J C Male
W T Norris
B J Maddock
J A Bonnell

B J Maddock

Childhood cancer in relation to distance from high voltage power lines in England and Wales: a case-control study

B J Maddock

Designers of overhead power lines have to contend with many technical and economic factors in their work. More recently, yet another has been added to the list – electric and magnetic fields. This article points out the principal design features which affect the fields from lines and summarises the guidelines and standards which exist around the world for power-frequency fields. It is based on the author’s lecture to the IEE Power Division on the 20th February 1991.

J. Radiol. Prot. 14 (March 1994) 67-75
Measurements of static magnetic fields in homes in the UK and their implication for epidemiological studies of exposure to alternating magnetic fields

J Swanson

Technol. & Sci Labs., Nat. Grid Res. & Dev. Centre, Leatherhead, UK

Abstract. The variation of static magnetic fields within and between 55 homes in an area of south east England was measured. In general, fields varied by up to +or-10 mu T from the unperturbed geomagnetic field. They varied more in the corners of rooms (standard deviation 2.6 mu T) than in the centres (1.2 mu T). The field in the bedroom was uncorrelated with the field in the living room of the same home. For this sample of homes, the spread of field within a typical home was greater than the spread between homes. Therefore it is not meaningful to categorise homes by a single value of static field. This finding casts doubt on the suggestion that confusing epidemiological results concerning childhood cancer and power-frequency magnetic fields could be explained by a resonance mechanism involving static fields.

J. Radiol. Prot. 14 (June 1994) 155-164
Occupational exposures to power-frequency magnetic fields in the electricity supply industry

C J Merchant, D C Renew and J Swanson

Nat. Grid Res. & Dev. Centre, Leatherhead, UK

Abstract. Reports a survey of the exposure of staff in the UK Electricity Supply Industry to power-frequency magnetic fields. Two hundred and fifty-eight staff from a variety of jobs and locations wore a monitor for 1 week each. The results form a substantial body of data which adds significantly to the understanding of occupational exposures. The results show that fields encountered in shops, offices and distribution sites were roughly half those in power stations, which in turn were half those in transmission sites. Office workers based on transmission sites experienced higher fields (geometric mean of individual time-weighted average fields: 0.48 mu T) than those at headquarters offices (0.18 mu T). In power stations, electrical workers experienced higher fields (0.46 mu T) than mechanical workers (0.25 mu T). Amongst transmission and distribution staff, the highest fields (1.16 mu T) were experienced by transmission substation attendants and the lowest fields (0.17 mu T) by staff working predominantly in domestic environments.

J. Radiol. Prot. 14 (March 1994) 77-87
Exposures to power-frequency magnetic fields in the home

C J Merchant, D C Renew and J Swanson

Technol. & Sci. Labs., Nat. Grid Res. & Dev. Centre, Leatherhead, UK

Abstract. Power-frequency magnetic fields in homes come from a variety of sources, internal (appliances and domestic wiring) and external (electricity distribution and transmission circuits). The authors present results from a survey of the fields encountered at home by 258 adults over one week each. Information on the major electrical features of each of the homes was collected and related to the exposures incurred. The strongest identified factor influencing exposure at home was the presence or absence of overhead lines at voltages of 132 kV or above within 100 m of the home (geometric-mean TWA field encountered by participants 208 nT near lines, 54 nT not near lines). Occupants of homes near overhead lines or supplies from 415 V to 66 kV did not on average encounter fields significantly different to those in homes without such lines (50 and 54 nT, respectively). Occupants of flats incurred greater exposures than those incurred by occupants of semi-detached and terraced houses, which were in turn greater than those incurred by occupants of detached houses (109, 60, 56 and 43 nT, respectively).

J. Radiol. Prot. 15 (September 1995) 259-260
Allocating extremely-low-frequency magnetic-field exposure between sources

C J Merchant

Nat. Grid Technol. & Sci. Lab., Leatherhead, UK

Abstract. Where a person is exposed to extremely-low frequency magnetic fields from a number of sources, the question of how to allocate their total exposure between the sources may arise. This is not trivial, because the total magnetic field is the square root of the sum of the squares of the resultants of the phasor addition of the three spatial components of field, i.e. is not simply the linear addition of the field from each source in isolation. Each source produces an alternating field in three dimensions of space with a certain phase. In other words, the resulting field is described by a vector. An expression attributes the field between sources in a way that treats all sources equivalently. It can be applied in carrying out detailed exposure assessments of a person subject to fields from a number of appliances and background sources.

J. Radiol. Prot. 15 (September 1995) 253-258
Correlation of residential magnetic fields, road type and traffic in the UK

C J Merchant

Nat. Grid Technol. & Sci. Lab, Leatherhead, UK

Abstract. Traffic has been proposed as a confounding factor in studies of residential power-frequency magnetic fields and ill health. For confounding to be possible, magnetic field and traffic must be correlated. This paper shows that there is some correlation in the UK. The power-frequency magnetic fields from electricity distribution are lower in homes on cul-de-sacs than in those on residential through roads. There is a tendency for there to be higher magnetic fields in homes on residential through roads expected to carry more traffic. This may reflect the fact that the routes followed by traffic and electric-power distribution within residential areas are often related. The correlation of magnetic field and traffic density was not shown to extend to homes on main roads.

J Swanson

Abstract. An experiment has been performed to compare the calculated and measured magnetic fields produced by a double-circuit 400 kV transmission line. The phase currents were measured in the substation at one end of the line, taking particular care to measure the zero-sequence currents accurately, and the earth-wire current was measured at the span where the experiment was conducted. These currents were used to calculate the magnetic fields, using a number of computer programs based on Ampere’s law. the magnetic field was measured at 22 positions ranging from 100 m on one side of the line to 500 m on the other side. Measured and calculated fields generally agreed well. The largest errors were ±7% ±1 nT. These errors are attributed to a mixture of random errors in the calibration, resolution and synchronisation of the measuring instruments, and systematic errors stemming from the measurement of zero-sequence currents.

First published in Physics World, November 1996
Do the electric and magnetic fields created by power lines cause cancer?

Power lines and health

Click here for the full text of this paper

J. Radiol. Prot. 16 (December 1996) 287-301
Long-term variations in the exposure of the population of England and Wales to power-frequency magnetic fields

J Swanson

National Grid Technology and Science Laboratories, Kelvin Avenue, Leatherhead, Surrey KT22 7ST, UK

Received 5 August 1996, accepted for publication 27 September 1996
Abstract. This paper estimates the change in the average exposure of the population of England and Wales to power-frequency magnetic fields between 1949 and 1989. If magnetic fields are causally linked to disease with a linear exposure - response relationship, this quantity is related to the incidence rate of the disease. The exposure is divided into components attributable to a number of sources, principally residential background fields and fields from domestic appliances and the transmission system. The 1989 average exposures from these sources are estimated as 45 nT, 20 nT and 4.2 nT respectively. For each source, an understanding of how fields arise is combined with statistics on the use of electricity and demographic statistics to estimate the change in exposure from that source. These individual changes are then combined, weighted according to the average exposure from that source. The estimated increase in overall average exposure is by a factor of 4.5, which applies to the whole population and also just to children. This increase is slightly greater than the result obtained by the simpler method of taking average domestic electricity demand per consumer, and can be treated with more confidence. There are still numerous approximations involved, some of which are identified and discussed, with the conclusion that the estimated increase is probably an underestimate.

J. Radiol. Prot. 16 (December 1996) 275-286
Net currents in underground distribution circuits in the UK: implications for assessing magnetic-field exposures

J Swanson

National Grid Technology and Science Laboratories, Kelvin Avenue, Leatherhead, Surrey KT22 7ST, UK

Received 2 May 1996, accepted for publication 15 September 1996
Abstract. In the majority of homes in the UK, background power-frequency magnetic fields come from currents in final distribution circuits. In these circuits, load currents produce a negligible external magnetic field. The fields in homes arise from net currents, produced when neutral currents divert out of the distribution cable through earth connections.

This paper reports statistics on the prevalence of neutral-to-earth connections and measurements of net currents. Neutral-to-earth connections occur as part of protective multiple earthing, which is applied to 64% of underground circuits and 32% of domestic consumers' installations, and also occur accidentally within up to 20% (and probably substantially more) of homes. The 48 h average net current in a sample of 21 circuits was 3.6 A.

Because net currents are produced by diverted neutral current, they vary as loads vary. However, neutral current is proportional not to total load but to the unbalance between the three phases, and this weakens the correlation between net currents and loads. Individual unbalanced loads can lead to disproportionately high net currents. These considerations suggest that the best way of assessing average magnetic fields in residences (which is necessary for epidemiological studies) remains by direct measurement over at least 24 h.

Int J Radiat Biol 1996 May;69(5):651-2; discussion 653-7
Comment on the paper: enhanced deposition of radon daughter nuclei in the vicinity of power frequency electromagnetic fields.

Jeffers DE.

J. Radiol. Prot. 17 (September 1997) 197-199
NOTE
Net currents in underground distribution circuits in the UK

R F Cook, D C Renew and J Swanson

The National Grid Company plc, Kelvin Avenue, Leatherhead, Surrey KT22 7ST, UK

Received 1 June 1997, accepted for publication 2 July 1997

J. Radiol. Prot. 17 (June 1997) 111-113
Does the exposure of children in the UK to background residential power-frequency magnetic fields differ from that of the whole population?

J Swanson

The National Grid Company plc, Kelvin Avenue, Leatherhead, Surrey KT22 7ST, UK

Received 6 March 1997, accepted for publication 11 March 1997
Abstract. Background power-frequency magnetic fields in homes in the UK vary with the category of accommodation. The categories considered, in ascending order of average field, are detached, semidetached and terraced houses and flats. Because children occupy a different distribution of accommodation compared with the population as a whole, they will on average be exposed to a different background field. Using data on fields from a survey of two hundred people and on distribution of accommodation types from national statistics, it is estimated that this effect leads to children being exposed to background fields which are lower than those experienced by the population as a whole, 54.2 nT, by an average of , a difference which is barely significant.

Int J Radiat Biol 1998 May;73(5):579-85
Comment on the paper: High-voltage overhead lines and radon daughter deposition.

Jeffers DE.

Bioelectromagnetics 1999;20(4):244-54
Comparison of residential power-frequency magnetic fields away from appliances in different countries.

Swanson J, Kaune WT.

The National Grid Company plc, Leatherhead, Surrey, United Kingdom. john.swanson@ngtuk.com

The purpose of this paper is to review measurements of residential power-frequency magnetic fields made in different countries and to determine whether average magnetic fields away from appliances are higher in some countries than in others. The paper includes 27 studies reporting measurements of residential magnetic fields in samples of homes: 14 from North America, 5 from the United Kingdom, and 8 from other European countries. Various factors that might make the results from individual studies unrepresentative of average fields in the relevant country are identified and discussed. Because distributions of magnetic fields generally are approximately log-normal, they are summarised by their geometric means. The best estimate of the geometric means of long-term average background fields in the United States is 60-70 nT and in the United Kingdom approximately 36-39 nT. In other countries, there are insufficient studies to draw firm conclusions on average fields. Measurements of personal exposure are higher than measurements of background fields, perhaps because they include exposures from appliances and other sources in the home. The ratio of personal exposure to background field seems, on average, to be approximately 1.4.

J Radiol Prot 1999 Sep;19(3):213-29
Possible mechanisms by which electric fields from power lines might affect airborne particles harmful to health.

Swanson J, Jeffers D.

The National Grid Company plc, Leatherhead, Surrey, UK.

Power lines produce alternating electric fields and modify static electric fields in their vicinity. These electric fields, if large enough, can alter the concentration or transport of airborne particles (including particles harmful to health), for example by causing deposition of charged particles, polarisation of neutral particles, or by production of ions. It has been suggested that this could lead to adverse health effects being associated with power lines. Theoretical considerations and experimental evidence relevant to eight separate postulated mechanisms involving power lines and airborne particles are examined. On theoretical grounds, none should lead to any adverse health effect, primarily because the effects produced are very small and are swamped by air currents or by gravity, and because people spend limited time in the relevant conditions. The experimental evidence also weighs against any adverse health effects. Further, even if significant health effects were produced, they would be different from those suggested by existing epidemiology concerning power lines.

Int J Radiat Biol 1999 Dec;75(12):1533-9
Effects of wind and electric fields on 218Po deposition from the atmosphere.

Jeffers D.

National Grid Company plc, Surrey, UK.

PURPOSE: To estimate, under atmospheric conditions, 218Po deposition on a sphere representing the human head and compare with the effects of the maximum electric field to be found under a transmission line. METHOD: The effect of the wind in the absence of electric fields was calculated using the Reynolds Analogy between heat and mass transfer. The effect of the electric field was shown to be large compared with that of turbulence. A 'capture radius' due to the field was then estimated and charged 218Po particles blown into this region were assumed to be captured. RESULTS: The deposition ratio was proportional to gammaV0.4E0(0.67), where gamma = charged fraction of 218Po, V = velocity and E0 = surface electric field. With the charged fraction ranging from 0.9% to 3.2%, a surface field on 280 kV m(-1) and a wind speed of 3 m s(-1), the deposition ratio ranged from 3.4 to 9.3. The surface field is several orders of magnitude higher than the average personal exposures that have been measured in epidemiological studies and the effect does not appear to be of epidemiological significance. At low velocities, the predictions of this model are in agreement with the measurements of Henshaw et al. CONCLUSIONS: 218Po deposition by environmental AC fields cannot be advanced as an explanation for the reported associations between childhood leukaemia and electrical installations.

Epidemiology 1999 Jul;10(4):465-7
Childhood leukemia and electrical appliances.

Jeffers DE.

Int J Radiat Biol 2000 Dec;76(12):1685-91
Comment on the papers: increased exposure to pollutant aerosols under high voltage power lines; and Corona ions from powerlines and increased exposure to pollutant aerosols.

Swanson J, Jeffers DE.

Radiat Prot Dosimetry 2001;95(2):181-3
A note on the charging of aerosols by overhead line corona.

Jeffers D.

Elevated concentrations of corona ions have been measured in the vicinity of high voltage overhead lines. It is shown that the integrated ion exposure of aerosols in the corona plume is of the order of (x/u)n(x) where n(x) is the ion concentration at distance x downwind from the line and u is the wind speed. Estimated ion exposures are of order 10(11) m(-3) x s, less then 1% of the 10(13) m(-3) x s needed to cause saturation charging of 20 nm aerosols. It is suggested that it is not valid to postulate that AC corona is a health hazard as a consequence of its charging aerosols in the size range 20-125 nm.

Health Phys 2002 Sep;83(3):395-401
Basic restrictions in EMF exposure guidelines.

Renew DC, Glover ID.

National Grid Company, Leatherhead, Surrey UK

Different bodies have set guidelines restricting exposure to electric and magnetic fields. The limits at power frequencies recommended by these guidelines and the scientific basis and rationale for setting them have been reviewed, starting with the WHO Environmental Health Criteria 69 on Magnetic Fields, published in 1987. These guidelines are all designed to limit the induced current density to 10 mA m(-2), sometimes reduced by an additional safety factor of five for the general public. While published guidelines have, to date, universally adopted a restriction based upon induced current density, the internal electric field is the more fundamental quantity in determining biological effects. It is recommended that consideration be given to using the internal electric field rather than current density in future guidelines. Those who are responsible for setting guidelines need good scientific information on which to be able to set their limits. While there is already a significant weight of scientific evidence upon which exposure restrictions can be based, there is a need for more research to reduce uncertainties and to enable greater precision in the setting of limits. Some suggestions for future research directions, particularly aimed at further understanding of the interaction of electric fields with the nervous system, are suggested in this paper and are developed more widely in the following papers covering the proceedings of the EPRI Guidelines Science Workshop held in Brussels in June 2000.

Sheppard AR, Kavet R, Renew DC.

The EMF Exposure Guidelines Science Workshop was held to evaluate the information base for guidelines for electric and magnetic field exposures at extremely low frequencies, to identify research needs, and to discuss how best to apply scientific knowledge in developing exposure recommendations. Although the existing guideline values are based on electrostimulation of nerve and muscle tissues, guidelines must also consider controversial studies of potential health effects from chronic exposures at environmental levels that are far too weak to cause electrostimulation. The size of the safety factor applied in specifying exposure limits reflects a variety of factors that introduce uncertainties. These include confidence in existing dose-effect relationships, population diversity, the reliability and precision of techniques to control over-exposure, and completeness of the information base, particularly regarding long-term effects. Specific research questions that can change the level of uncertainty meaningfully were identified in these areas: biophysical mechanisms of electrostimulation; the range of thresholds throughout exposed populations; cancer causation in children and adults, including a possible role for contact currents; cognitive, behavioral, and physiological effects on the central nervous system; improved dosimetry; and better understanding of electromagnetic interference with implanted biomedical devices such as pacemakers, defibrillators, and physiological monitors. This report introduces ten papers from the workshop that address these and related topics in detail.

Swanson J.

National Grid, Leatherhead, Surrey, UK. john.swanson@ngtuk.com

Exposure standards for power-frequency electric and magnetic fields are often structured in terms of basic restrictions and investigation levels. For uniform exposures, investigation levels help the user operate within the standard without concern about exceeding the basic restriction. For non-uniform fields, however, numerical calculations of induced currents in the body may be necessary to determine compliance with the basic restriction. Utilities have strong incentives to apply exposure standards so that they do not impose unnecessary and unjustified constraints on their operations. They are therefore likely to be prepared to invest considerable effort in assessing compliance. However, the standards are based on round numbers, the values of tissue conductivity are not well characterized, and the calculations needed to apply the standards are still evolving. This suggests that, scientifically, this level of effort devoted to distinguishing compliant and non-compliant exposure scenarios is not justifiable.

J Radiol Prot. 2003 Sep;23(3):279-303.
A method for assessing occupational exposure to power-frequency magnetic fields for electricity generation and transmission workers.

Renew DC, Cook RF, Ball MC.

A new method for assessing both current and historical occupational exposures to magnetic fields has been developed and used in health studies involving a cohort of electricity generation and transmission workers in England and Wales. The exposure values are derived by calculation from engineering and operational data about the power stations rather than from measurements. They are provided for each of 11 job categories for each year of operation of each power station represented in the cohort. The engineering data are used to determine the average magnetic fields in specified areas of work within the power station and then applied to information about the time spent in these areas by each of the job categories. The operational data are used to adjust the exposures for each year according to the power station output for the year. Earlier methods used measurements or the advice of panels of experts to provide exposure scores for a number of job categories across all power stations and years. Such methods were not able to distinguish exposures from different power facilities or during the different years of their operation. Measurement surveys at 10 power stations of the magnetic fields in the work areas gave confidence that the calculations were realistic. Exposure measurements on 215 workers at three power stations were compared in job groups with the exposures predicted by the method. The Pearson correlation coefficient was 0.86 and the slope and intercept of the line of best fit were 0.87 and 0.07 microT respectively. The method gives a good prediction of measured exposure and is being used for studies of occupational exposure to magnetic fields and leukaemia, and of cardiovascular disease, and a reanalysis of brain cancer.

BMJ 2005;330:1290-3
Childhood cancer in relation to distance from high voltage power lines in England and Wales: a case-control study

Gerald Draper, Tim Vincent, Mary E Kroll, John Swanson

Objective To determine whether there is an association between distance of home address at birth from high voltage power lines and the incidence of leukaemia and other cancers in children in England and Wales.

Design
Case-control study.

Setting
Cancer registry and National Grid records.

Participants
29081 children with cancer, including 9700 with leukaemia. Children were aged 0-14 years and born in England and Wales, 1962-95. Controls were individually matched for sex, approximate date of birth, and birth registration district. No active participation by cases or controls was required.

Main outcome measures
Distance from home address at birth to the nearest high voltage overhead power line in existence at the time. Results Compared with those who lived >600 m from a line at birth, children who lived within 200 m had a relative risk of leukaemia of 1.69 (95% confidence interval 1.13 to 2.53); those born between 200 and 600 m had a relative risk of 1.23 (1.02 to 1.49). There was a significant (P<0.01) trend in risk in relation to the reciprocal of distance from the line. No excess risk in relation to proximity to lines was found for other childhood cancers.

Conclusions
There is an association between childhood leukaemia and proximity of home address at birth to high voltage power lines, and the apparent risk extends to a greater distance than would have been expected from previous studies. About 4% of children in England and Wales live within 600 m of high voltage lines at birth. If the association is causal, about 1% of childhood leukaemia in England and Wales would be attributable to these lines, though this estimate has considerable statistical uncertainty. There is no accepted biological mechanism to explain the epidemiological results; indeed, the relation may be due to chance or confounding.

Radiation Research 2006 Apr;165(4):470-8
Biophysical mechanisms: a component in the weight of evidence for health effects of power-frequency electric and magnetic fields.

Swanson J, Kheifets L.

Comparatively high exposures to power-frequency electric and magnetic fields produce established biological effects that are explained by accepted mechanisms and that form the basis of exposure guidelines. Lower exposures to magnetic fields (< 1 microT average in the home) are classified as "possibly carcinogenic" on the basis of epidemiological studies of childhood leukemia. This classification takes into consideration largely negative laboratory data. Lack of biophysical mechanisms operating at such low levels also argues against causality. We survey around 20 biophysical mechanisms that have been proposed to explain effects at such low levels, with particular emphasis on plausibility: the principle that to produce biological effects, a mechanism must produce a "signal" larger than the "noise" that exists naturally. 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.

Bioelectromagnetics. 2006 May 24;27(7):545-552
Childhood leukemia, electric and magnetic fields, and temporal trends.

Kheifets L, Swanson J, Greenland S.

During the past 25 years concern has been raised about the possible health effects of extremely low frequency (ELF) electric and magnetic fields (EMFs), particularly regarding childhood leukemia. Comparison of changes in electricity consumption (a surrogate for exposure) to changes in childhood-leukemia rates, known as ecologic correlation, have been used to argue both for and against the association between magnetic fields and childhood leukemia. In this paper we explore what can be learned from such an ecologic approach. We first examine separately the evidence on trends in exposure to EMFs and on trends in leukemia rates, and then compare the two. Both incidence rates and exposures have increased, but there are so many approximations and assumptions involved in connecting the two trends that we cannot regard the ecologic evidence as providing any meaningful evidence for or against a causal link.

Ann N Y Acad Sci. 2006 Sep;1076:318-30
Power-frequency electric and magnetic fields in the light of Draper et al. 2005.

Swanson J, Vincent T, Kroll M, Draper G.

National Grid, London, WC2N 5EH, UK. john.swanson@physics.org

Power-frequency electric and magnetic fields are produced wherever electricity is used; exposure is ubiquitous. Epidemiologic studies find an association between children living in homes with the highest magnetic fields and childhood leukemia, but bias is a possible alternative to a causal explanation. A new study, Draper et al., looks at residence close to high-voltage power lines, one source of exposure to such fields, and its design avoids any obvious bias. It finds elevated childhood leukemia rates, but extending too far from the power lines to be straightforwardly compatible with the existing literature. This leads to an examination of alternative explanations: magnetic fields, other physical factors, such as corona ions, the characteristics of the areas power lines pass through, bias, and chance. The conclusion is that there is currently no single preferred explanation, but that this is a serious body of science that needs further work until an explanation is found.

J Radiol Prot. 2007 Mar;27(1):41-58. Epub 2007 Mar 6
Investigation of the sources of residential power frequency magnetic field exposure in the UK Childhood Cancer Study.

Maslanyj MP, Mee TJ, Renew DC, Simpson J, Ansell P, Allen SG, Roman E.

HealthProtection Agency, Radiation Protection Division, Centre for Radiation, Chemical and Environmental Hazards, Chilton, DidcotOX11 0RQ, UK.

Thereis an unexplained association between exposure to the magnetic fields arising from the supply and use of electricity, and increase in risk of childhood leukaemia. The UK Childhood Cancer Study (UKCCS) provides a large and unique source of information on residential magnetic field exposure in the UK. The purpose of this supplementary study was to investigate a sample of UKCCS homes in order to identify the particular sources that contribute to elevated time-averaged exposure. In all, 196 homes have been investigated, 102 with exposures estimated on the basis of the original study to be above 0.2 microT, and 21 higher than 0.4 microT, a threshold above which a raised risk has been observed. First, surveys were carried out outside the property boundaries of all 196 study homes, and then, where informed consent had been obtained, assessments were conducted inside the properties of 19 homes. The study found that low-voltage (LV)sources associated with the final electricity supply accounted together for 77% of exposures above 0.2 microT, and 57% of those above 0.4 microT. Most of these exposures were linked to net currents in circuits inside and/or around the home. High-voltage (HV) sources, including the HV overhead power lines that are the focus of public concern, accounted for 23% of the exposures above 0.2 microT, and 43% of those above 0.4 microT. Public health interest has focused on the consideration of precautionary measures that would reduce exposure to power frequency magnetic fields. Our study provides a basis for considering the options for exposure mitigation in the UK. For instance, in elevated-exposure homes where net currents are higher than usual, if it is possible to reduce the net currents, then the exposure could be reduced for a sizeable proportion of these homes. Further investigations would be necessary to determine whether this is feasible.