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Natural and human -made electric and magnetic fields are, in fact, present everywhere in our <br />environment. The Earth's natural static background electric field is approximately 120 to 150 volts <br />per meter (V/m). Natural electric fields are also produced by the local build-up of electric charges in <br />the atmosphere that are associated with thunderstorms. The Earth itself has a magnetic field that <br />ranges from approximately 300 to 700 milligauss (mG), the field is a steady-state or static (zero <br />hertz) magnetic field, but has similar characteristics to the magnetic fields emanating from human - <br />made sources. <br />Electric and magnetic fields created by humans include X-rays and magnetic resonance imaging <br />(MRIs) machines, electric and magnetic passenger trains, electric cars, and cellular telephones. The <br />general wiring and appliances located in a typical home can produce an average background <br />magnetic field of 0.5 mG to 4 mG (EPA, 1992). <br />Electric and magnetic fields arise from the voltage and the flow of electricity (current) through a <br />conductor. The intensity of the electric field is related to the voltage of the line and the intensity of <br />the magnetic field is related to the electric current. The electric field associated with high-voltage <br />transmission lines "extend" from the energized conductors to other nearby objects whereas the <br />magnetic field "surrounds" the conductor. Together, these fields are generally referred to electric <br />and magnetic fields or EMF. A summary of electric and magnetic field properties is summarized in <br />Table 8. <br />Table 8: Summary of Electric and Magnetic Field Properties <br />Electric Fields <br />Magnetic Fields <br />Electric fields arise from voltage. <br />Magnetic fields arise from current flows. <br />Their strength is measured in kilovolts per meter <br />Their strength is measured in milligauss (mG) or microtesla <br />(kv/m)• <br />QT) - <br />An electric field can be present even when a device is <br />Magnetic fields exist as soon as a device is switched on and <br />switched off. <br />current flows. <br />Field strength decreases with distance from the <br />Field strength decreases with distance from the source. <br />source. <br />Most building materials shield electric fields to some <br />Magnetic fields are not attenuated by most materials. <br />extent. <br />Source: WHO, 1999 <br />This section of the EA specifically addresses electric and magnetic fields produced through <br />transmission of electric power at 60 Hz or cycles per second. <br />Electric Fields <br />Electrical fields are created by voltage. Voltage can be described as the potential difference between <br />two points and will always try to drive an electric current. The voltage on any conductor produces <br />an electric field that extends from the wire in all directions. The intensity of electric fields is <br />Environmental Assessment <br />PUC Docket E002/TL-11-223 Page 30 <br />