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BLOG: March 2009
Power-frequency field exposure
Power-frequency field (PFF) is produced by the standard 50/60Hz electricity in power and distribution lines, indoor and outdoor wiring and electrical devices of all types. Average 24-hour residential PFF exposure in the U.S. is around 1.2mG, with 95% of the population being in the 0.3-3.4mG range (Zaffanella, 1998). Similar levels of exposure have been found for children in Canada (Deadman et al. 1999) and in American woman (McCurdy et al. 2001).
This magnetic field strength corresponds, in the so called far field (over 10 wavelengths away from the source), to the electric field average intensity of 0.038kV, and 0.009-0.1kV 95% range.
Actual exposure to these two fields, however, varies unpredictably due to nearly all of it taking place in the near field (less than 1 wavelength from the source, with the wavelength at 60Hz frequency being 5,000km). Main sources of exposure are power lines, electrical wiring, electric appliances and electrical devices in general.
Electromagnetic fields weaken rapidly with the distance from the source. Thus, the strength of energy field created by power lines depends on the power line voltage and distance from the line, as shown in the table below (EPA 1992).
*since magnetic field fluctuates with the current flow (usage), it is given as a mean value
Distribution lines operate at lower voltage, but are also generally closer. According to NJDE (New Jersey Department of Energy), some underground 69kV distribution lines may produce as high as 55mG fields directly above the line; at 50m distance, it is down to about 1mG (also, the highest NJDE measurement under power lines was 130mG, significantly more than the typical maximum above; the lowest was 8mG).
Interior wiring voltage level, at 0.11/0.22kV, is a small fraction of that in power and distribution lines. However, it can be very close to the body, and its field strength may not be negligible. There is no official figures for it, but judging on the voltage/distance/field relationships for the power and distribution lines, it could reach up to a few milligauss (mG) at 1 foot distance. Enough to pay attention not to have it in the wall next to your bed - much less next to your child's.
Electrical devices are intermittent source of exposure. Duration-wise, for most people these exposures are insignificant compared to those to power and distribution lines, if present, and even compared to exposure from interior wiring. However, energy fields produced by these devices are often much stronger, and one should be aware of them. Typical emissions for some common electrical devices are given in the following table (EPA 1992). Note that 1mG=0.1μT (microtesla).
These are, of course, average values; any particular device can deviate from the average value for its type, possibly significantly. The list is also far from being complete, and some devices emit much stronger field than one would reasonably expect. For instance, clock radios, as small as they are, can generate magnetic fields
in excess of 1000mG
at small distances (Vistnes, 2001).
Also, many electrical devices emit at more than a single, 60Hz frequency. In addition to this basic frequency, TV and PC screens produce 10,000-30,000Hz fields, microwaves produce 2.45MHz fields, and so on.
Keep in mind that magnetic field strength
increases exponentially with the reduced distance.
Some devices are used very close to the body, in which case the level of radiation skyrockets. For instance, at 3cm distance (1.2 inch) magnetic field can reach 20,000mG with hair dryer, 15,000mG with electric shaver, 4,000mG from fluorescent light, or 560mG with portable radio.
The longer duration of exposure, the more important becomes its level. Home sawing machine produces 12mG field at the chest level, and 5mG at the head level; industrial machines can reach 35mG at the chest level and 215mG at the knee level (Sobel 1994). More recent study (Kelsh et al. 2003) puts mean exposure from sawing machines at several hundreds milligauss.
There are also other sources of electromagnetic fields that can significantly add up to your total exposure. Security systems (metal detectors, airport security, badges, stores, libraries, etc.) can create fields thousands of milligauss strong (Kjellsson, 2002). Magnetic fields in 60Hz electric trains have been reported to be as high as 500mG in the passenger areas at the seat height.
Think electric cars, elevators, store escalators, air conditioners, portable heathers, electrical heating systems, toasters...
And it doesn't end with the energy field produced by electricity. Another two significant forms of exposure to electrical energy are not even covered officially. One are irregular high-frequency harmonics and transient currents - giving to it the affectionate nickname dirty electricity - that form in increasingly complex electrical circuitry. The other is stray voltage, the "regular" electricity that leaks out of the system. They both escape electrical lines and wiring and travel through conductive media, such as water pipes, sink, bathtub, floors, or human body. There is a solid body of evidence that the exposure to these currents
can make sensitive individuals very sick.
Information on measuring and minimizing these currents can be found in Protection from EMF.
Switching back to the magnetic fields created by standard electricity, most of exposure figures don't look really alarming if referenced to the official safety limit for this frequency, which is 1,000mG. But it becomes entirely different story when referenced to the levels of exposure linked to significantly increased risk of childhood leukemia (from around 2mG up), or even occupational exposures that have been linked to significantly increased risk of various diseases, including cancer (from about 10mG up).
How is this magnitude of discrepancy between what is officially proclaimed "safe" and prevailing scientific data even possible? The answer is complex, and requires becoming more familiar with the official reasoning, research results, and powerful interests shaping up this controversy. That comes up next.