This page is based on an article first published in Physics World and a review paper published in Radiation Research and assumes a certain level of scientific understanding from the reader.

If fields are to have any effect on people, there must be a mechanism of interaction with a simple physical system involving electric charges or magnetic moments, and that mechanism must produce a “signal” in the biological system which is greater than whatever “noise” level exists naturally. This requirement is not unique to electric or magnetic fields, but applies to any physical stimulus which produces an effect on the whole organism. Through the evolution of specialist sensory organs, such as the eye or ear, the human body can detect signals that are comparable to the fundamental noise levels, but it cannot detect signals below that limit.

This page cannot give a comprehensive survey of every mechanism ever proposed. We will consider some of the more interesting, however, as an indication of the mechanisms proposed and the issues raised.

• Alternating fields induce currents, and at high fields this is the basis of the known effect on the human body. But the currents induced by environmental field levels (the “signal”) seem to be less than the naturally ocurring current densities present in the body (in this context, the “noise”).
• The induced alternating currents could modulate the unipolar voltages that exist across cell membranes. The noise is partly thermal noise, which can be calculated, and for a single cell the voltage produced by the field is much less than the noise. It might be possible for the signal to exceed the noise if a large number of cells acted cooperatively, but this would suggest a special structure optimised for the purpose, analogous to the eye or ear, for which there is no evidence in humans. In contrast, sharks do have such structures, the ampullae of Lorenzini, and use them to detect microvolt-per-metre electric fields in water.
• The noise level could be reduced by reducing the bandwidth of the interaction mechanism, that is, by a resonance. In fact, to achieve an adequate signal-to-noise ratio, the bandwidth might have to be so narrow that the effect could be tuned to 50 Hz or 60 Hz (the power frequencies on the two sides of the Atlantic) but not both! Various resonant mechanisms involving the earth’s static field have been proposed. One of these, ion cyclotron resonance (for example of the calcium ion) cannot actually occur in a cell as the orbit would have to be over 1 m. Others, such as Larmor precession and ion parametric resonance, require unfeasibly infrequent collisions and are open to other objections.
• 50 Hz fields do not have enough energy to break chemical bonds directly - see more detail on this.
• Magnetic fields exert moments on ferromagnetic particles. Such particles are found in humans, but when the maximum plausible magnetic moment and the viscosity of the cell matter surrounding it are quantified, the amplitude of oscillation induced by environmental fields is less than the Brownian motion.
• Many biochemical reactions involve free radicals, highly reactive entities produced in pairs each with unpaired electron spins. Magnetic fields can affect singlet-triplet conversion and hence affect the concentration of free radicals by altering the recombination probabilities. The appropriate “signal-to-noise ratio” for this process has never been properly addressed. However, there is another reason why it seems that it cannot underlie the epidemiological results. Free-radical reactions typically have timescales of tens of nanoseconds, compared to which 50 Hz fields are effectively static. The relevant magnetic field is therefore the instantaneous total field, which is usually dominated not by the power-frequency component but by the geomagnetic field, which is around 50 µT in the UK.
• It has been suggested that the fields produced by transmission lines could deflect cosmic rays, concentrating them on people living nearby. The two most obvious objections to this are that with an alternating field the deflection will average to nearly zero, and the size of deflection can be estimated as a metre at most.
• Most recently, Professor Denis Henshaw at Bristol University has pointed out that electric fields can cause movement and concentration of radon daughter products (and possibly other carcinogens as well). Daughter products attached to polarisable particles will drift towards the source of a non-uniform field, and some daughter products are attached to charged particles which oscillate in the field. Corona ions produced by high electric fields on the surface of some power-line conductors can interact with existing airborne pollutants. Whether this could lead to health consequences and in particular to an increase in leukaemia is still controversial.

Conclusion

The review paper by Swanson and Kheifets concluded:

"Some of the mechanisms are impossible, and some require specific conditions for which there is limited or no evidence as to their existence in a way that would make them relevant to human exposure. Others are predicted to become plausible above some level of field. We conclude that effects below 5 microT are implausible. At about 50 microT, no specific mechanism has been identified, but the basic problem of implausibility is removed. Above about 500 microT, there are established or likely effects from accepted mechanisms. The absence of a plausible biophysical mechanism at lower fields cannot be taken as proof that health effects of environmental electric and magnetic fields are impossible. Nevertheless, it is a relevant consideration in assessing the overall evidence on these fields."

Clearly, it would be both arrogant and rash of physicists to argue that because we have not yet been able to think of a possible physical mechanism it is impossible for there to be an effect. However, the absence of a mechanism and the absence of replicable laboratory results inevitably - and correctly - means that the epidemiological results are viewed with greater caution.