RF Safety is a growing concern in the Wireless / Electronics Industry.
This page is intended to give you some of the background information on RF Safety.
Please explore the links below to gain a more complete understanding of this
important topic!
Although Amateur Radio is basically a safe activity, in recent
years there has been considerable discussion and concern about
the possible hazards of electromagnetic radiation (EMR), including
both RF energy and power frequency (50-60 Hz) electromagnetic
fields. Extensive research on this topic is underway in many countries.
This section was prepared by members of the ARRL RF Safety Committee
and coordinated by Dr Robert E. Gold, WBØKIZ. It summarizes what
is now known and offers safety precautions based on the research
to date.
All life on Earth has adapted to survive in an environment of
weak, natural low-frequency electromagnetic fields (in addition
to the Earth's static geomagnetic field). Natural low-frequency
EM fields come from two main sources: the sun, and thunderstorm
activity. But in the last 100 years, man-made fields at much higher
intensities and with a very different spectral distribution have
altered this natural EM background in ways that are not yet fully
understood. Much more research is needed to assess the biological
effects of EMR.
Both RF and 60-Hz fields are classified as nonionizing radiation
because the frequency is too low for there to be enough photon
energy to ionize atoms. Still, at sufficiently high power densities,
EMR poses certain health hazards. It has been known since the
early days of radio that RF energy can cause injuries by heating
body tissue. In extreme cases, RF-induced heating can cause blindness,
sterility and other serious health problems. These heat-related
health hazards are called thermal effects. In addition,
there is evidence that magnetic fields may produce biologic effects
at energy levels too low to cause body heating. The proposition
that these athermal effects may produce harmful health consequences
has produced a great deal of research.
In addition to the ongoing research, much else has been done to
address this issue. For example, the American National Standards
Institute, among others, has recommended voluntary guidelines
to limit human exposure to RF energy. And the ARRL has established
the RF Safety Committee, a committee of concerned medical doctors
and scientists, serving voluntarily to monitor scientific research
in the fields and to recommend safe practices for radio amateurs.
Thermal Effects of RF Energy
Body tissues that are subjected to very high levels of RF energy
may suffer serious heat damage. These effects depend upon the
frequency of the energy, the power density of the RF field that
strikes the body, and even on factors such as the polarization
of the wave.
At frequencies near the body's natural resonant frequency, RF
energy is absorbed more efficiently, and maximum heating occurs.
In adults, this frequency usually is about 35 MHz if the person
is grounded, and about 70 MHz if the person's body is insulated
from the ground. Also, body parts may be resonant; the adult head,
for example is resonant around 400 MHz, while a baby's smaller
head resonates near 700 MHz. Body size thus determines the frequency
at which most RF energy is absorbed. As the frequency is increased
above resonance, less RF heating generally occurs. However, additional
longitudinal resonances occur at about 1 GHz near the body surface.
Nevertheless, thermal effects of RF energy should not be a major
concern for most radio amateurs because of the relatively low
RF power we normally use and intermittent nature of most amateur
transmissions. Amateurs spend more time listening than transmitting,
and many amateur transmissions such as CW and SSB use low-duty-cycle
modes. (With FM or RTTY, though, the RF is present continuously
at its maximum level during each transmission.) In any event,
it is rare for radio amateurs to be subjected to RF fields strong
enough to produce thermal effects unless they are fairly close
to an energized antenna or unshielded power amplifier. Specific
suggestions for avoiding excessive exposure are offered later.
Athermal Effects of EMR
Nonthermal effects of EMR may be of greater concern to most amateurs
because they involve lower level energy fields. Research about
possible health effects resulting from exposure to the lower level
energy fields, the athermal effects, has been of two basic types:
epidemiological research and laboratory research.
Scientists conduct laboratory research into biological mechanisms
by which EMR may affect animals including humans. Epidemiologists
look at the health patterns of large groups of people using statistical
methods. These epidemiological studies have been inconclusive.
By their basic design, these studies do not demonstrate cause
and effect, nor do they postulate mechanisms of disease. Instead,
epidemiologists look for associations between an environmental
factor and an observed pattern of illness. For example, in the
earliest research on malaria, epidemiologists observed the association
between populations with high prevalence of the disease and the
proximity of mosquito infested swamplands. It was left to the
biological and medical scientists to isolate the organism causing
malaria in the blood of those with the disease and identify the
same organisms in the mosquito population.
In the case of athermal effects, some studies have identified
a weak association between exposure to EMF at home or at work
and various malignant conditions including leukemia and brain
cancer. However, a larger number of equally well designed and
performed studies have found no association. A risk ratio of between
1.5 and 2.0 has been observed in positive studies (the number
of observed cases of malignancy being 1.5 to 2.0 times the "expected"
number in the population). Epidemiologists generally regard a
risk ratio of 4.0 or greater to be indicative of a strong association
between the cause and effect under study. For example, men who
smoke one pack of cigarettes per day increase their risk for lung
cancer tenfold compared to nonsmokers, and two packs per day increase
the risk to more than 25 times the nonsmokers' risk.
However, epidemiological research by itself is rarely conclusive.
Epidemiology only identifies health patterns in groups-it does
not ordinarily determine their cause. And there are often confounding
factors: Most of us are exposed to many different environmental
hazards that may affect our health in various ways. Moreover,
not all studies of persons likely to be exposed to high levels
of EMR have yielded the same results.
There has also been considerable laboratory research about the
biological effects of EMR in recent years. For example, it has
been shown that even fairly low levels of EMR can alter the human
body's circadian rhythms, affect the manner in which cancer-fighting
T lymphocytes function in the immune system, and alter the nature
of the electrical and chemical signals communicated through the
cell membrane and between cells, among other things.
Much of this research has focused on low-frequency magnetic fields,
or on RF fields that are keyed, pulsed or modulated at a low audio
frequency (often below 100 Hz). Several studies suggested that
humans and animals can adapt to the presence of a steady RF carrier
more readily than to an intermittent, keyed or modulated energy
source. There is some evidence that while EMR may not directly
cause cancer, it may sometimes combine with chemical agents to
promote its growth or inhibit the work of the body's immune system.
None of the research to date conclusively proves that low-level
EMR causes adverse health effects. Given the fact that there is
a great deal of research ongoing to examine the health consequences
of exposure to EMF, the American Physical Society (a national
group of highly respected scientists) issued a statement in May
1995 based on its review of available data pertaining to the possible
connections of cancer to 60-Hz EMF exposure. This report is exhaustive
and should be reviewed by anyone with a serious interest in the
field. Among its general conclusions were the following:
"The scientific literature and the reports of reviews
by other panels show no consistent, significant link between cancer
and powerline fields."
"No plausible biophysical mechanisms for the systematic
initiation or promotion of cancer by these extremely weak 60-Hz
fields has been identified."
"While it is impossible to prove that no deleterious health
effects occur from exposure to any environmental factor, it is
necessary to demonstrate a consistent, significant, and causal
relationship before one can conclude that such effects do occur."
The APS study is limited to exposure to 60-Hz EMF. Amateurs will
also be interested in exposure to EMF in the RF range. A 1995
publication entitled Radio Frequency and ELF Electromagnetic
Energies, A Handbook for Health Professionals includes a chapter
called "Biologic Effects of RF Fields." In it the authors
state: "In conclusion, the data do not support the finding
that exposure to RF fields is a causal agent for any type of cancer"
(page 176). Later in the same chapter they write: "Although
the data base has grown substantially over the past decades, much
of the information concerning nonthermal effects is generally
inconclusive, incomplete, and sometimes contradictory. Studies
of human populations have not demonstrated any reliably effected
end point." (page 186).
Readers may want to follow this topic as further studies are reported.
Amateurs should be aware that exposure to RF and ELF (60 Hz) electromagnetic
fields at all power levels and frequencies may not be completely
safe. Prudent avoidance of any avoidable EMR is always a good
idea. However, an Amateur Radio operator should not be fearful
of using his equipment. If any risk does exist, it will almost
surely fall well down on the list of causes that may be harmful
to your health (on the other end of the list from your automobile).
Safe Exposure Levels
How much EM energy is safe? Scientists have devoted a great deal
of effort to deciding upon safe RF-exposure limits. This is a
very complex problem, involving difficult public health and economic
considerations. The recommended safe levels have been revised
downward several times in recent years-and not all scientific
bodies agree on this question even today. A new Institute of Electrical
and Electronics Engineers (IEEE) guideline for recommended EM
exposure limits went into effect in 1991 (see references). It
replaced a 1982 American National Standards Institute guideline
that permitted somewhat higher exposure levels. ANSI-recommended
exposure limits before 1982 were higher still.
This new IEEE guideline recommends frequency-dependent and time-dependent
maximum permissible exposure levels. Unlike earlier versions of
the standard, the 1991 standard recommends different RF exposure
limits in controlled environments (that is, where energy
levels can be accurately determined and everyone on the premises
is aware of the presence of EM fields) and in uncontrolled
environments (where energy levels are not known or where some
persons present may not be aware of the EM fields).
The graph depicts the new IEEE standard. It
is necessarily a complex graph because the standards differ not
only for controlled and uncontrolled environments but also for
electric fields (E fields) and magnetic fields (H fields). Basically,
the lowest E-field exposure limits occur at frequencies between
30 and 300 MHz. The lowest H-field exposure levels occur at 100-300
MHz. The ANSI standard sets the maximum E-field limits between
30 and 300 MHz at a power density of 1 mW/cm2 (61.4 V/m) in controlled
environments-but at one-fifth that level (0.2 mW/cm2 or 27.5 V/m)
in uncontrolled environments. The H-field limit drops to 1 mW/cm2
(0.163 A/m) at 100-300 MHz in controlled environments and 0.2
mW/cm2 (0.0728 A/m) in uncontrolled environments. Higher power
densities are permitted at frequencies below 30 MHz (below 100
MHz for H fields) and above 300 MHz, based on the concept that
the body will not be resonant at those frequencies and will therefore
absorb less energy.
In general, the IEEE guideline requires averaging the power level
over time periods ranging from 6 to 30 minutes for power-density
calculations, depending on the frequency and other variables.
The ANSI exposure limits for uncontrolled environments are lower
than those for controlled environments, but to compensate for
that the guideline allows exposure levels in those environments
to be averaged over much longer time periods (generally 30 minutes).
This long averaging time means that an intermittently operating
RF source (such as an Amateur Radio transmitter) will show a much
lower power density than a continuous-duty station for a given
power level and antenna configuration.
Time averaging is based on the concept that the human body can
withstand a greater rate of body heating (and thus, a higher level
of RF energy) for a short time than for a longer period. However,
time averaging may not be appropriate in considerations of nonthermal
effects of RF energy.
The IEEE guideline excludes any transmitter with an output below
7 W because such low-power transmitters would not be able to produce
significant whole-body heating. (However, recent studies show
that hand-held transceivers often produce power densities in excess
of the IEEE standard within the head.)
There is disagreement within the scientific community about these
RF exposure guidelines. The IEEE guideline is still intended primarily
to deal with thermal effects, not exposure to energy at lower
levels. A small but significant number of researchers now believe
athermal effects should also be taken into consideration. Several
European countries and localities in the United States have adopted
stricter standards than the recently updated IEEE standard.
Another national body in the United States, the National Council
for Radiation Protection and Measurement (NCRP), has also adopted
recommended exposure guidelines. NCRP urges a limit of 0.2 mW/cm2
for nonoccupational exposure in the 30-300 MHz range. The NCRP
guideline differs from IEEE in two notable ways: It takes into
account the effects of modulation on an RF carrier, and it does
not exempt transmitters with outputs below 7 W.
It is a widely held belief that cardiac pacemakers may be adversely
affected in their function by exposure to electromagnetic fields.
Amateurs with pacemakers may ask whether their operating might
endanger themselves or visitors to their shacks who have a pacemaker.
Because of this and similar concerns regarding other sources of
electromagnetic fields, pacemaker manufacturers apply design methods
that for the most part shield the pacemaker circuitry from even
relatively high EM field strengths.
It is recommended that any amateur who has a pacemaker or is being
considered for one discuss this matter with his or her physician.
The physician will probably put the amateur into contact with
the technical representative of the pacemaker manufacturer. These
representatives are generally excellent resources and may have
data from laboratory or "in the field" studies with
pacemaker units of the type the amateur needs to know about.
One study examined the function of a modern (dual chamber) pacemaker
in and around an Amateur Radio station. The pacemaker generator
has circuits that receive and process electrical signals produced
by the heart and also generate electrical signals that stimulate
(pace) the heart. In one series of experiments the pacemaker was
connected to a heart simulator. The system was placed on top of
the cabinet of a 1-kW HF linear amplifier during SSB and CW operation.
In addition, the system was placed in close proximity to several
1 to 5-W 2-meter hand-held transceivers. The test pacemaker connected
to the heart simulator was also placed on the ground 9 meters
below and 5 meters in front of a three-element Yagi HF antenna.
No interference with pacemaker function was observed in this experimental
system.
Although the possibility of interference cannot be entirely ruled
out by these few observations, these tests represent more severe
exposure to EM fields than would ordinarily be encountered by
an amateur with an average amount of common sense. Of course prudence
dictates that amateurs with pacemakers using hand-held VHF transceivers
keep the antenna as far from the site of the implanted pacemaker
generator as possible and use the lowest transmitter output required
for adequate communication. For high power HF transmission, the
antenna should be as far from the operating position as possible
and all equipment should be properly grounded.
Low-Frequency Fields
Recently, much concern about EMR has focused on low-frequency
energy rather than RF. Amateur Radio equipment can be a significant
source of low-frequency magnetic fields, although there are many
other sources of this kind of energy in the typical home. Magnetic
fields can be measured relatively accurately with inexpensive
60-Hz dosimeters that are made by several manufacturers.
Table 9.1 shows typical magnetic field intensities of Amateur
Radio equipment and various household items. Because these fields
dissipate rapidly with distance, "prudent avoidance"
would mean staying perhaps 12 to 18 inches away from most Amateur
Radio equipment (and 24 inches from power supplies with 1-kW RF
amplifiers) whenever the ac power is turned on. The old custom
of leaning over a linear amplifier on a cold winter night to keep
warm may not be the best idea!
There are currently no non-occupational US standards for exposure
to low-frequency fields. However, some epidemiological evidence
suggests that when the general level of 60-Hz fields exceeds 2
milligauss, there is an increased cancer risk in both domestic
environments and industrial environments. Typical home environments
(not close to appliances or power lines) are in the range of 0.1-0.5
milligauss.
Determining RF Power Density
Unfortunately, determining the power density of the RF fields
generated by an amateur station is not as simple as measuring
low-frequency magnetic fields. Although sophisticated instruments
can be used to measure RF power densities quite accurately, they
are costly and require frequent recalibration. Most amateurs don't
have access to such equipment, and the inexpensive field-strength
meters that we do have are not suitable for measuring RF power
density. The best we can usually do is to estimate our own RF
power density based on measurements made by others or, given sufficient
computer programming skills, use computer modeling techniques.
Table 9.2 shows a sampling of measurements made at Amateur
Radio stations by the Federal Communications Commission and the
Environmental Protection Agency in 1990. As this table indicates,
a good antenna well removed from inhabited areas poses no hazard
under any of the various exposure guidelines. However, the FCC/EPA
survey also indicates that amateurs must be careful about using
indoor or attic-mounted antennas, mobile antennas, low directional
arrays or any other antenna that is close to inhabited areas,
especially when moderate to high power is used.
Ideally, before using any antenna that is in close proximity to
an inhabited area, you should measure the RF power density. If
that is not feasible, the next best option is make the installation
as safe as possible by observing the safety suggestions listed
in Table 9.3.
It is also possible, of course, to calculate the probable power
density near an antenna using simple equations. However, such
calculations have many pitfalls. For one, most of the situations
in which the power density would be high enough to be of concern
are in the near field-an area roughly bounded by several wavelengths
of the antenna. In the near field, ground interactions and other
variables produce power densities that cannot be determined by
simple arithmetic.
Computer antenna-modeling programs such as MININEC or other codes
derived from NEC (Numerical Electromagnetics Code) are suitable
for estimating RF magnetic and electric fields around amateur
antenna systems. (See the Handbook's Propagation chapter for more
information about MININEC.) And yet, these too have limitations.
Ground interactions must be considered in estimating near-field
power densities. Also, computer modeling is not sophisticated
enough to predict "hot spots" in the near field-places
where the field intensity may be far higher than would be expected.
Intensely elevated but localized fields often can be detected
by professional measuring instruments. These "hot spots"
are often found near wiring in the shack and metal objects such
as antenna masts or equipment cabinets. But even with the best
instrumentation, these measurements may also be misleading in
the near field.
One need not make precise measurements or model the exact antenna
system, however, to develop some idea of the relative fields around
an antenna. Computer modeling using close approximations of the
geometry and power input of the antenna will generally suffice.
Those who are familiar with MININEC can estimate their power densities
by computer modeling, and those who have access to professional
power-density meters can make useful measurements.
While our primary concern is ordinarily the intensity of the signal
radiated by an antenna, we should also remember that there are
other potential energy sources to be considered. You can also
be exposed to RF radiation directly from a power amplifier if
it is operated without proper shielding. Transmission lines may
also radiate a significant amount of energy under some conditions.
Further RF Exposure Suggestions
Potential exposure situations should be taken seriously. Based
on the FCC/EPA measurements and other data, the "RF awareness"
guidelines of Table 9.3 were developed by the ARRL RF Safety Committee.
A longer version of these guidelines, along with a complete list
of references, appeared in a QST article by Ivan Shulman,
MD, WC2S (see References).
In addition, QST carries information regarding the latest
developments for RF safety precautions and regulations at the
local and federal levels.
