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Regulatory information
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.
What levels of RF energy are considered safe? Various
organizations and countries have developed standards for
exposure to radiofrequency energy. These standards
recommend safe levels of exposure for both the general public
and for workers. In the United States, the FCC has used safety
guidelines for RF environmental exposure since 1985.
The FCC guidelines for human exposure to RF electromagnetic
fields are derived from the recommendations of two expert
organizations, the National Council on Radiation Protection and
Measurements (NCRP) and the Institute of Electrical and
Electronics Engineers (IEEE). In both cases, the
recommendations were developed by scientific and engineering
experts drawn from industry, government, and academia after
extensive reviews of the scientific literature related to the
biological effects of RF energy.
Many countries in Europe and elsewhere use exposure
guidelines developed by the International Commission on Non-
Ionizing Radiation Protection (ICNIRP). The ICNIRP safety limits
are generally similar to those of the NCRP and IEEE, with a few
exceptions. For example, ICNIRP recommends different exposure
levels in the lower and upper frequency ranges and for localized
exposure from certain products such as hand-held wireless
telephones. Currently, the World Health Organization is working
to provide a framework for international harmonization of RF
safety standards.
The NCRP, IEEE, and ICNIRP all have identified a whole-body
Specific Absorption Rate (SAR) value of 4 watts per kilogram (4
W/kg) as a threshold level of exposure at which harmful
biological effects may occur. Exposure guidelines in terms of field
strength, power density and localized SAR were then derived
from this threshold value. In addition, the NCRP, IEEE, and ICNIRP
guidelines vary depending on the frequency of the RF exposure.