National Policies and Precautionary Limits
Regulatory agencies in the U.S. and Canada have not established national health-based standards limiting exposure to EMF, although several states in the U.S. have established limits for electric fields within the ROW and for both electric and magnetic fields at the edge of the ROW. More than 50 countries worldwide have set exposure limits in some manner that vary widely from country-to-country (Compilation of EMF limits and standards worldwide; note: this link provides the latest update posted). Some countries have adopted the ICNIRP limits, some have country-specific safety limits similar to ICNIRP or IEEE, and still others have limits that apply to the ROW. Some countries have adopted more conservative limits for certain circumstances, such as for new residential construction.
With regard to field mitigation, WHO stated in its 2007 report, “…it is not recommended that the limit values in exposure guidelines be reduced to some arbitrary level in the name of precaution. Such practice undermines the scientific foundation on which the limits are based and is likely to be an expensive and not necessarily effective way of providing protection.” WHO further recommended that field reduction could be considered when at “little or no cost.”
The National Radiological Protection Board (now absorbed into PHE) in the United Kingdom reviewed the EMF literature in 2004, stating “the results of epidemiological studies, taken individually or as collectively reviewed by expert groups cannot be used as a basis for restrictions on exposure to EMFs.” The clear message here was that the existing guidelines and standards provide protection against known effects with established mechanisms, and limits need not be reduced any further.
Prior to the WHO and NRPB recommendations, the California Public Utilities Commission (CPUC) set a policy in 1993, reaffirmed in 2006, “to mitigate EMF exposure for new utility transmission and substation projects. As a measure of low-cost mitigation, we [the CPUC] continue to use the benchmark of 4% of transmission and substation project costs for EMF mitigation, and combine linked transmission and substation projects in the calculation of this 4% benchmark.” The CPUC currently uses a 15% reduction in EMF at the ROW edge as the minimum level of mitigation.
An example of a low-cost intervention is illustrated in Figure 8, which shows a double-circuit 345-kV transmission line (Figure 3 illustrated a single-circuit 345-kV transmission line). As is evident from Figure 3, the cables (or conductors) on transmission lines come in groups of three, each of which is identified as a ‘phase’, A, B, and C. A double circuit line has two groups of three conductors. When the line is ‘like’ phased with phases A, B, and C symmetrically placed on the tower (A opposite A, etc.), the magnetic field is maximized. At virtually no cost (and if operationally feasible and implemented during the initial construction) the double circuit can be phased in an ‘unlike’ manner, which drives down both the electric field and the magnetic field. The reason is because the unlike phases opposite each other have a cancelling effect on the field (whereas with like phasing the fields are reinforced and therefore greater). This same effect was shown in Figure 1 for a 230-kV double-circuit line in which the field profile for unlike phasing (blue curve) is considerably lower than the profile for like phasing (burgundy curve).
Occupational Studies: Studies of workers can offer a useful opportunity to examine environmental EMF exposures at higher levels than occur in residential settings. Many occupational studies of electrical workers and others exposed to higher magnetic fields have examined both cancer and other diseases. Overall, the occupational studies do not support a link between magnetic fields exposure and any form of cancer or other adverse effects.
Cancer Clusters: When several cancers occur close in time and space – that is, in a cluster, such as in a given location – people seek a reason, and at times EMF has been thought to be a possible culprit. Most often, upon further investigation, no actual cancer cluster is identified. The perception of a cluster arises partly because people do not always understand how common cancer is. In industrialized countries, about one in 3 people will develop some type of cancer during their lifetimes. Cancer clusters can and do occur by chance, but distinguishing a chance occurrence from an occurrence with a common cause is difficult. As a result, cancer cluster investigations are rarely productive, and none have linked a cancer cluster to magnetic field exposure.
Electromagnetic Hypersensitivity (EHS): Some individuals experience a wide range of nonspecific symptoms such as headaches and sleep disturbance that can be quite debilitating, which they ascribe to EMF exposure. Further, some of these individuals believe that they can sense the presence of high fields, which trigger their symptoms. The consensus of the scientific community is that while some of these individuals clearly have health conditions and may react to factors in their environment, their symptoms are not related to EMF. This conclusion is based mostly on carefully conducted tests in the laboratory in which individuals self-identified as EHS cannot reliably detect the presence of fields, and their symptoms cannot be attributed to EMF. Several studies have indicated that the observed effects may be caused by an expectation that something harmful is going to happen. In light of the fact that an EMF basis for these individuals’ conditions has not been observed, the condition has more recently been labeled ‘Idiopathic Environmental Intolerance Attributed to Electromagnetic Fields’.
Pacemakers and Other Medical Devices: Cardiac pacemakers and defibrillators are the most commonly implanted medical devices, and research has indicated that they may be susceptible to interference under certain high field conditions. The sensitivity of these devices depends on manufacturer, design, device settings, and how they are used by a patient. Metallic case shielding, robust filtering, and bipolar sensing have contributed to improved immunity to interference. In practice, interference is extremely rare, if at all. Many other medical assist devices are now deployed in patients, such as insulin pumps and brain stimulators, but interference to them from power frequency fields has not been addressed. Manufacturer product standards generally call for implanted medical devices to maintain immunity (no interference) from power frequency (50/60 Hz) magnetic fields of 1000 mG and 6-10 kV/m.
Animals and Vegetation: Research on how animals and plants might be affected by exposure to EMF has been conducted since the 1970’s. EMF exposure has not been shown to have any consistent detectable, adverse effects on plant growth, crop yield or animal health. A separate issue is sometimes raised about potential harm to farm animals from ‘stray voltages’. Stray voltage is a general term used to describe the small voltages that may exist at contact locations where they would not be necessarily expected. These voltages may arise from the normal operation of a ‘multi-grounded’ power system, and may originate from electricity systems both on and off a farm. Stray voltages may be enhanced by various abnormal and correctible situations, such as poor insulation or wiring errors.
Questions have arisen as to whether the environments within transmission line rights-of-way are inhospitable to native bees and honey bees, both crucial to agricultural production. The U.S. Geological Survey states (Featured Story ) that:
“According to the USDA [U.S. Department of Agriculture], bees of all sorts pollinate approximately 75 percent of the fruits, nuts and vegetables grown in the United States…bee pollination is responsible for more than $15 billion in increased crop value each year.” Recent research has shown that high voltage transmission line easements can provide quality habitat for native bees, particularly when these areas are managed in a way that promotes the growth of native shrubs and flowering perennials. Honeybees in commercial hives with metallic components in high electric fields under high voltage transmission lines may experience tiny electrical discharges within the hives. These effects can be mitigated by not using metal parts for hive construction, shielding with a simple wire mesh, and grounding or moving the hives a short distance away from directly underneath the line.
Theories of Mechanisms: Over the years, many theories have been advanced to explain how low level magnetic fields may interact with the cells and tissues within our bodies. For example, in the 1980s the ‘cyclotron resonance’ theory was introduced predicting how certain ions like calcium and lithium would be affected by magnetic fields of specific frequency and magnitude. Although the theory attracted attention at the time, further analyses and experiments did not support its plausibility, and scientific interest in it faded.
Another hypothesis suggested that tiny magnetic particles in the surface of cells in the human brain could be physically rotated in a magnetic field (like a compass) thereby altering signaling in the brain. However, the presence of such deposits in the human brain was never ascertained. Magnetic deposits, present in some animals, such as honey bees, may help them navigate using the earth’s natural field as a guide, and we know for certain that magnetotactic bacteria contain large magnetic crystals that guide them to their source of nutrients.
A third example concerns a biological pathway through a small structure in the brain called the pineal gland that secretes melatonin, a substance that is instrumental in regulating our 24-hour biological cycle (called the ‘circadian rhythm’). A suppression of melatonin in animal experiments increased the occurrence of hormonally dependent cancers, such as breast cancer. Early experiments reported promising results that magnetic fields suppressed melatonin, but after different scientists across different laboratories attempted replications, the effect was no longer apparent. In any case, the proponents of the melatonin hypothesis were unable to explain how a low level magnetic field could interact at the cellular level to set this proposed pathway in motion.
The one established mechanism in humans is electrostimulation, the stimulation of nerve tissue by magnetic or electric fields (or by direct contact with an electrical conductor), which occur above threshold exposure levels that are much greater than those present in our daily lives. As described under Exposure Guidelines and Standards, published exposure limits are structured to protect people against adverse electrostimulation.