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The source document for this Digest states:
Before man-made electricity, humans were exposed only to the magnetic field of the earth, electric fields caused by charges in the clouds or by the static electricity of two objects rubbing together, or the sudden electric and magnetic fields caused by lightning. Since the advent of commercial electricity in the last century we have been increasingly surrounded by man-made EMF generated by our power grid (composed of powerlines, other electrical equipment, electrical wiring in buildings, power tools, and appliances) as well as by higher frequency sources such as radio and television waves and, more recently, cellular telephone antennas.
Source & ©: California Department of Health Services and
the Public
Health Institute of California "Electric and Magnetic Fields
Program" (2000)
The source document for this Digest states:
Wherever there is electricity, there are also electric and magnetic fields, invisible lines of force created by the electric charges. Electric fields result from the strength of the charge while magnetic fields result from the motion of the charge, or the current. Electric fields are easily shielded: they may be weakened, distorted or blocked by conducting objects such as earth, trees, and buildings, but magnetic fields are not as readily blocked. Electric charges with opposite signs (positive and negative) attract each other, while charges with the same sign repel each other. The forces of attraction and repulsion create electric fields whose strength is related to "voltage" (electrical pressure). These forces of attraction or repulsion are carried through space from charge to charge by the electric field. The electric field is measured in volts per meter (V/m) or in kilovolts per meter (kV/m). A group of charges moving in the same direction is called an "electric current." When charges move they create additional forces known as a "magnetic field." The strength of a magnetic field is measured in "gauss" (G) or "tesla" (T), while the electric current is measured in "amperes" (amps).
The strength of both electric and magnetic fields decrease as one moves away from the source of these fields.
Fields vary in time
An important feature of electric and magnetic fields is the way they vary in time. Fields that are steady with respect to direction, rate of flow, and strength are called "direct current" (DC) fields. Others, called "alternating current" (AC) fields, change their direction, rate of flow, and strength regularly over time. The magnetic field of the earth is DC because it changes so little in one year that it can be considered constant.
However, the most commonly used type of electricity found in power lines and in our homes and work places is the AC field. AC current does not flow steadily in one direction, but moves back and forth. In the U.S. electrical distribution system it reverses direction 120 times per second or cycles" 60 times per second (the direction reverses twice in one complete cycle). The rate at which the AC current flow changes direction is expressed in "cycles per second" or "Hertz" (Hz).
The power systems in the United States operate at 60 Hz, while 50 Hz is commonplace elsewhere. This fact sheet focuses on "power frequency" 60 Hz fields and not the higher frequency fields generated by sources such as cellular phone antennas.
Describing magnetic fields
The concentration of a chemical in water can be described by citing a single number. Unlike chemicals, alternating electric and magnetic fields have wave-like properties and can be described in several different ways, like sound. A sound can be loud or soft (strength), high or low-pitched (frequency), have periods of sudden loudness or a constant tone, and can be pure or jarring. Similarly, magnetic fields can be strong or weak, be of high frequency (radio waves) or low frequency (powerline waves), have sudden increases ("transients") or a constant strength, consist of one pure frequency or a single dominant frequency with some distortion of other higher frequencies ("harmonics").
It is also important to describe the direction of magnetic fields in relation to the flow of current. For instance, if a magnetic field oscillates back and forth in a line it is "linearly polarized." It may also be important to describe how a field's direction relates to other physical conditions such as the earth's static magnetic fields.
Measuring magnetic field strength
The strength or intensity of magnetic fields is commonly measured in a unit called a or by magnetic field meters called "gaussmeters." A milligauss (mG) is a thousandth of a gauss, and a microtesla (µT) is a millionth of a tesla (one milligauss is the same as 0.1 microtesla). The magnetic field strength in the middle of a typical living room measures about 0.7 milligauss or 0.07 microtesla. As noted above, the strength of the magnetic field is only one component of the mixture that characterizes the field in a particular area. Measuring only magnetic field strength may not capture all the relevant information any more than the decibel volume of the music you are playing captures the music's full impact. The main health studies to date have only measured magnetic field strength directly or indirectly and assessed its association with disease. Some scientists wonder if the weak association between measured magnetic fields and cancer in these studies might appear stronger if we knew which aspect of the EMF mixture to measure. Other scientists wonder if any such aspect exists.
Source & ©: California Department of Health Services and
the Public
Health Institute of California "Electric and Magnetic Fields
Program" (2000)
See question 2.1 for definition of ELF
The source document for this Digest states:
There are "power frequency" electric and magnetic fields almost everywhere we go because 60 Hz electric power is so widely used. Exposure to magnetic fields comes from many sources, like high voltage "transmission" lines (usually on metal towers) carrying electricity from generating plants to communities and "distribution" lines (usually on wooden poles) bringing electricity to our homes, schools, and work places. Other sources of exposure are internal wiring in buildings, currents in grounding paths (where low voltage electricity returns to the system in plumbing pipes), and electric appliances such as TV monitors, radios, hair dryers and electric blankets.
Sources with high voltage produce strong electric fields, while sources with strong currents produce strong magnetic fields. The strength of both electric and magnetic fields weakens with increasing distance from the source (table 1). Magnetic field strength falls off more rapidly with distance from "point" sources such as appliances than from "line" sources (powerlines). The magnetic field is down to "background" level supposed to be no greater than that found in nature) 3-4 feet from an appliance, while it reaches background level around 60-200 feet from a distribution line and 300-1000 feet from a transmission line. Fields and currents that occur at the same place can interact to strengthen or weaken the total effect. Hence, the strength of the fields depends not only on the distance of the source but also the distance and location of other nearby sources.
Table 1. Examples of magnetic field strengths at particular distances from appliance surfaces
MILLIGAUSS (mG) at 1 foot at 3 feet.
MILLIGAUSS (mG) at 1foot at 3foot Source: L. Zaffanella, School Exposure Assessment Survey, California EMF Program, interim results, November, 1997 GreenFacts note: 1 ft = 30.48 cm; 3 ft = 91.44 cm ; 1 mG = 0.1 µT (see toolbox on SI units) aquarium pump 0.35-18.21 0.01-1.17 band saw 0.51-14.24 0.05-0.75 can opener 7.19-163.02 1.30-6.44 clock 0.34-13.18 0.03-0.68 clothes iron 1.66-2.93 0.25-0.37 coffee machine 0.09-7.30 0-0.61 computer monitor 0.20-134.7 0.01-9.37 copier 0.05-18.38 0-2.39 desktop light 32.81 1.21 dishwasher 4.98-8.91 0.84-1.63 drill press 0.21-33.33 0.03-8.35 fax machine 0.16 0.03 food processor 6.19 0.35 garbage disposal 2.72-7.79 0.19-1.51 microwave oven 0.59-54.33 0.11-4.66 mixer 0.49-41.21 0.09-3.93 portable heater 0.11-19.60 0-1.38 printer 0.74-43.11 0.18-2.45 portable fan 0.04-85.64 0.03-3.12 radio 0.43-4.07 0.03-0.98 range 0.60-35.93 0.05-2.83 refrigerator 0.12-2.99 0.01-0.60 scanner 2.18-26.91 0.09-3.48 sewing machine 3.79-7.70 0.35-0.45 tape player 0.13-6.01 0.01-1.66 television 1.80-12.99 0.07-1.11 toaster 0.29-4.63 0.01-0.47 vacuum cleaner 7.06-22.62 0.51-1.28 VCR 0.19-4.63 0.01-0.41 vending machine 0.46-5.05 0.02-0.59
Source & ©: California Department of Health Services and
the Public
Health Institute of California "Electric and Magnetic Fields
Program" (2000)
GreenFacts note: 1 ft = 30.48 cm; 3 ft = 91.44 cm ; 1 mG = 0.1
µT
(see toolbox on SI units)
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