently
damage biological tissues including DNA, the genetic material. Ionization only
occurs with very high levels of electromagnetic energy such as X-rays and gamma
rays. Often the term radiation is used when discussing ionizing radiation (such as
that associated with nuclear power plants).
The energy levels associated with radiofrequency energy, including both radio waves
and microwaves, are not great enough to cause the ionization of atoms and
molecules. Therefore, RF energy is a type of non-ionizing radiation. Other types of
non-ionizing radiation include visible light, infrared radiation (heat) and other forms
of electromagnetic radiation with relatively low frequencies.
Large amounts of RF energy can heat tissue. This can damage tissues and increase
body temperatures. Two areas of the body, the eyes and the testes, are particularly
vulnerable to RF heating because there is relatively little blood flow in them to carry
away excess heat.
The amount of RF radiation routinely encountered by the general public is too low to
produce significant heating or increased body temperature. Still, some people have
questions about the possible health effects of low levels of RF energy. It is generally
agreed that further research is needed to determine what effects actually occur and
whether they are dangerous to people. In the meantime, standards-setting
organizations and government agencies are continuing to monitor the latest
scientific findings to determine whether changes in safety limits are needed to
protect human health.