Magnetic Fields

Characterisation of Magnetic fields

In the assessments of health effects, the question arises as to which metrics of magnetic field exposure are appropriate. Some of the commonly used epidemiological exposure parameters include, time-weighted average (TWA) magnetic field strength, personal 24-hour measurements and single location 24-hour and spot measurements.

Bowman and Methner (2000) carried out a study of the field characteristics that should be investigated in industrial hazard surveillance. The authors concluded that biological effects resulting from the postulated mechanisms could also vary in ways that cannot be predicted by measurements of the ELF magnetic field magnitude, and therefore more detailed measurements of field characteristics are required for assessing the potential of magnetic fields to cause health effects.

Where most of the epidemiological studies look at central tendency, several recent studies have shown how important it is to consider field stability, and also biological relevance of the measurements being carried out - Remember increase in risk of miscarriage is associated with exposures above 1.6microT, not average exposure, so it is important to consider transients and harmonics by looking at rate of change metric of total harmoic distortion for example

Polarisation is one metric that is of biological interest. Click here for more information on polarised magnetic fields.

Magnetic Fields in the Home

We examined the degree of vector ellipticity of extremely low frequency magnetic fields to which members of the public are exposed from common domestic appliances found in the home. We used instruments called the EMDEX and the Multiwave II to carry out tri-axial measurements of the magnitude and phase of the 0 - 3000 Hz magnetic fields, produced by 226 domestic mains-fed appliances of 32 different types, in 16 homes.

Overall we found that magnetic field strengths were low, with RMS values of 0.03 + 0.02 T across all residences. In contrast, background field ellipticities were high, on average 47+11%. Microwave and electric ovens produced the highest ellipticities: mean respective values of 21 + 21 % and 21 + 17 % were observed 20 cm away from these appliances. There was a negative correlation between field strength and field polarisation, which we attributed to the higher relative field contribution close to each individual (single-phase) appliance.

Magnetic Fields in the Vicinity of High Voltage Powerlines

We have been investigating the characterisation of the 50 Hz magnetic fields in the vicinity of high voltage powerlines. Again we took tri-axial measurements of magnetic field strength and vector ellipticity, this time directly underneath and close to six 132 kV distribution lines in the Bristol area. Magnetic field strengths of up to 6.57 + 0.08 T, with up to 89 + 5 % ellipticity, were observed in the vicinity of these powerlines. The values of the dispersion metrics, such as rate of change metric and total harmonic distortion were, as expected, low.

So large magnetic fields, with near-circular polarisation, do exist close to high voltage powerlines.