Nonionizing Electromagnetic Fields and Cancer: A Review

Introduction

Oncologists are being asked to evaluate the possibility that a patient's malignancy, or a malignancy cluster, is causally related to exposure to nonionizing electric and magnetic fields. The answer to this question is unresolved, due to an incomplete and inconclusive preliminary research data base. There is a deceiving quietness about these fields compared with known carcinogenic agents like chemicals, which are generally easier to detect and more amenable to experimentation. Dose-response studies of chemical and physical carcinogens usually show a linear dose response, a characteristic that may not be true of electromagnetic fields. Some studies have suggested distinctive frequency zones of effect instead of linearity [1].

Although we are constantly bathed in electromagnetic fields, especially 60-Hz fields (Hz = cycles per second) related to electric power, field strengths generally reach high levels before we are aware of interaction with our physiology. Magnetic fields can induce electric currents in the body, but those too may be undetected. The human body generates its own internal electric currents and fields, measured routinely by ECG and EEG. The brain generates measurable electrical activity at four frequency ranges: alpha at 8-13 Hz; beta at greater than 13 Hz; theta at 4-7 Hz; delta at less than 4 Hz.

Nonionizing electromagnetic fields have been used in medical practice for many years; examples include ultrasound therapy of musculoskeletal disease [2], radiofrequency radiation ablation of abnormal heart rhythms [3], pulsed electromagnetic field treatment of orthopedic diseases [4], and radiofrequency- or microwave frequency-induced hyperthermia in antineoplastic therapy, either alone or in conjunction with ionizing radiation therapy [5] or chemotherapy [6].

Recent attention has focused on nonionizing, nonthermal electromagnetic fields as an environmental etiology of human disease. Research suggests that electromagnetic fields may influence the immune system [7], behavior [8], pregnancy [9], chronobiology [10], and cancer [11]. A number of case reports of diseases or symptoms related to exposure to these fields have been discussed [12]. Reaction to available information by the scientific-medical community and the public has been mixed, generating legal and regulatory activity and, within some groups, a rapid acceptance of the idea that these fields fit into the carcinogenic cascade.

In the United States and other countries, safe exposure guidelines for the fields generated by high-voltage electric power transmission lines, as well as other nonionizing electromagnetic fields, are available, but these are revised periodically [13]. Through the courts, legal action has attempted to associate exposure to nonionizing electromagnetic fields with the development of leukemia, lymphoma, and primary brain neoplasm. There are no good theoretical correlates at present to explain this wide range of neoplasias believed to be related to electromagnetic field exposure. Although most researchers in the field believe that electromagnetic fields are not a neoplastic initiator, considerable research has focused on whether these fields are involved in the promotional stages of human carcinogenesis [14].

Nonionizing Electromagnetic Fields: Background

Living organisms have evolved on the Earth while exposed to nonionizing electromagnetic fields produced by the Earth's natural conditions. The Earth generates a geoelectrostatic field of 130 to 150 volts/meter (V/m), which can vary by about 15%; much larger changes occur during geoelectrostatic disturbances [15]. Atmospheric disturbances may produce fields increasing to thousands of volts per meter [16]. The Earth also normally generates a weak static (non-time varying) magnetic field with a fairly stable magnetic flux density of 30 to 50 microtesla (mcT), varying by only about 0.1% daily [15,16]. Clinicians routinely expose patients to magnetic fields of 1 to 2 T during diagnostic magnetic resonance imaging, and these are considered safe [17].

Of the electromagnetic fields studied, those of 60-Hz electric power transmission frequency have received considerable attention because of their omnipresence and epidemiologic studies suggesting a relationship between these fields and the development of cancer [18,19]. The spectrum of frequencies comprising electromagnetic radiation extends from the lowest frequencies, represented by electric power transmission at 50 (Europe) and 60 (USA) Hz, to higher-end frequencies of ionizing radiation at 10²ºHz)(Table 1).

Ionization potential indicates that the radiation possesses sufficient energy to cause ejection or displacement of orbital electrons from atoms or molecules [20], and the energy content of electromagnetic radiation ("photon energy") is directly proportional to its frequency [21]. At frequencies greater than 1 million gigahertz, electromagnetic radiation is ionizing. Table 2 shows the relationship between electromagnetic frequency and another measure of the energy content of the electromagnetic radiation, electron-volts (eV).

The amount of electron-volt energy of the electric power transmission frequencies and microwaves is minuscule when compared with that of ionizing radiation or higher frequency nonionizing electromagnetic radiation. This is one reason that nonionizing electromagnetic fields are not considered to be carcinogenic initiators.

Common daily exposure to low-frequency electromagnetic fields occurs not only from electric power transmission lines and electric wiring inside and outside the home, but from in-home and occupational devices and appliances that use 60-Hz electric power frequencies, such as electric blankets, kitchen and bathroom appliances, computers and video display terminals (VDTs), and telephones (cellular, cordless). Although these sources generate both electric and magnetic fields, it is the magnetic field component that researchers believe may be involved in the promotional stage of carcinogenesis. The strength of a magnetic field is commonly measured in milligauss (mG) or microtesla (Table 3).

Table 4 shows a representative sampling of magnetic fields, measured in microtesla, produced by various devices and appliances at two distances. The important point to be gleaned from Table 4 is how rapidly field strength falls off when moving away from the field source. The number of persons actually exposed to the very highest kilovolt (kV) electric power transmission lines is low. Table 5 shows the rapid fall off of magnetic field strength in milligauss and electric field strength in kilovolt/meter from high-voltage electric power transmission lines carrying different voltages.

A well-known effect of the application of electromagnetic fields in science and clinical medicine is the generation of heat. This is particularly important in deep heating treatment for physical therapy and in the generation of antineoplastic hyperthermia. Although this is accomplished at frequencies much greater than the 60-Hz electric power transmission frequencies, it is believed that the possible involvement in the carcinogenic process by electromagnetic fields is in the nonthermal mode.

Ionizing radiation is either an electromagnetic wave-like radiation or particulate radiation. Particulate radiations, including charged particles such as electrons, protons, alpha particles, or heavy ions, appear to ionize directly; ionizing electromagnetic radiations (ie, x-rays and gamma radiation) energize orbital electrons, producing fast recoil electrons and resulting in ionization [20]. Studies of mutational spectra have shown that the greatest percentage of changes induced by ionizing radiation are deletions [22].