Comments by electricity industry scientists on electric fields and ions from power lines
Possible mechanisms for a health risk from power lines have been proposed by Professor Henshaw and co-workers at the Human Radiation Effects Group in the Bristol University Physics department.
This page summarises the suggestions put forward by Professor Henshaw’s group and the response to them by electricity industry scientists (published in various scientific papers). The NRPB has also published a report on these suggestions; see a summary.
The two main suggestions (detailed in two papers published in 1999) are:
- Electric fields produced by high-voltage power lines cause existing charged airborne particles to oscillate; the larger the amplitude of oscillation the greater the probability of hitting a surface such as human skin and sticking to it; once stuck to the skin (“plated out”) they can cause harm. The main example proposed is the radioactive decay products of radon (“radon daughter products”), which are suggested to cause skin cancer by their subsequent radioactive decay when attached to the skin, but other examples, such as viruses or bacteria, are suggested as well.
Comments by electricity industry scientists:
- Fields high enough to cause significant increases (e.g. a factor of 3) in skin deposition certainly occur, but only within say 20 m of >132 kV power lines (the actual distance depends on the voltage and wind conditions); do people spend enough time outdoors that close to power lines for a significant effect?
- It is by no means established that radon daughter products can cause skin cancer; the evidence is not strong and is disputed. Even if they do, they are likely to be only a small contribution to skin cancer, and therefore any increase in deposition would make only a small difference to skin cancer rates. The NRPB say any health effects from deposited particles appear to be negligible.
- There is contrversy over whether the mechanism works only for smaller, more mobile airborne particles, and it is not certain whether it would work for most bacteria or viruses.
- High-voltage power lines produce corona ions. The corona ions are blown away downwind from the power line and transfer charge to existing airborne particles (“aerosols”). When those particles are breathed in, the increased charge results in an increased probability of retention in the airway or lung with consequently increased disease rates when the particle concerned is a cause of disease.
Comments by electricity industry scientists:
- Power lines do indeed produce corona ions and not just in bad weather, but really large quantities of ions are produced only in bad weather (when people are not generally present) or by occasional spans with inappropriate conductors or fittings.
- Increased charge can indeed result in increased retention of particles in airways, but this is not a general phenomenon and depends on particular combinations of particle size and airway size.
- The key parameter is not the total charge transferred from ions to airborne particles but the change in the number of charged particles, a subtle distinction that can potentially make a big difference.
- There is no accepted way of doing accurate calculations, but simple calculations suggest that even for spans of line in heavy corona, the quantity of ions produced may not be sufficient to produce more than a small change to the number of charged aerosols, and therefore a small (possibly negligible) change in disease rates.
Other proposals previously put by the Bristol group
- The suggestion: Polarised particles are attracted to the source of an electric field; therefore airborne particles migrate towards a power line, resulting in an increased airborne concentration and hence increased disease rates.
Comment: the forces seem to be too small and any effect is probably swamped by gravity and air currents. Direct measurements have not shown any such increase in radon daughter products near power lines.
- The suggestion: The electric field acting directly on charged particles causes increased deposition in the airway or lungs
Comment: the body screens electric fields, and the field in the airways or lungs is almost certainly too small to cause increased deposition.
- The suggestion: Electric field close to conductors cause airborne particles to be deposited on the conductors or fittings; these are then washed off by rain onto passers by.
Comment: the deposition does happen, but the circumstances for a person to get increased exposure seem unlikely in practice.
- The diseases for which these theories would primarily predict increases around power lines are skin cancer, lung cancer, or other skin or respiratory diseases. There is no particular epidemiological support for this, though neither have these suggestions ever been tested particularly carefully. Conversely, the epidemiological evidence on EMF is strongest in relation to childhood leukaemia. Professor Henshaw has suggested that increased lung deposition of radioactivity can significantly increase leukaemia rates but this is not widely accepted.
- Having said that childhood leukaemia is not perhaps the disease you would most expect to be caused by corona ions, this has now been tested by the Childhood Cancer Research Groupo at the University of Oxford, who concluded their data on childhood leukaemia did not support the corona-ion hypothesis. See full details of this test
- If increased deposition of air pollution in the lungs is to cause increased childhood cancer, clearly there must be a link between air pollution and childhood cancer to start with.See more on the evidence about pollution.