Power lines that operate with DC (direct current) produce DC electric and magnetic fields (most power lines operate with AC, alternating currents, and produce AC fields). But we are all also exposed to the earth's DC magnetic field, the geomagnetic field.
How big is the geomagnetic field?
The geomagnetic fields varies from about 25 µT towards the equator to 65 µT towards the poles. See for example one of many different maps available on the internet. In the UK it varies from about 47 to 50 µT from south to north. There is at least one calculator to work out the geomagnetic field at any given latitude and longitude (and year).
This is a similar magnitude to the magnetic field produced by a typical high-voltage DC underground cable.
How much does it vary?
As well as the broad variation over the earth's surface - tens of microteslas from equatorial lattitutes to the poles, a microtesla or two across the UK - there are two sorts of more local perturbations:
- caused by underlying geology, producing variations typically up to a microtesla over distance scales of a few km
- caused by ferrous objects within homes, producing variations of a microtesla or so - see measurements of this in the UK
Is it directly linked to health effects?
It would be very difficult to disentangle any health effects of the geomagnetic field on a global scale. The geomagnetic field essentially distinguishes the tropical lattitudes - low - from the temperate lattitudes - high. So many other factors vary in the same way that it would be almost impossible to work out whether an effect was due to the geomagnetic field or something else.
It might be possible to look just at the geomagnetic variation within one country, where other factors might vary less. The problem then is whether the underlying geomagnetic variation is swamped by the perturbations within each home.
Does it interact with AC fields?
AC fields, for example from power lines, form a "ripple" on top of the geomagnetic field. This diagram shows a 0.4 µT (rms) AC field superimposed on a 48 µT DC field (but not to scale, or the ripple would be barely visible):
This means the AC field makes no difference to the average value of the total field - the AC field averages out and the total field is just the DC field. This has implications for possible mechanisms - they will behave differently according to whether they depend on the total field or just the AC field.
Free radicals and the geomagnetic field
One example of a mechanism that depends on the total field, not the AC field is free radicals. So we might expect that if free radicals are a mechanism for producing effects from AC fields, they should produce even bigger effects from DC fields. See more information on one test of this.