Sources of magnetic fields in the home

A small fraction (less than 2.5%) of UK homes have background magnetic fields above 0.2 µT (microtesla), associated with the electricity power supply, and most of these are not close to major power lines or electricity substations. This is the conclusion1 of an extensive study of sources of power frequency magnetic fields associated with nearly 200 homes identified from the UK Childhood Cancer Study (UKCCS).

The most common source of magnetic field exposure above 0.2 µT in homes was found to be due to currents flowing in the supply to the home and possible wiring faults (32%) followed by nearby high voltage power lines of 132 kV (kilovolts) and above (20%). The effect of currents in low voltage distribution mains cables located near to homes was also identified as a source of exposure (16%). For homes with exposures of 0.4 µT and above, the most common source of exposure was high voltage overhead lines (43%).

The UKCCS published the results of exposure to power frequency magnetic fields in 1999 followed by a paper on residential proximity to power lines in 2000. The study showed that most UK homes have background magnetic fields in the range 0.01 to 0.1 µT and less than 2.5% have levels above 0.2 µT. The UKCCS identified that not all of the homes above 0.2 µT were close to major power lines, which have been a source of public concern.

The study reported here was set up to investigate the sources of exposure in and around the homes that were measured in the UKCCS. This involved a review of the original UKCCS data, external site inspections of 196 homes and the acquisition of detailed circuit information from the electricity industry. Consent was obtained from a small number of householders to make internal house inspections.

In 102 homes with magnetic fields above 0.2 µT the effect of currents in service wiring and possible internal wiring faults was the most common source (33 homes). The next most common source was nearby high voltage power lines of 132 kV and above (20 homes). The effect of currents in low voltage distribution mains cables was identified as the most likely source for 16 out of 102 homes. These residences were often characterised by little or no front garden and were therefore more likely to be closer to the cables. The other individual sources identified were 275 kV cables, low voltage overhead lines, electrical appliances and an electrified train line close to one house. The magnetic fields in the remaining homes were due to a possible combination of low voltage sources.

Of 21 homes with exposures of 0.4 µT and above, the most common single source was high voltage overhead lines (43%), followed by currents in the supply to the home and possible internal wiring faults (33%). The various low voltage sources taken together accounted for 57% of the exposures.

The National Radiological Protection Board (now the Radiation Protection Division of HPA) carried out this study on behalf of the Leukaemia Research Fund (Epidemiology and Genetics Unit, University of York). The research was funded by the Department of Trade and Industry and the Energy Networks Association.