It is my understanding that many of the proposed mechanisms for magnetic field/biological interaction [like those that affect the calcium ion during cell mitosis, or neuron activity] are NOT related to magnitude, but rather specific 'bands' of magnitudes. That is one of the justifications for discounting much of the epidemological relationships purported to be found. It's just that people think in terms of amplitude and not specific amplitudes. Like adding fertilizer to the lawn, a little is ok, too much causes death to the plants. In magnetics a little goes unnoticed and at certain amplitudes some wild interchanges take place, then at way more, again, nothing much happens. So trying to blindly correlate magnetic field to interaction has not resulted in much success.
There was potential evidence of this phenomenon in downtown San Jose, CA. There is a govt building there that consists of two identical wings, built same time, built same materials, serviced by same heating/etc, even hit by the same radar beams from the nearby airport, YET the west wing was deemed a 'sick' building and had 15 cases of one of the rarest forms of cancer brain tumours and the east wing was clean. What was completely left out of the ensuing investigation was the fact that people in the west wing usually parked their cars in an informal area across Guadalupe Pkwy, directly under high tension lines coming into the city and people in the east wing usually used the allocated parking, an isolated parking lot far away from lines. Every day the people who parked under the lines would get in and out of their cars exposing themselves to wildly gyrating values of magnetic field 'sucked' in by the metal of the cars. Shortly after a widow appeared on the Heraldo Show as part of gaining public sympathy for her 'wrongful death' case; the utilities company stripped out the high tension lines and put in very expensive buried ground distribution - and all symptoms stopped.
The epidemiologist at that time told me the cancer was actually a slow growing cancer appearing in a wide population of many, many people, but that people usually died of old age before the cancer advanced enough to have any symptoms appear. She felt something had 'triggered' the cancer to grow at a faster rate, thus young people were getting the 'observable' tumours. That falls in line with some scientists' claims that magnetic field/biological interaction is that the field encourages certain types of cell growths.