Occupational setting

The four types of devices that generate RF and MW energy are power grid tubes, linear beam tubes (klystrons), crossed-field devices, and solid-state devices. Sources of RF and MW energy can operate in three modes: continuous, intermittent, and pulsed. The continuous mode is used in some communication devices, the intermittent mode is used in heating devices, while the pulsed mode is used in radar and digital communication.

MW energy can be transmitted from the generating device through a wave guide or through a transmission line to an applicator or antenna. Microwaves are used to transmit signals in telecommunications, navigation, radar, and broadcasting (i.e., radio and television); they can also be used to produce heat in industrial and home microwave ovens and dielectric heaters (i.e., heaters used to heat electrically nonconductive materials by means of a rapidly alternating electromagnetic field).

Cellular telephones operate at frequencies between 800 and 2 200 MHz. Home microwave ovens use a microwave frequency of 2.45 GHz. Dielectric heaters are used in the manufacture of automobiles, furniture, glass fiber, paper products, rubber products, and textiles. RF dielectric heater applications include sealing and molding plastics; drying glues after manufacturing; drying textiles, paper, plastic, and leather; and curing materials such as epoxy resins, polymers, and rubber. Video display terminals (VDT) can generate RF radiation (at ~10–30 kHz) because they have a cathode ray tube, which is a source of electrons (measured levels have been extremely low). Industrial welding also generates RF radiation, typically ~400 kHz. RF radiation is also used for diathermy (deep-tissue heating) applications in medical treatment.

Measurement issues

An electromagnetic wave results from the combination of electric and magnetic field vectors, each perpendicular to the other, that produce a wave that is propagated perpendicular to the first two vectors (see Chapter 1, Figure 1.1). The power density or energy of the wave is derived from the measured intensity of the electric and magnetic field vectors. The total energy of the wave is expressed in milliwatts per square centimeter (mW/cm²). The individual field strengths can be measured; the electric field strength is measured in volts per meter (V/m), and the magnetic field strength is measured in amperes per meter (A/m). Whole body absorption is also measured/estimated as specific absorption rate (SAR) and expressed as watts per kilogram (W/kg).

Measurements are most often made using meters with frequency ranges from 2 kHz to 40 GHz. These meters yield point/spot measurements of the strength of either the electric or the magnetic fields. From there, the power density in mW/cm² is calculated. The instruments do not directly measure power density but have sensing probes that measure voltages or currents. These are usually displayed in V/m or A/m, which are then converted to power density as W/m² or mW/cm².

At frequencies above 300 MHz, the primary measurement is the electric field. Below 300 MHz, separate electric and magnetic field measurements must be made and combined. The reasoning behind this methodology is complicated, but it is related to whether the measurement is taken in the “near” field or the “far” field. In the far field (greater than one wavelength from the emitter), the ratio between the magnetic and electric fields is constant. When measured in the near field (less than one wavelength from the emitter), the ratio between the electric and magnetic field varies and both need to be measured. At 300 MHz, the corresponding wavelength is 1 m. As the frequency decreases, the wavelength (and the length of the near field) increases. This increases the chance that measurements are made in the near field. Personal dosimeters are also available to measure exposures.

Exposure guidelines

The exposure standards for RF and MW radiation are based on the assumption that the primary way that energy is absorbed for these frequencies is by heat deposition. Power absorption and heat deposition are affected by the following factors:

• Frequency of the radiation
• Body position relative to wave direction
• Distance between body (target) and source (MW energy generally decreases with the inverse of the square of the distance from the energy source)
• Exposure environment (surrounding objects may reflect, resonate, or modify incident waves)
• Electrical properties of the tissue (conductivity and dielectric constant).

The dielectric constant is a measure of the “permittivity” of the tissue; it measures the ratio of the amount of electric current that will flow in a specific medium versus the amount that will flow in a vacuum. Tissue electrical properties are constant and depend on water content. Higher energy absorption occurs in tissues with higher water (higher dielectric constant) contents, such as brain, muscle, and skin; lower energy absorption occurs in tissues with lower water (lower dielectric constants) contents, such as bone and fat.

The boundaries between tissues can reflect the energy waves differently, resulting in “hot spots” that can cause localized injury. Changes in position can change the amount of energy absorbed, because the body acts as an antenna to receive the MW or RF energy.

The Occupational Safety and Health Administration (OSHA)’s Nonionizing radiation regulation (29 CFR 1910.97)¹ specifies an exposure limit of 10 mW/cm² over a 6-min period (or longer) for frequencies between 10 MHz and 100 GHz.² There is also a consensus ACGIH standard (Table 15.1) that applies to a much broader frequency range. The ACGIH standards are derived on the basis of an SAR equivalent to 4 W/kg to which a safety factor of 10 is applied. This standard is based on data that show no effects from exposures at 4 W/kg over the 6-min period. Accordingly, the resulting standard is based on an SAR of 0.4 W/kg. This relates to comparison energy values of 1 W/kg for an individual at rest and 5 W/kg when the person is exercising.

Because it is difficult to measure SAR directly, the standard is expressed in measurable quantities such as power density in mW/cm² or field strengths in V/m (or V²/m²) or A/m (or A²/m²). These power densities and field strengths represent allowable exposures that do not exceed the SAR. The standard, in effect, allows an equivalent power density of 1 mW/cm² for frequencies between 100 and 300 MHz. The exposures are averaged over 6 min; and the electric and magnetic fields must be measured separately below 300 MHz. Exposures at frequencies from 300 MHz to 3 GHz are calculated according to the formula P = f/300 (allowable power density in mW/cm² equals the frequency in MHz divided by 300).

The penetration of energy is a function of frequency with penetration depth of ~1.7 cm at 2.45 GHz in comparison with 2–4 cm of penetration at 0.915 GHz. Although there is greater penetration depth at lower frequencies, the resulting heating decreases with frequency. Below 10 MHz, the body is essentially transparent to RF radiation, and little heating takes place. The resonance frequency, or frequency that generates the greatest energy deposition, occurs at ~70 MHz.

Pathophysiology of injury

Biological tissues respond to MW and RF radiation exposure with the induction of their own electric and magnetic fields. Depending on the polarity of the biological molecules, rotation and agitation of molecules can occur, resulting in heat generation. Thermal injury can occur as a result of RF/MW exposure when exposures are in excess of 10 mW/cm². Because internal hot spots may result from internal resonance due to differences in dielectric properties or radiation reflection, there may be localized increases in energy absorption and heating. The net heating of the body is related to the amount of energy absorbed minus the amount lost through the usual heat-dissipating mechanisms (blood flow, evaporation, radiation, convection, and conduction). The phenomenon of MW clicking (“hearing” MW radiation), originally thought to be a nonthermal response, appears to be caused by thermal elastic expansion and contraction of the cochlea.

The least likely tissues and organs to be affected are those with greater thermal regulatory ability, usually due to increased blood flow and greater heat-dissipation potential. RF burns, which are the most frequently encountered industrial effects, usually involve the skin. Subcutaneous tissue heating usually occurs simultaneously with skin exposure. Where full-thickness skin burns occur and subcutaneous tissues are involved, healing may be prolonged due to the lack of base granulation tissue. Skin burns appear similar to sunburn. The patient may initially present with a feeling of warmth, as if the skin or exposed portion were being heated. Within hours, redness and slight induration can occur. The course is usually characteristic of any thermal burn, but it may include vesiculation and ulceration. Similarly, thermogenic exposures have been associated with cataract formation in exposed workers, but cataracts have not occurred following exposures below recommended limits. There are case reports of massive exposures to MW and other RF sources that resulted not only in eye or skin burns but also in symptoms of neurasthenia or posttraumatic stress-like disorders (recurrent headache, malaise, fatigue, depression). Hypertension and/or peripheral neuropathy has also been reported.^2,3

The proliferation of cellular telephones and their potential MW emission when held within a few centimeters from the brain have resulted in concern and investigation of possible health effects. Cell phones are now used by over 90% of American adults. The conclusion of Corle’s (2012) review and analysis of studies of cell phone use and glioma risk summarizes the current knowledge relating to cell phones and brain cancer risk:

Despite the results pointing to an association in one direction or another, it is clear that there is no definite answer to the question of whether cell phone use is associated with increased brain cancer risk. Notwithstanding the inconsistencies in the epidemiological studies, a few of the human studies do suggest an association between cell phone use and brain tumors for a 10 year or greater induction period and/or a high number of cumulative call hours.⁴

There have been anecdotal reports of nonthermal effects from MW and RF exposures. These include carcinogenic, reproductive, hematopoietic, immunologic, neurologic, neuroendocrine, and psychological effects. These bioeffects have been reviewed extensively, but a specific nonthermal mechanism has not been identified. Because of inconsistent and conflicting animal and human data, genotoxicity, carcinogenicity, reproductive toxicity, and other systemic/organ effects have not been clearly linked to nonthermal exposures.^5,6 Similarly, extensive research of reproductive effects in relation to VDT use has not found an association.⁷ However, continued positive findings in some cancer studies have led IARC to designate radio frequency radiation as a category “2-B” carcinogen. Specifically, IARC concluded: “Radiofrequency electromagnetic fields are possibly carcinogenic to humans (Group 2B).”⁸

Treatment

The medical response to MW/RF radiation exposure should involve (i) removal from exposure, (ii) determination of radiation frequency and exposure intensity, and (iii) medical treatment for thermal injury to skin or subcutaneous tissues. High-intensity exposures can lead to deep-tissue injury. Localized subcutaneous hot spots and deeper-penetration heating may make such thermal injury more difficult to evaluate. There have been reports of burning of the skin with undamaged subcutaneous fat and burned muscle tissue below the fat layer. If a high-intensity exposure is suspected, tests for deeper-tissue injury, such as creatine phosphokinase (CPK) to evaluate muscle injury, can be performed. Tests of specific organ function can also be ordered if injury is suspected. Routine burn management can be followed for superficial burns. Deeper or more serious burns should be referred to a burn specialist for specific medical treatment and follow-up.

Medical surveillance

Because no effects have been consistently demonstrated following long-term low-intensity exposures, periodic examination would not yield findings that would indicate a need for preventive actions. Given the inconsistencies and lack of scientific consensus regarding nonthermal effects, there is no basis for any periodic monitoring. Following an acute high-intensity exposure that results in thermal injury, appropriate follow-up should be instituted. Other than that predicted based on the thermal tissue effects, sequelae to that injury would not be expected.⁹

Prevention

Identification of RF/MW exposure in excess of recommended levels should be accomplished using available instrumentation that creates a plot of the potential fields and intensities. These measurements should occur at the time of initial equipment use and following any equipment changes thereafter.

Engineering controls include partial enclosure and elimination of leakage. Enclosing an area with wire mesh and sealing the seams with copper tape is a common engineering measure. Care must be taken to ensure that enclosures do not allow leakage. Where enclosures are not sufficient to reduce potential exposures, identification of the distance necessary for adequate energy dissipation can be effective. For example, hazard zones can be clearly marked surrounding the MW source. Although some personal protective equipment, such as eyewear and clothing, has been developed, its effectiveness is controversial, and it is not usually recommended.

There are no specific pregnancy-related recommendations for RF and MW exposures. The current recommendation of a SAR of 0.4 V/kg limits exposures to levels below those that cause significant thermal effects. Studies of the reproductive effects of MW and RF radiation have shown fetal loss and teratogenesis postexposure. However, most positive studies used exposures in the >100-V/kg range and were associated with internal temperature increases of 2–10°C.¹⁰

Heat has adverse effects on the testis and can also cause decreased spermatogenesis.¹¹ Exposures incapable of thermal effects are not considered reproductive hazards.

Occupational setting

ELF energy is generated from electric power transmission and most household appliances. Workers with potential ELF exposure include electrical and electronic engineers and technicians; electric power line, telephone, and cable workers; electric arc welders; electricians; television and radio repair workers; power station operators; and motion picture projectionists. Magnetic fields and ELF electromagnetic radiation form the lowest end of the electromagnetic spectrum and include radiation at frequencies from 0 to 300 Hz. Concern regarding health effects in this portion of the electromagnetic spectrum centers around the power frequencies of 50 or 60 Hz (wavelength of 5000 km). Although it is composed of an electric and a magnetic field, the electric field does not have significant human tissue penetration.

Measurement issues

A magnetic field is formed whenever there is a flow of electric current. A static magnetic field occurs as a result of direct voltage or direct current (DC), while a time-varying magnetic field results from alternating current (AC). Magnetic field intensities are measured with a unit of magnetic flux called tesla or gauss (10 000 gauss = 1 T, 10 gauss = 1 mT). Magnetic fields freely penetrate many materials, including biological systems, and are very difficult to shield. Time-varying magnetic fields are of greatest interest because of their suggested association with molecular biological perturbation and health effects. Sources of time-varying magnetic fields include any device that utilizes an AC energy source, such as appliances, electrical equipment, and high-power transmission lines.

Two types of instruments are available for the measurement of magnetic fields: one is a multiturn loop used with a portable voltmeter and the other is a gauss meter. Otherwise, the measurement issues are similar to those discussed in the MW/RF section. Because cell membranes are relatively poor electrical conductors, the internal electric field is reduced between l0⁶- and 10⁸-fold compared to the external field. External fields of 1 million V/m would be needed to produce an internal field on the order of the existing transmembrane potential. Because of the tremendous attenuation of the electric field, it is not measured when evaluating ELF exposures. Personal dosimeters (EMDEX models A, B, and C; Electric Field Measurements, West Stockton, MA, United States) are also available to measure exposures in the 35–300-Hz band of frequencies.

Exposure guidelines

The ACGIH has recommended threshold limit values for electric and magnetic fields in the 1–30-kHz range. At exposures between 1 and 300 Hz, whole body exposure limits are determined by the formula B = 60/f where B is the limit in millitesla (mT) and f is frequency. At 60 Hz, this would result in a 1-mT exposure (10 gauss). For frequencies between 300 Hz and 30 kHz, the whole body exposure limit is 0.2 mT (2 gauss) (1 gauss = 0.1 mT).

The recommended standard for the electric field varies with frequency and is designed to both prevent induced internal currents and eliminate spark discharges and other safety hazards that take place at field strengths >5–7 kV/m.

Natural background exposure from the static Earth’s field is ~450 mG, and it may change by as much as 0.5 mG per day due to changes in solar activity. The Earth’s electric field is ~120 V/m, which is comparable to the field found under a typical 12-kV urban power distribution line. Disturbances in the Earth’s local electric field are commonly found in the form of lightning, which needs at least 3 million V/m to ionize the air.

Electric and magnetic fields at the edges of a restricted right-of-way have been characterized by many public utilities. Typical electric and magnetic fields from the Bonneville Power Administration are reviewed in Table 15.2.¹² Table 15.3 summarizes some of the other typical electric and magnetic fields that might be encountered.

Pathophysiology and health effects

Many biological effects have been proposed to be associated with ELF radiation. Savitz and Calle first aroused interest in this area with their 1979 review of the incidence of leukemia in workers exposed to high electromagnetic fields.¹³ Thereafter, this interest was heightened when a two to fourfold increase in childhood leukemia in the Denver area was attributed to ELF exposures in a report, also published in 1979, by Wertheimer and Leeper.¹⁴ Their study linked high childhood cancer rates to ELF exposures by wiring code configurations (WCC), which were used as a surrogate for ELF exposure.

Good examples of childhood residential studies have been published by Savitz et al. (Denver) in 1988 and London et al. (Los Angeles) in 1991.^15,16 In the Denver study, spot magnetic field measurements were used in addition to WCC. In the Los Angeles study, spot measurements and 24-hour magnetic field measurements were recorded. Based on WCC data, the relative risk for leukemia in the high- versus low-current classification was 1.54 (95% confidence interval 0.9–2.63) in the Denver study and 1.73 (95% confidence interval 0.82–3.66) in the Los Angeles study. Both these studies show a rise in the relative risk with increasing WCC. However, a significantly increased risk was not demonstrated in either study when the risk was assessed in relation to measured magnetic fields.

Since 1979, over 1000 articles have been published in the area of ELF effects. Research papers can generally be divided into the following areas: (i) human epidemiologic studies of cancer, focusing on childhood and adult residential exposures and on adult occupational exposures; (ii) effects on growth control; (iii) neurobehavioral effects; and (iv) other physiologic effects.

Overall, research has yielded conflicting results about the presence (or absence) of an association between ELF and health effects. For example, residential/nonoccupational studies of adults have a number of epidemiologic shortcomings centered on the confounding factors of occupational versus home/other exposures, and they have not consistently shown an increased risk with measured ELF exposures. Nonoccupational exposures include exposures from home wiring, electrical distribution lines and substations, transportation equipment (i.e., electric trains and buses), and home appliances (i.e., hot plates, refrigerators, hair dryers, electric blankets, and any other electrically powered device). Many of the studies negative for cancer were performed with field strengths well above what should have been considered typical for an occupational or residential exposure. The two effects that have been reported at typical residential exposure levels (Table 15.4) are changes in calcium ion flux and inhibition of melatonin secretion.^17,18

Epidemiologic studies suggest a weak association between ELF fields and childhood leukemia. Kheifets et al. reviewed childhood leukemia ELF research and found small overall elevations in relative risks (<1.5).¹⁹ Studies of ELF association with other cancers (e.g., breast) have been negative or inconclusive.^20,21 The effect of ELF on reproduction has also been studied. Huuskonen et al.’s review concluded that “the epidemiologic evidence does not, taken as a whole, suggest strong associations between exposure to ELF magnetic fields and adverse reproductive outcome. An effect at high levels of exposure cannot be excluded, however.”²² Other conditions, including multiple sclerosis and Alzheimer’s disease, have not been shown to be associated with ELF magnetic field exposure.²³ The biological plausibility of ELF effects rests on the ability of the magnetic field to interact with the body at the cellular level. Although the induced ELF fields are weak, the evidences of changes in calcium ion fluxes, increased rates of bone healing, and changes in melatonin secretion suggest that magnetic fields are biologically active.

IARC’s most recent (2002) review of the available ELF and cancer data concluded as follows:

There is limited evidence in humans for the carcinogenicity of extremely lowfrequency magnetic fields in relation to childhood leukaemia. There is inadequate evidence in humans for the carcinogenicity of extremely lowfrequency magnetic fields in relation to all other cancers. There is inadequate evidence in humans for the carcinogenicity of static electric or magnetic fields and extremely low-frequency electric fields. There is inadequate evidence in experimental animals for the carcinogenicity of extremely low-frequency magnetic fields. No data relevant to the carcinogenicity of static electric or magnetic fields and extremely low-frequency electric fields in experimental animals were available.

Overall evaluation

Extremely low-frequency magnetic fields are possibly carcinogenic to humans (Group 2B).

Static electric and magnetic fields and extremely low-frequency electric fields are not classifiable as to their carcinogenicity to humans (Group 3).²⁴

The 2007 report published by the World Health Organization reflects the current knowledge regarding the biological effects of ELF magnetic fields.²⁵ The summary of the WHO report concludes:

Although a causal relationship between magnetic field exposure and childhood leukaemia has not been established, the possible public health impact has been calculated assuming causality in order to provide a potentially useful input into policy. However, these calculations are highly dependent on the exposure distributions and other assumptions, and are therefore very imprecise. Assuming that the association is causal, the number of cases of childhood leukaemia worldwide that might be attributable to exposure can be estimated to range from 100 to 2400 cases per year. However, this represents 0.2 to 4.9% of the total annual incidence of leukaemia cases, estimated to be 49 000 worldwide in 2000. Thus, in a global context, the impact on public health, if any, would be limited and uncertain.

A number of other diseases have been investigated for possible association with ELF magnetic field exposure. These include cancers in both children and adults, depression, suicide, reproductive dysfunction, developmental disorders, immunological modifications and neurological disease. The scientific evidence supporting a linkage between ELF magnetic fields and any of these diseases is much weaker than for childhood leukaemia and in some cases (for example, for cardiovascular disease or breast cancer) the evidence is sufficient to give confidence that magnetic fields do not cause the disease.

In their review published in 2015, the European Commission’s Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR) incorporated newer research in their analysis and reached similar conclusions.²⁶ The main findings of their report are²⁷:

Overall, the epidemiological studies on radiofrequency EMF exposure do not show an increased risk of brain tumours. Furthermore, they do not indicate an increased risk for other cancers of the head and neck region.

Previous studies also suggested an association of EMF with an increased risk of Alzheimer’s disease. New studies on that subject did not confirm this link.

Epidemiological studies associate exposure to Extremely Low Frequency (ELF) fields, from long-term living in close proximity to power lines to a higher rate of childhood leukaemia. No mechanisms have been identified and no support from experimental studies could explain these findings, which, together with shortcomings of the epidemiological studies prevent a causal interpretation.

Concerning EMF hypersensitivity (idiopathic environmental intolerance attributed to EMF), research consistently shows that there is no causal link between self-reported symptoms and EMF exposure.

Prevention

Even though there are no proven adverse health effects related to EMF and ELF, a number of simple steps may be taken to reduce exposure without significant expense. This is known as the strategy of “prudent avoidance.” Identifying high-voltage transmission equipment at the worksite will help to focus on the kinds of monitoring that may be necessary. It would be prudent to reduce exposures to below the recommended levels. Distance is the best control since the field strength falls inversely with the square of the distance. Shielding is more difficult because magnetic fields react differently to different metals and different metal configurations (e.g., screen, mesh, sheet metal). In some cases, rewiring to oppose adjoining field polarities may diminish exposure, but this procedure can often be prohibitively expensive.