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Nonionizing Electromagnetic Fields and Cancer: A Review

Nonionizing Electromagnetic Fields and Cancer: A Review

ABSTRACT: Low-frequency electromagnetic radiation had previously been thought to cause human injury only by generation of excess heat or by shock from direct contact with electric current. Information accumulating over the past few decades, however, suggests that nonionizing electric and magnetic fields associated with this radiation may be an environmental etiology in human disease. Human beings are affected not only by natural background nonionizing electromagnetic fields produced by the Earth, but also by a host of manmade sources. Of the diseases believed related to these fields, cancer and participation of these fields in the carcinogenic process have received considerable attention. This paper is a review of the basic science that points to this possible association. [ONCOLOGY 10(4):563-578, 1996]

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].

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