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Section 4A: Safety Guidelines 179
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.
FDA, EPA and other US government agencies responsible for public health and
safety have worked together and in connection with WHO to monitor
developments and identify research needs related to RF biological effects.