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