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Power Lines

Low Frequency

Characteristics of the application and its use

Different types of power lines serve to transport electricity from the generation site to the end-users. Most commonly, and for efficiency reasons, the electricity is transported over long distances as alternating current at 50 or 60 Hz, carried overhead or underground using high voltage power lines or cables. Over very long distances or underwater, transmission occurs sometimes with high voltage direct current (HVDC).

Power lines transmitting alternating current (AC) are surrounded by low frequency electric and magnetic fields. HVDC lines emit static electric and magnetic fields. The strength of the fields emitted by these lines depends mainly on the transmitted current and line voltage.

Materials such as brick and clay are very efficient at shielding the electric field. In underground lines, the construction design is such that the electric field is completely shielded. The static electric field from overhead HVDC lines can expand further into the surroundings compared to AC lines (corona effects). The magnetic field, in contrast, passes unobstructed through most materials. However, the fields' strength diminishes quickly with distance from the line.

LF effects on the body and health implications

When people are exposed to LF fields, electric fields and currents are generated inside the body and they can interfere with the body’s own electric fields and current flows that are related to normal biological functioning. The LF or static electric field interacts as a surface charge on the body. At low levels these interactions go mostly unnoticed by the body and do not compromise health. In addition these fields can charge objects that are not electrically connected to ground. If a charged object and an object connected to ground (where either of the two might be a person) come into contact, a discharge sometimes through a spark occurs. The effect is similar to that of a field generated by electric fields and currents.

Above a certain level of exposure, referred to as threshold, the induced internal fields provoke reversible effects on excitable cells in the body such as a faint light flickering in the periphery of the visual field (phosphenes), electric charge effects at the skin level (similar to that experienced when you comb your hair, causing your hair to rise) or a stimulation of nerves and muscles experienced as a tingling sensation. At higher levels LF fields cause irreversible cardio-vascular effects or tissue burns.

Long-term effects of low-level exposure stemming from the power distribution system including power lines and their relevance to health have been extensively studied over the last few decades.

Epidemiological studies have suggested that long-term low-level exposure to 50-60 Hz magnetic fields might be associated with an increased risk of childhood leukemia. However, a combination of selection bias, some degree of confounding and chance could possibly explain these results. In addition, no biophysical mechanism has been identified and results from animal and cellular laboratory studies do not support the notion that exposure to 50-60 Hz magnetic fields is a cause of childhood leukemia. Therefore, the currently existing scientific evidence does not lead to the conclusion that prolonged exposure to LF is a cause of childhood leukemia. Evidence for cancer in adults from LF exposure is very weak. There is no substantial scientific evidence for an association between LF exposure and Parkinson’s disease, multiple sclerosis, developmental and reproductive effects, and cardiovascular diseases, while for Alzheimer´s disease and amyotrophic lateral sclerosis the evidence is inconclusive. Studies of symptoms, sleep quality, cognitive function have not provided consistent evidence of an effect from this type of exposure.

Overall research has not shown to date that long-term low-level LF exposure has detrimental effects on health.

Effects of static fields on the body and health implications

There are several known mechanisms by which magnetic fields can influence biological systems. Magnetic fields not only exert physical forces on metallic objects but also on moving electric charges. With respect to biological functioning, exposure to static magnetic fields will affect the electrically charged particles and cells in the blood, especially when moving through the magnetic field. The magnetic force can accelerate or reduce the movement of charged particles. An example is a reduction in the velocity of blood cells flowing through blood vessels. A further mechanism is via complex electronic interactions that may affect the rate of specific chemical reactions in the body.

There is no evidence for adverse effects of exposure to fields up to 8 T, except for limited information on minor effects such as on hand-eye coordination and visual contrast. Magnetic fields of 2-3 T or higher (such as those generated by equipment in some industrial and medical settings or in some specialist research facilities - i.e. MRI) can evoke transient sensations such as vertigo and nausea. These occur as the result of the generation of small electrical currents in the ear's balance organ. The currents generate signals to the brain that provide different information to that obtained through vision, resulting in the sensations of vertigo and nausea. These effects are not adverse health effects in themselves, but they can be annoying and they may impair normal functioning.

Static electric fields do not penetrate the human body. They interact only indirectly through surface charge effects like those described above. Furthermore, very high electric fields from HVDC lines can charge particles in the air, including particles of pollutants. There is a hypothesis that charged particles might be better absorbed in the lung than uncharged ones and so, raise people’s exposure to air pollution. Current knowledge, however, suggests that an increased health risk from such charging of particles is very unlikely.

Overall research has not shown to date that exposure to low-level static electric and magnetic fields have detrimental effects on health.

Protection

To prevent health-relevant interactions with static or LF fields, ICNIRP recommends limiting exposure to such fields so that the threshold at which the interactions between the body and the external electric and magnetic field shows adverse effects is never reached inside the body.

In relation to static electric fields no specific exposure limit is recommended, as they only interact at the surface of the body. For static magnetic fields the exposure limits are set to avoid the occurence of perceptible effects on the human body causing transient sensations such as vertigo and nausea. They are expressed in terms of magnetic flux density in tesla (T). In relation to the LF fields, the exposure limits, called basic restrictions, are related to the threshold showing adverse effects, with an additional reduction factor to account for any scientific uncertainties pertaining to the determination of the threshold. They are expressed in terms of the induced internal electric field strength in V/m. The exposure levels outside the body, called reference levels, are derived from the basic restrictions using worst-case assumptions, in such a way that remaining below the reference levels (in the air) implies that the basic restrictions will also be met (in the body). In addition, ICNIRP recommends avoidance of any annoying or detrimental indirect effects from discharge currents. Refer to the below ICNIRP Guidelines for all figures and more information.

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