Electricity Towers (Pylons) and High Voltage Lines
Electricity towers are sources of AC magnetic fields, and homes should be sited at least ~150 meters from the larger 400 kV electricity towers. Large and small power lines both need to be considered, although the former requires more distance than the latter. PowerWatch approximates that to drop below 2mG, one should be ~150 meters away from the strongest 400 kV lines as opposed to only ~15 meters for 11 kV lines. However, it recommends measurement, since fields could vary over time.
The graph below shows the extend and magnitude of the magnetic field from various overhead power lines in Ireland.