RF Radiation and Electromagnetic Field Safety (I)

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.

First, let's define some terms in order to well clarify the subject and prevent any interpretation in a physics matter not always easy to vizualise. It constitutes a good reminder too.

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Definitions

An electromagnetic field (EM) is generated when charged particles, such as electrons are accelerated. By nature these charged particles are surrounded by an electric field. Once these particles are in motion they produce a magnetic field. So, when these particles change of velocity (accelerate or slow down), an EM field is produced.

The electromagnetic spectrum is divided in frequencies. From an electrical point of view, the spectrum has been arbitrarily divided into three main bands or fields : Extremely Low Frequency (ELF) fields usually concern all frequencies up to 300 Hz. Intermediate Frequency (IF) fields concern all frequencies from 300 Hz to 10 MHz and Radiofrequency (RF) fields concern frequencies between 10 MHz and 300 GHz. The effects of electromagnetic fields on the human body depend not only on the concerned field intensity but on its frequency and energy as well.

The electric field (or E component of an EM field) exist whenever charge is present. Its strength is measured in volt per metre (V/m or dBμV/m). An electric field of 1 V/m is represented by a potential difference of 1 V existing between two points that are 1 m apart. The V/m is primarly used to express the intensity of the EM field.

Magnetic field (or B component of an EM field) arises from current flow. Its flux density (intensity per surface unit) ) is measured in weber/m2 but more often in tesla (T) in the International System (SI), and in gauss (G) is the old CGS system of units (1 G = 10-4 T or 1 μT = 10 milligauss). A flux density of 1 G represents 1 maxwell/cm2. The tesla (or the gauss) is mainly used to expres the flux density produced by magnets commonly encountered in consumer products (monitor, microwave oven, etc). On the earth' surface, the flux density of the magnetic field is always less than 1 G.

At radio frequencies, electric and magnetic fields are closely interrelated and we measure their power densities in watt per square metre (W/m2, usually expressed in mW/cm2).

Electromagnetic fields

Life on Earth has adapted to survive in an environment of weak, natural low frequency electromagnetic fields (in addition to the static geomagnetic field and natural radioactivity). Natural low-frequency EM fields come from two main sources: the sun, and thunderstorm activity. But since the begin of the XXth century, man-made fields of much higher intensities and distributed in a very wide spectrum have superimposed to this natural EM background in ways that are not yet fully understood. Much more research is needed to assess the biological effects of EMR.

 Some applications of the electromagnetic spectrum. Document HowStuffWorks modified by the author.

Both RF and 50 or 60-Hz fields are classified as non-ionizing 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.

 Thermal image of an aluminium housing to shield a RF module. Document Compix.

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 carrier is present continuously at its maximum level during each transmission). In any event, it is rare for radio amateurs to be subject 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.

Temperature of semiconductors : never too hot

In a pure technical point of view, remember that the temperature of a CPU and any other electronic semiconductor components is a critical factor that affects the good functioning of your equipment, hence the use of a fan on both the CPU and on the cabinet of your transceiver, amplifier or the one of your computer, to extract the excess of heat. These are not simple accessories; they are mandatory and must remain operational if you want that your system works in good conditions.

 Thermal radiation from electronic components. In domestic devices like a radio or a computer there is no risk for the health but avoid to touch them as temperature can reach 95°C (e.g. on a fast CPU) ! Remember that a 10°C increase in junction temperature reduces by half the lifespan of a semiconductor. Documents Thermal Imaging Survey and Sierra Pacific Corp.

We usually say that every 10°C rise in junction temperature will cut the mean time between failure (MTBF) of a semiconductor in half. So, as we all expect a very long life for our ham equipment, better to use oversized and powerful fans, and why not adding an extra fan on the cabinet if you noticed that your system is rather hot after have used it heavily.

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 EMR 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. Their conclusions have not changed with years.

 Electromagnetic field of a dipole tight 1/2λ high. At left a sideview, at right from the end. Plots calculated with MathCad by KB7QHC.

Among its general conclusions were the following:

1. The scientific literature and the reports of reviews by other panels show no consistent, significant link between cancer and powerline fields.

2. No plausible biophysical mechanisms for the systematic initiation or promotion of cancer by these extremely weak 60-Hz fields has been identified.

3. 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 (50/60 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 or her 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).

 Electromagnetic field of a 1/4λ vertical. At left a sideview, at right from top. Plots calculated with MathCad by KB7QHC.

Safe Exposure Levels

Under what energy level an EM 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.

The main bodies involved in these measurements are next :

- ANSI, American National Standards Institute

- IEEE,  Institute of Electrical and Electronics Engineers

- ICNIRP, International Commission on Non-Ionizing Radiation Protection

- NCRP, National Council on Radiation Protection and Measurements.

Among the supranational institutions having a consultative or legal function, name :

- WHO, World Health Organization

- Underwriters Laboratory that tests and certify devices

- European Parliament and European Commission that edicte general rules and impose their application in Europe.

A new IEEE guideline for recommended EM exposure limits went into effect in 1991 (see IEEE C95.3-2002). It replaced a 1982 ANSI 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 next 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 and magnetic fields.

 This graph is extracted from 2002 (data 1991) RF guide titled "IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz", ref. IEEE Standard C95.3-2002.

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.

 Predicted radiation directivity pattern (RDP) of an S-band antenna array (3.4 GHz). This chart represents the front hemisphere RDP for the entire feed+dish system. Document Lambda Science, Inc.

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 US body is the NCRP that 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 watts.

Cardiac Pacemakers and RF Safety

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 as briefly described at left.

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.

Second part

Low-Frequency Fields

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