Mechanisms
Possible mechanisms for the interactions of EMFs with humans
If EMFs have an effect on humans, there must be a mechanism of interaction that produces this effect. To start with, a biophysical mechanism is needed whereby the fields interact at the fundamental physical level with a charge or a molecule. There is no known biophysical mechanism that produces a big enough effect to be significant. Many have been proposed in the scientific literature, but none have been demonstrated to provide a plausible mechanism. Some of those proposed are discussed below.
Ionising radiation is known to cause health effects because they contain enough energy to pass through our bodies and break chemical bonds, damaging DNA. Power-frequency fields (at 50 or 60 Hz) do not have enough quantum energy directly to break chemical bonds and therefore they do not have enough energy to damage DNA.
Some biological molecules can be split into two free radicals - reactive species with single, unpaired electrons which when separated spin in a given direction. Whether they recombine or stay separate depends on whether the two spins change direction or not. A magnetic field can change the spins. So, this is a mechanism whereby magnetic fields can affect the outcome of a chemical reaction and change the concentrations of these reactive species.
For this to happen, there must be the right combination of conditions: the recombination time, the strength of the external magnetic field compared to internal magnetic fields, and so on. The effect has been demonstrated many times at relatively high fields, 1000 µT or more. But it has not yet been demonstrated at the lower fields, of order one microtesla, as implicated by the epidemiology.
Melatonin is a hormone produced rhythmically by the pineal gland, normally highest at night. It has been linked to a number of important physiological functions such as influencing sleep propensity, and it is suppressed by light.
In 2010, shift work was classified as 'probably' carcinogenic by the International Agency for Research on Cancer (Group 2A), specifically for breast cancer. This classification comes from 'sufficient evidence' from animal experiments and 'limited evidence' in humans demonstrating that light during the normal dark period of the day increases carcinogenicity.
Melatonin has been proposed as a potential mechanism by which EMF may cause biological effects. One proposal by Stevens et al., (1987) is that EMF may suppress melatonin production in a similar way to light at night, providing a potential mechanism for explaining increased risk of cancer.
There have been many human, animal, and cellular studies investigating EMF and melatonin suppression since this hypothesis. The Health Protection Agency (HPA) produced a specific review of EMF, melatonin and breast cancer looking at the evidence and it has also been considered by other expert bodies such as the WHO. The summary on melatonin suppression from the HPA concluded:
"Investigations using cell, animal and humans have not given consistent or convincing evidence that EMF exposure affects melatonin production or action. However there are deficiencies in the existing research, which leave open the possibility of an effect."
and on breast cancer:
"No consistent evidence, from research using cells, animals and humans, that EMF exposure is a cause of breast cancer, nor has any mechanism for such an association been demonstrated."
At the surface of high voltage conductors, electrical breakdown of the surrounding air can occur, known as corona discharge. Where corona discharge occurs clouds of positive or negative ions are produced which are blown downwind of overhead lines. An increase of charge density downwind of power lines is well established and can be measured at distances up to several kilometres. These ion clouds charge pollutant particles that pass through them.
The corona ion theory hypothesised that these charges make harmful particles, such a radon, more likely to stick to the lungs or skin and concentrate their presence in a particular area and would provide a mechanism for overhead lines to cause disease. This has been subject to much research and was independently reviewed by the National Radiological Board in 2004. They concluded:
“…it seems unlikely that corona ions would have more than a small effect on the long-term health risks associated with particulate air pollutants, even in the individuals who are most affected. In public health terms, the proportionate impact will be even lower because only a small fraction of the general population live or work close to sources of corona ions.”
and
“Any health risks from the deposition of environmental particulate air pollutants on the skin appear to be negligible.”
More details on some of the studies and conclusions can be found here.
One suggested mechanism that relates directly to childhood leukaemia is "contact currents". The theory is that small voltages impressed on plumbing systems in homes could cause currents to flow in the body and in particular through the bone marrow in children that might be large enough to cause biological effects. In practice, UK plumbing and earthing systems do not result in the voltages occurring, although they are possible in other countries.
When this theory was tested in an epidemiological study, there was no support for it being linked to childhood leukaemia.