Cutaneous Perception

Cutaneous perception is one of the best-studied of the effects of electromagnetic fields (EMFs) on humans. At extremely low frequencies, one perceives the presence of electric fields through stimulation of vibration or pressure sensors. Studies of electric-field perception at 50-60 Hz have yielded thresholds of 2-30 kV/m, depending on the subject, posture, and humidity.^6,7,30 The threshold for cutaneous perception of ELF magnetic fields is unknown, but Graham et al.⁶ report that seated subjects were unable to detect the presence of 60-Hz magnetic fields up to 32 A/m (= 400 mG = 40/µT).

The detection of LF currents flowing through the skin is reviewed in Chapter 3. Such currents can arise from contacts with ungrounded metal structures near GWEN relay nodes (RNs). Briefly, the sensation of LF currents passed through either a fingertip or a gripped contact is one of warmth, with perception thresholds of 20-300 mA, depending on the subject and type of contact.^4,5 Chapter 3 also shows that currents arising from contact with vehicles, fences, and other large conducting objects that might be in the vicinity of a GWEN RN are below the threshold of perception for any type of contact. Cutaneous-perception thresholds for incident EMFs at ultra-high frequencies (UHF) comparable to the GWEN transmitters (225-400 MHz) have not been measured, but, because of differences in penetration depth, are expected to be somewhat above the 15-to 44 mW/cm² threshold measured by Justesen for 2,450-MHz fields applied to the forearm.¹¹ That is roughly 10,000 times the UHF power density at the boundary of GWEN RNs.

Phosphenes

ELF currents passing through the retina cause the perception of images called phosphenes. The effect is noted only in the ELF band and can be induced by either direct coupling or induction. Thresholds for magnetically-induced phosphenes range from 8 mT at 20 Hz to 20 mT at 60 Hz.^12,31-33 The minimum retinal current density required to produce the effect is less than 1 µA/cm² at 20 Hz. Although GWEN LF and UHF fields can induce current densities up to 1 µA/cm², GWEN frequencies are too high for phosphenes to be observed.

Pacemaker Interference

Many older models of cardiac pacemakers are susceptible to electromagnetic interference. The threshold of susceptibility depends on the device design and on the frequency and amplitude of the interference. Measurements with a variety of pacemakers available around 1975, for instance, showed interference thresholds as low as 6 V/m for pulsed fields at 450 MHz.³⁴ Newer pacemakers incorporate rejection circuitry that makes them immune to RF interference. Because there is a finite lifetime for pacemakers, most of the older models have been replaced by the new types.

Microwave Auditory Effect

Humans exposed to rectangular microwave pulses of microsecond duration perceive a sound if the energy flux of the pulse is high enough (about 40 µJ/cm²).¹³ The phenomenon has been shown to arise from an acoustic wave generated in the head by the small but rapid thermoelastic expansion of tissue caused by microwave-energy absorption. The energy flux from GWEN LF or UHF emissions is much too small to elicit this auditory effect.

Circadian Rhythms

Wever^35-37 used a specially constructed living environment with no time cues to demonstrate that human circadian periods are lengthened by an average of 20 min when the living space is shielded against natural electric and magnetic fields. Introduction of a weak 2.5-V/m, 10-Hz square-wave electric field into the shielded environment was found to shorten circadian periods by 1.3 h relative to the field-free condition. The same square-wave field was also found to weakly entrain free-running circadian rhythms. Sinusoidal electric-field exposures of the same frequency and intensity were not as effective as the square-wave fields. The experiments have not been repeated elsewhere.

Brain-Evoked Potentials

In an extensive series of experiments, Graham et al.¹⁴ repeatedly observed an influence of combined 60-Hz electric and magnetic fields on the late components of the auditory-evoked potential. The effects were observed for both continuous and intermittent exposures and were more pronounced at combined fields of 9 kV/m and 20µT than at higher or lower combined-field strengths. Because only late components are affected, Graham and colleagues conclude that the effect is on the cognitive process of stimulus evaluation and not on overall neural conduction velocity.

Alterations in visually-evoked potentials (VEPs) were also reported by Silny³⁸ to occur in response to sinusoidal 50-Hz magnetic fields with flux densities exceeding 5 mT. The change in VEP was characterized by a reversal of polarity and a decrease in the amplitude of the three major evoked potentials. The effects were observed within 3 min after onset of the magnetic-field exposure, and the VEP returned to normal by approximately 30-70 min after termination of the exposure.

Heart Rate

Lowering of heart rate has been often, but not universally, observed in experiments involving human exposure to ELF electric fields, magnetic fields, and injected currents.¹⁴ Table 8-1 lists relevant findings. The studies reporting heart-rate decreases generally characterize them as temporary, noncumulative, and within the range of normal physiologic variation. Extrapolation of the findings to GWEN LF and UHF exposures is difficult. The current density induced by LF exposures at the GWEN site boundary is hundreds of times larger than the ELF current densities associated with changes in heart rate (0.1/µA/cm²). As the discussion in Chapter 3 points out, however, the sensitivity of muscle and nerve tissue at LF and UHF is likely to be much lower than that at ELF. Although experiments on the effects of exposure to radiofrequency (RF) fields on human heart rate are lacking, evidence from animal studies shows no acute effects of RF irradiation on heart rate in situ²⁶ (also see Chapter 6). Long-term effects of small RF exposure on cardiovascular function remain largely unstudied.

TABLE 8-1

Effects of ELF Fields and Currents on Human.

Reaction Time

The effects of ELF electric fields on human reaction time in response to auditory or visual stimuli have been extensively studied, but the results have been inconsistent.^14,27 Increased auditory or visual reaction times have been noted at thresholds ranging from 1-2 V/m in a 3-Hz field⁸ to 10 kV/m in a 50-Hz field.⁴⁵ Decreased visual reaction times were found by Hauf⁴⁶ in 50-Hz fields of I and 15 kV/m, but were not verified by later experiments in the same laboratory.^41,47

Gamberale and colleagues⁴⁴ found that a normal increase in reaction time throughout the day was smaller for high-voltage linemen when they were working on energized lines than when they were working on de-energized lines.

In an extensive series of experiments involving exposures to 60-Hz electric fields up to 12 kV/m and 60-Hz magnetic fields up to 30µT (300 mG), Graham et al. found no consistent effect of field exposures on reaction times.¹⁴

Mood and Cognitive Function

Stollery¹⁶ measured the acute effects of 50-Hz current on mood and verbal reasoning. Subjects were exposed or sham-exposed to a 36-kV/m field for 5.5 h on each of 2 days. Five tests of mood and verbal reasoning were administered throughout the experiment. Exposure-related effects were noted on the second day in stress level, as measured by a mood-adjective checklist, and in response times in tests of syntactic reasoning. In a second series of experiments involving an identical exposure regimen, Stollery measured the effects of 50-Hz current on vigilance and concentration.⁹ No effects on those end points were observed, and the suggestion of exposure-induced stress raised by the earlier study was not confirmed.

In their investigation of high-voltage linemen doing simulated line inspections, Gamberale and colleagues found no effect on wakefulness or stress, as measured by a mood-adjective questionnaire.⁴⁴ They also observed no effects on vigilance, perceptional speed, or short-term memory.

Blood Composition

Human laboratory studies examining field effects on blood composition are limited to the ELF range. Sander and colleagues⁴⁸ exposed volunteers to 50-Hz electric fields of 10-20 kV/m for 6-22 h/d for a week and found no effects of exposure on the concentrations of 33 constituents of blood. Gamberale⁴⁴ found no effects on serum concentrations of seven hormones in linemen performing simulated inspections of a 400-kV transmission line. In an experiment involving 11 subjects, Beischer, et al.¹⁵ found increases in serum triglycerides among subjects exposed to a 1-G (100-µT) 45-Hz magnetic field. In contrast, Rupilius⁴⁷ found no difference in triglyceride concentrations between a control group and subjects exposed for 3 h to combined 3-G (300-µT) and 20-kV/m 50-Hz fields.

Bone Repair and Growth Stimulation

Induced or injected currents have been widely used to treat recalcitrant nonunions of fractured bone by stimulating bone growth. Sinusoidal, DC, and pulsed currents have each produced some measure of success.^24,49 Brighton et al. have observed enhancements in bone-fracture repair with 60-kHz CW currents.^19-22 The threshold electric field in bone tissue^19,50 needed to produce the 60-kHz effects ranges from 5 to 20 mV/cm, corresponding to a range of induced current of 2.5-10 µA/cm². That is comparable with the average LF electric field and current density induced in bone tissue of a person standing at the boundary of a GWEN RN site. There are few data on the frequency dependence of the bone-healing effects reported by Brighton and colleagues. One study of healing of rabbit fibula showed significant field-induced healing at 60 kHz, but not at 10 kHz or 250 kHz with induced currents of the same intensity.²⁰ Limited evidence of the role of duty cycle in the bone-healing effect at 60 kHz shows no clear pattern. Field-induced enhancements in thymidine incorporation in vitro were found at duty cycles as short as 0.25%²² and 0.5%,²³ but disuse osteoporosis in vivo was reversed only by duty cycles greater than about 10%.²¹

Although studies in different biological systems have shown that 60-kHz tissue fields around 2 V/m can evoke measurable biological effects, there is little evidence with which to judge the strength of such effects at GWEN LF frequencies and duty cycles. If GWEN LF fields can induce biological responses in bone tissue, evidence is too scant to conclude whether the responses would be beneficial or harmful.

Conclusions

As with the animal and in vitro evidence reviewed by the committee, few human laboratory data are directly relevant to GWEN-related exposures. However, the many laboratory studies of human exposure to ELF fields have not identified any deleterious acute effects, although data on chronic exposure are lacking. Studies demonstrating bone growth induced by kilohertz fields are interesting, but there is no evidence of effects of such exposures on healthy tissues.

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