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Reducing your exposure from powerlines
Powerlines come in different shapes and sizes. The highest power ones (400 kilovolts or kV) are the long-distance lines from the generating power station to the places where the power is needed. The pylons supporting these power cables are large, metal structures, which have long strings of insulators from which the cables hang. The smallest 230 volt lines start from local substations and supply the power needs for a relatively small area. In between these two extremes are a variety of other types of lines carrying different voltages.
There are two types of electromagnetic fields (EMFs) associated with powerlines; electric fields and magnetic fields. These are at their highest to either side of the central cable of large lines, and underneath the cables (which tend to be one on top of the other, or 'plaited' together) of smaller lines. The EMFs come from the cables, not the pylons as the pylons are insulated from the fields generated. As you move away from the line, the fields begin to drop away. How far they extend depends on the line voltage for the electric field, and the power being supplied for the magnetic field.
Reducing your exposure from Electric fields
Homes will be screened from electric fields by the building materials used in their construction. All materials reduce electric fields, some better than others. The exception to this is window glass (whether single or double glazed) which hardly reduces electric fields at all.
Gardens will not be screened by building materials.
Reducing your exposure from Magnetic fields
There is absolutely nothing you can do about magnetic fields. Lead or steel sheets are ineffective. There is a metal called mu-metal which does reduce the fields, but it is very expensive. Mu-metal foil is for use in audio processes and is not suitable for screening magnetic fields from powerlines.
The cables on local supply lines can be either 4 individual ones, or sometimes, they are twisted together forming what is known as ABC (aerial bundled conductor) cabling. Fields are lower from ABC cabling than from the 4 individual cables.
Power cables can be undergrounded. Electric fields will be absorbed by the earth above a buried cable. Magnetic fields will be higher immediately above an underground cable than they will be below an overhead line, because you are closer, but the fields reduce much more quickly from an underground cable. The Electricity Association reckons the cost of putting cables underground is twenty times higher than allowing them to go overhead, less for lower-voltage lines.
High electric fields around power cables attract all sorts of airborne pollutants, if you live in an area where these are generated, such as near a main road, chemical factory, sprayed fields, etc. These are then deposited downwind or in rain.
If you would like more information on this topic, you could read our articles, "Buying an "EMF safe" property", "Powerfrequency EMFs and health risks", "Powerfrequency protection for you and your family" which contain far more information on this subject. We extensively cover this area in our article library.