The “contact current” hypothesis

The epidemiological evidence suggests an association between magnetic fields and childhood leukaemia.

Because there is no robust laboratory support for a direct effect of magnetic fields, nor any known biophysical mechanism, we need to ask: is there anything else that could explain this association?  And because magnetic fields (as assessed in most epidemiological studies) come from two different main sources, low-voltage distribution wiring and high-voltage power lines, we need to ask specifically: is there any other way in which both low-voltage distribution and high-voltage power lines could possibly affect living tissues?

One sugestion is "contact currents": the idea that small voltages impressed on e.g. 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.  We give a simple summary first then we go into more details.

Simple summary of the hypothesis

The theory

The theory runs like this:

  • Both low-voltage distribution wiring and high voltage power lines can produce small voltages, typically less than a volt, between a home's plumbing system and the ground.
    • Low-voltage distribution does this because of the way the earth of the electricity system is bonded to the water system as it enters the home.
    • High-voltage power lines do this by the magnetic field inducing voltages in the loops formed by distribution wiring and plumbing systems.
  • There are times when a child is exposed to this voltage, e.g. when they are in the bath (at ground potential because it is earthed through the waste pipe) and they touch a tap.
  • That voltage causes a current to flow through the child, and where the current is concentrated through the bone marrow in the limbs, it may be high enough to cause effects in the cells that populate the bone marrow.
  • Because the bone marrow is where our blood cells are produced, these effects may go on to lead to childhood leukaemia.

And does this theory work?

  • We know that the two mechanisms for producing the voltages can indeed work.  But they don't necessarily produce voltages in all homes, and, from house to house, the voltage isn't always very well correlated with the magnetic field.
  • We know that, if a voltage is present at all, a child is likely to be exposed to it sometimes.  But it may not be very often or for very long.
  • We know that, when the child is exposed to the voltage, it will indeed produce a contact current through them, which induces a voltage over cells, and which is concentrated in the bone marrow, and can be higher there than elsewhere in the body.  But we don't know whether even that higher voltage in the bone marrow is high enough to cause an effect.
  • And we really don't know, if those voltages in the bone marrow are large enough to cause effects in the first place, whether this would lead to childhood leukaemia or not.

So, overall, we'd say there are quite a lot of unknowns still.  It remains a hypothesis - worth investigating further, but certainly not established.

And, the only time it has been tested in an epidemiological study, there was no support for it being linked to childhood leukaemia.

That was the simple version...

We now give more detail on each of the stages in the theory.

How are these contact voltages produced?

From the distribution system

Consider first the distribution system (the low-voltage wiring supplying electricity to homes).  The mechanism depends on multiple earthing of the neutral: see a more detailed explanation of this.

diagram showing how contact voltages are produced

With multiple earthing of the neutral conductor, some of the neutral current can divert out of the neutral conductor through one of these earth connections (1).  The earth conections are not perfect, so the current flowing through the earth connections creates a small voltage.  This means the voltage or "potential" of the neutral conductor entering the home, which also serves as the earth connection, is raised a small amount above earth potential.

Within the home, the electrical earth point is bonded to the water pipes (2).  So now the water pipes in the home are also a small potential above earth.

But also, through this bonding to the water pipes (2), some of the neutral current can flow back through the water pipe (3).  That also creates a small voltage on the water pipes.

If we now assume that the waste pipes are at true earth potential, the voltage VWG appears between the tap (connected to the water pipes) (4) and the water in the bath (connected to earth) (5).  A child sitting in the bath and touching the tap would experience this contact voltage and it would cause a small current to flow.

Because the contact voltage stems from the diverted neutral current, and because this diverted neutral current - the "net current" - is also a significant source of magnetic field in most homes - often the main source - we expect there to be a linkage between the contact voltage and the magnetic field.  That's an essential part of this theory - it's why a true effect of the contact voltage could look as if it is an effect of the magnetic field.

For the contact voltage to appear in this way, several things have to apply.  For example, both the water pipes in the home and the waste pipe inside and outside have to be continuous and conducting, which won't always be the case.  So there will be some homes where this mechanism can't operate, and in those homes, there is consequently no exposure.

Note also that we have described how the mechanism works on a simplified model - in practice there are lots of variants as to where the earth connections are and where the various currents flow and voltages appear.  But the basic principles stay the same.

From high-voltage power lines

A high-voltage power line produces a magnetic field, which is an alternating field.  Alternating magnetic fields induce voltages in conducting loops.  The water pipes and the distribution conductors form all sorts of conducting loops because of the way they are bonded, and all these loops will therefore have small voltages induced in them, and this means the water pipes will have voltages with respect to true ground - the VWG in the diagram above.  It can be quite difficult to define exactly where the loops are, because there are so many different connections and wires and pipes.  But whatever the loop in question is, the higher the magnetic field from the power line, the higher the voltage.  So again, the VWG is linked to the magnetic field.

And we might expect these induction effects to extend quite a way from the power line, because of the way earthing systems are connected across residences - so this could link with the CCRG finding of raised childhood leukaemia rates at unexpectedly large distances.

How well correlated with magnetic fields are they?

For this theory to work, it not just has to produce effects in the body that can cause childhood leukaemia.  It also has to explain the reported association of residential magnetic fields with childhood leukaemia.  That means that the higher contact voltages have to correspond pretty well with the higher magnetic fields, so that what looks like a magnetic field effect is in fact a contact-voltage effect.  So we need ask: how closely linked are magnetic fields and contact voltages?

An epidemiological study in Northern California found a correlation coefficient of 0.10 between the indoor contact voltage and the magnetic field.

Another paper pooled the data from this study with others and found a correlation coefficient of 0.29.

These correlation coefficients are not very good.  So we might conclude this is not very strong support for the hypothesis.  But the authors of the pooled analysis did some further analysis looking not just at whether all contact voltages are correlated with all magnetic fields, but specifically at whether the higher contact voltages were associated with the higher magnetic fields, and concluded that the correlation might in fact be big enough.  So  this issue alone probably does not rule out the hypothesis.

Radiat Res. 2011 Mar;175(3):390-6.
Exposure to electrical contact currents and the risk of childhood leukemia.
Does M, Scélo G, Metayer C, Selvin S, Kavet R, Buffler P.The objectives of this study were to examine the association between contact current exposure and the risk of childhood leukemia and to investigate the relationship between residential contact currents and magnetic fields. Indoor and outdoor contact voltage and magnetic-field measurements were collected for the diagnosis residence of 245 cases and 269 controls recruited in the Northern California Childhood Leukemia Study (2000-2007). Logistic regression techniques produced odds ratios (OR) adjusted for age, sex, Hispanic ethnicity, mother's race and household income. No statistically significant associations were seen between childhood leukemia and indoor contact voltage level [exposure ≥90th percentile (10.5 mV): OR  =  0.83, 95% confidence interval (CI): 0.45, 1.54], outdoor contact voltage level [exposure ≥90th percentile (291.2 mV): OR  =  0.89, 95% CI: 0.48, 1.63], or indoor magnetic-field levels (>0.20 µT: OR  =  0.76, 95% CI: 0.30, 1.93). Contact voltage was weakly correlated with magnetic field; correlation coefficients were r  =  0.10 (P  =  0.02) for indoor contact voltage and r  =  0.15 (P  =  0.001) for outdoor contact voltage. In conclusion, in this California population, there was no evidence of an association between childhood leukemia and exposure to contact currents or magnetic fields and a weak correlation between measures of contact current and magnetic fields.
Radiat Res. 2011 Dec;176(6):807-15.
The relationship between residential magnetic fields and contact voltage: a pooled analysis.
Kavet R, Hooper C, Buffler P, Does M.It has been suggested that residential exposure to contact currents may be more directly associated with the potential for an increased risk of leukemia in childhood than magnetic fields. Contact current exposure occurs when a child contacts a bathtub's water fixtures, which are usually contiguous with a residence's electrical ground, and when the drainpipe is conductive. The Northern California Childhood Leukemia Study (NCCLS) is the only epidemiological study known to address whether contact current may confound the reported association between residential magnetic fields and childhood leukemia. The study contributed contact voltage and magnetic-field data for over 500 residences of leukemia cases and control children. We combined these data with the results of previous measurement studies of contact voltage in other communities to conduct an analysis of the relationship of magnetic fields with contact voltage for a total sample of 702 residences. The Spearman correlation of magnetic field with contact voltage was 0.29 (Spearman, P < 0.0001). Magnetic-field and contact voltage data were both divided into tertiles, with an upper magnetic-field cutpoint of 0.3 μT suggested by values used in epidemiological results and an upper contact voltage cutpoint of 60 mV based on dosimetric considerations. Expressed as an exposure odds ratios (EOR), we report an association of contact voltage with magnetic fields of 15.1 (95% CI 3.6-61) as well as a statistically significant positive trend across magnetic-field strata (EOR of 4.2 per stratum with 95% CI 2.4-7.4). The associations appear to be large enough to support the possibility that contact current could be responsible for the association of childhood leukemia with magnetic fields. 

 

What currents and voltages do they produce in the body?

The calculations and measurements suggest that:

  • Not all homes have contact currents in the first place (if they have insulating drains or water pipes, no contact currents will flow).
    • In the California epidemiological study, 47% of homes had zero contact current measured at the bath.

When contact currents flow at all:

  • The voltage VWG that drives the current is typically less than one volt.
    • some measurements studies have actually found a lot less than one volt - for example a study of 191 homes in Denver found a median water-pipe voltage of 201 mV, but a median voltage measured between tap and drain in the bath of only 1 mV.
  • Although there is a lot of variability in the electrical properties of children, given that in the scenario of sitting in the bath touching a tap, they would have wet skin, their resistance could be of order of 1 kΩ.
  • So the currents that flow in the arms of the children could be a few tens of microamps.
  • Dosimetry studies tell us that a few microamps corresponds to a few tens of mV/m in the bone marrow of a child's arm.
    • more specifically: 10 μA of current is calculated to produce 50 mV/m averaged over the marrow in the lower arm of a small child and 130 mV/m or more in 5% of that tissue.
IEEE Trans Biomed Eng. 2001 Sep;48(9):1020-6.
Electric fields in the human body resulting from 60-Hz contact currents.
Dawson TW, Caputa K, Stuchly MA, Kavet R.Contact currents occur when a person touches conductive surfaces at different potentials and completes a path for current flow through the body. Such currents provide an additional coupling mechanism to that, due to the direct field effect between the human body and low-frequency external fields. The scalar potential finite difference method, with minor modifications, is applied to assess current density and electric field within excitable tissue and bone marrow due to contact current. An anatomically correct adult model is used, as well as a proportionally downsized child model. Three pathways of contact current are modeled: hand to opposite hand and both feet, hand to hand only, and hand to both feet. Because of its larger size relative to the child, the adult model has lower electric field and current-density values in tissues/unit of contact current. For a contact current of 1 mA [the occupational reference level set by the International Commission on Non-ionizing Protection (ICNIRP)], the current density in brain does not exceed the basic restriction of 10 mA/m2. The restriction is exceeded slightly in the spine, and by a factor of more than 2 in the heart. For a contact current of 0.5 mA (ICNIRP general public reference level), the basic restriction of 2 mA/m2 is exceeded several-fold in the spine and heart. Several microamperes of contact current produces tens of mV/m within the child's lower arm bone marrow. 

Could exposure to these voltages contribute to developing leukaemia?

We believe that leukaemic cells originate in the active, or red, marrow.  In children, this is found in the hands, feet and long bones of the limbs, but in adults, it is mainly in the sternum and hips.  So a mechanism such as contact currents that predicts effects in the bone marrow of the limbs would fit with associations having been found for childhood leukaemia but not for adult leukaemia.

The contact current hypothesis suggests that, in homes where it operates at all, it could produce electric fields in the bone marrow of tens of mV/m, or, in homes with higher contact voltages, perhaps hundreds of mV/m.

We usually think about these induced fields as having an effect in the retina or the central nervous system.  That is where the most sensitive established effect of magnetic fields - magnetophosphenes - occurs.  ICNIRP set the general public exposure limit, designed to prevent these effects, at 20 mV/m.

So if the threshold in bone marrow were the same as in the central nervous system, we'd say that the contact current hypothesis does produce fields and voltages big enough potentially to have effects.

There are three big unknowns however:

  • We don't know whether bone marrow is as sensitive as nerves in the brain are.  In fact, we often say that we expect the central nervous system to be particularly sensitive because of the way the nerves are connected, so we might expect the bone marrow to be less sensitive.
  • Even if the threshold for effects in bone marrow is reached by the contact currents, we really have no idea whether those effects, whatever they are, might lead on to leukaemia.
  • It's perfectly plausible that children would be exposed to the contact voltages when they have baths - maybe not every time, but certainly quite often - but that would still only be once a day or so for a few seconds at a time, and we have no idea if this is enough to add up to a significant effect.

What is the direct evidence linking contact voltages with childhood leukaemia?

So far, only one epidemiology study has measured contact currents and looked at the risk of childhood leukaemia, a study in Northern California.  See the abstract below.

It did not find any association of childhood leukaemia with contact voltage:

  • indoor contact voltage level,exposure ≥90th percentile (10.5 mV): OR  =  0.83, 95% confidence interval (CI): 0.45, 1.54

To be fair, this particular study did not find any correlation with magnetic fields either, unlike most other studies:

  • indoor magnetic-field levels, >0.20 µT: OR  =  0.76, 95% CI: 0.30, 1.93

So it could just be that in this particular study population, there were no effects of any sort, meaning that the failure to find an association with contact voltage is not significant.  The study turned out to have an unexpectedly low proportion of homes either with high magnetic fields or with high contact currents, which is an interesting finding in its own right, but means the study was never going to be very informative about these effects.

However, the bottom line remains, in the only study to have looked directly for an association with contact voltage, no association was found.

Radiat Res. 2011 Mar;175(3):390-6.
Exposure to electrical contact currents and the risk of childhood leukemia.
Does M, Scélo G, Metayer C, Selvin S, Kavet R, Buffler P.The objectives of this study were to examine the association between contact current exposure and the risk of childhood leukemia and to investigate the relationship between residential contact currents and magnetic fields. Indoor and outdoor contact voltage and magnetic-field measurements were collected for the diagnosis residence of 245 cases and 269 controls recruited in the Northern California Childhood Leukemia Study (2000-2007). Logistic regression techniques produced odds ratios (OR) adjusted for age, sex, Hispanic ethnicity, mother's race and household income. No statistically significant associations were seen between childhood leukemia and indoor contact voltage level [exposure ≥90th percentile (10.5 mV): OR  =  0.83, 95% confidence interval (CI): 0.45, 1.54], outdoor contact voltage level [exposure ≥90th percentile (291.2 mV): OR  =  0.89, 95% CI: 0.48, 1.63], or indoor magnetic-field levels (>0.20 µT: OR  =  0.76, 95% CI: 0.30, 1.93). Contact voltage was weakly correlated with magnetic field; correlation coefficients were r  =  0.10 (P  =  0.02) for indoor contact voltage and r  =  0.15 (P  =  0.001) for outdoor contact voltage. In conclusion, in this California population, there was no evidence of an association between childhood leukemia and exposure to contact currents or magnetic fields and a weak correlation between measures of contact current and magnetic fields.

 

A note on changes over time

When we look at the epidemiology of magnetic fields and childhood leukaemia, there is just a hint that the more recent studies may not be finding the same association as the earlier ones (for instance, compare the Kheifets pooled analysis to the earlier Ahlbohm pooled analysis).  More concretely, the CCRG study found a declining risk with high-voltage power lines in the UK over the decades from the 1960s to the present.

Over that same period, it seems likely that conducting - metalic - drain pipes have become less common and have been replaced by more plastic pipes.  In houses where that has happened, the contact current hypothesis could not operate.

So this may be a piece of supporting evidence for the contact current hypothesis - that it predicts a decline in risk over time.  But this is all a bit speculative and can't really be used as strong support for the hypothesis.

Research papers on this subject

See the abstracts of scientific papers published about this hypothesis.

Other possible mechanisms

This page is about contact currents, a suggested alternative to magnetic fields as a mechanism for causing disease.  Other alternative mechanisms include:

Terminology

The contact current hypothesis originated in America ... but this website is UK-based.  This affects two terms used in particular:

  • Americans tend to talk about "grounding" and the UK uses "earthing"
  • A "faucet" in the US is a "tap" in the UK

We use these terms interchangeably.  Please don't read too much significance into which we use!