Magnetic Field is caused by the electric current flowing in the cable when people use electrical power and can therefore vary considerably from one household to another. While, Electric fields are stopped by most building materials and can be shielded, magnetic fields penetrate most materials as if it wasn't there. The main and possibly the factor that reduces magnetic fields is distance from the source.
In the UK, the largest power lines (400 kV and 275 kV) are owned and maintained by National Grid are often 'unbalanced', that is, the current on one side of the line cables is very different to that on the other. This leads to much higher electric and magnetic fields than if both sides carried equal currents.
The electric fields for underground power cables will be zero as they are screened by earth, concrete, sand etc. Usually underground cables are buried rather shallowly. As a result, the magnetic fields at the surface of the ground are very high near to the buried cable, higher than from overhead cables because they are closer to you. They fall off more rapidly than the fields from overhead wires, because the cables are closer together and cancel out each other's effects more quickly.
Housing developments are often built near an existing line, and cables can be routed above, or sometimes fixed to, houses. The distance you need to be away from power lines depends on how large the load is and whether the load is balanced. Underground cables also emit high magnetic fields, which can extend outwards for a considerable distance.
Due to the varied and often unpredictable paths that electric currents in our electricity systems use it is not possible to calculate the field levels in a typical situation. The only way of knowing your exposure is to actually measure the fields and compare them with the average background magnetic field level (about 50 Nanotesla for a UK home).
What does the Research Data say?
Through extensive research, the adverse health effects that result from too much exposure to higher levels of Magnetic field were found to be too many to list. Below are just few:
- Childhood Leukemia and magnetic fields - Here
- Breast Cancer and Melatonin: 66Hz Magnetic Fields Block Melatonin's oncostatic action - Here
- Environment Magnetic fields inhibit the antiproliferative action of tamoxifen and melatonin in a human breast cancer cell line - Here
- Biological effects due to weak magnetic fields on plants - Here