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

We thank everyone who has commented on our paper; we respond here
only where we feel we can add anything to what we have already said in the
paper.

Various commentators have criticised us for publishing alarming
results that we are unable to explain. We should have preferred to delay
publication until we could offer a definitive explanation for our results.
However, once the analysis was complete, it would have been unethical not
to publish results of potential public health significance. Moreover, a
partial version of these results had been leaked and it became clear that
the only satisfactory way to respond to these leaks was to publish the
complete results.

We address first the responses concerning problems of methodology and
interpretation of the results, and then those that suggest possible
explanations of the results.

We do not agree with the statement by Hepworth et al that “the
findings are inconsistent with [the UKCCS1] study” – the only other UK
study with which comparison can be made. We consider that our results are
entirely consistent with that study: their relative risk (for acute
lymphoblastic leukaemia) of 1.42 for 0-400 m seems to agree rather well
with ours of 1.69 for 0-200 m and 1.23 for 200-600 m. This conclusion is
not weakened by the fact that the UKCCS estimate was not statistically
significant; this lack of statistical significance could be a consequence
of the smaller sample size in that study.

Hepworth et al and Kheifets et al raise questions concerning the
controls. As we stated in our paper, it seems possible that the elevated
relative risk for leukaemia depends, at least partly, on an
unrepresentative set of controls, since the addresses of the leukaemia
controls tend to be further from power lines than those of the controls
for the other diagnostic groups. We are puzzled by the suggestion by
Hepworth et al that findings can be considered to be less robust if the
estimates are noticeably different when the matching is broken, though, as
Kheifets et al point out, in the present analysis the estimates in fact
remain essentially unaffected. This is, however, quite separate from the
question of whether the complete set of controls should have been used.
Kheifets et al show that different estimates are then obtained. Although
these authors do not say so, these estimates would provide little evidence
for a relation between distance and leukaemia risk. There are two reasons
for regarding these latter estimates as unsatisfactory. First, they do not
take account of the original matching factors, particularly year of birth
and birth registration district. In fact, adjusting for birth year has
little effect on the estimates whichever set of controls is used. One
cannot, however, allow for a possible effect of birth registration
district in the unmatched analysis. Secondly, and in our view more
importantly, it is invalid to re-analyse the data using alternative
controls if this is done simply because the first set gives unexpected
results. (The situation is different if the original analysis is subject
to bias. It is extremely unlikely that there is any important source of
bias here.)

Hepworth et al suggest that adjustment for confounding factors might
explain our results. Neither our (admittedly less than adequate) measure
of socio-economic status (reported in the paper) nor population density
(not reported) explains the findings. We considered the question of
population mixing but it is not clear that an appropriate measure is
available for the whole of England and Wales over a period of 34 years. We
agree there might be other confounding factors that could explain our
results were we able to identify them.

Whitlock raises the question of bias arising from possible
differences in the likelihood of omitting cases near and far from lines.
We think this is unlikely, but such an effect would presumably apply also
to the controls and to the other diagnostic groups.

Coghill, Hepworth et al and O’Carroll refer to our calculation that
the association with distance that we reported implies that five cases of
childhood leukaemia a year in England and Wales would be attributable to
high voltage power lines if the association is causal. None of these
writers repeats our distinction between (chance) association and
causality. Coghill makes suggestions about the numbers of cases
attributable to 132kV lines. We do not agree with all his reasoning, but
in any event he goes beyond our data. O’Carroll’s calculations are based
on the assumption that the results at 0-200 metres are due to magnetic
fields while those at 200-600 metres are due to chance. We do not think it
justifiable to make an arbitrary division of our results into two bands.

Some of our correspondents over-interpreted, perhaps misinterpreted,
the findings. Burgess draws attention to the finding of a decreased
relative risk for CNS/brain tumours near the lines. But this decrease,
unlike the increase for leukaemia, is not part of a statistically
significant trend, nor does it correspond to any prior hypothesis. We
agree with Gaylord’s suggestion that the pattern of results for CNS/brain
and other tumours appears to be due to chance; this particular argument
cannot be applied to leukaemia though we have emphasised that, for other
reasons, we regard it as possible that the results are in fact due to
chance. Phillips appears to place too much emphasis on small, probably
chance, increases in relative risks at greater distances. His graph
appears to contain some inaccuracies and compares our leukaemia results
with the UKCCS ‘all malignancies’. As explained above, we do however agree
with him that our results relating to distance and leukaemia risk are
consistent with those of the UKCCS.

We, and our respondents, have considered a number of alternative
explanations for our results. Henshaw and Preece refer to Henshaw’s corona
ions hypothesis. Coulton questions its plausibility; we tested for it
without taking any view on its plausibility. We described our test as
“oversimplified”. Preece, who devised the method, points out the
simplification that all addresses in the north-east quadrant from the line
are considered “exposed”, i.e. that the wind that transports these ions is
assumed to be from the south-west, whereas one ought to consider actual
wind directions. Additionally, all addresses within 600 m are considered
equally exposed, without taking account of the actual distance or the
different propensity of different lines to produce ions, and the method
considers the closest point of the line only. We are analysing our data
using a better test, agreed with both Preece and Henshaw, avoiding these
simplifications.

Coghill, Juli, Lincoln and Preece raise questions about the
measurement of fields and about other sources of EMFs. We made no
assumption about a direct equivalence between field and distance. We shall
analyse calculated fields in a further paper; these fields, which are
still being checked, take into account the line characteristics mentioned
by Juli. We are investigating the possibility of analysing proximity to,
and calculated fields from, lower voltage distribution systems, but not
those from domestic appliances. Electric fields, suggested by Coghill,
appear no more likely to explain risks at 600 m than magnetic fields.

We shall investigate as many as possible of the various suggested
explanations put forward by Koenigbuescher, Netter, Poston, Coghill,
Henshaw, Preece and McDevitt, though in many cases the relevant data
will not be available.

1 UK Childhood Cancer Study Investigators. Childhood cancer and
residential proximity to power lines. Br J Cancer 2000; 83:1573-80.

Competing interests:
GJD, TJV and MEK: no conflict of interest. JS is employed by National Grid Transco and worked on this project with their permission

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Competing interests:
For LK work with EPRI and consulting with utilities

Leeka Kheifets1, Maria Feychting2, Joachim Schuz3

1. Department of Epidemiology, School of Public Health, UCLA, CA, USA

2. Institute of Environmental Medicine, Karolinska Institutet, Sweden

3. Institute of Cancer Epidemiology, Danish Cancer Society, Denmark

We have read with interest the paper from Draper and colleagues [1].
Given its large size the risk estimates in the paper should be stable.
Furthermore, because contact with the subject was not necessary selection
bias due to the differential participation among cases and controls, which
plagued some of the previous studies [2], has been avoided. Thus we were
particularly surprised by the dependence of the results on the chosen
control group noted by the authors, (who used CNS and other cancer
controls for leukaemia cases in one of the comparisons). To explore this
further we combined all controls into one group and used it for
comparison. We felt this is justified based on both theoretical and
empirical grounds: exposure at birth among controls chosen for leukaemia,
brain tumours and other cancers should not depend on the cancer subtype;
crude odds ratios calculated by us did not differ (beyond first decimal)
from the matched results presented by authors (data not shown).

Use of the combined control group revealed a pattern different than
the one presented in the original paper (Table 1). As would be expected,
results for all cancers combined show no relation to the distance. For
both leukaemia and brain cancer results at two distances are noteworthy:
for the 50-100 meters category an excess of leukaemia and a deficit for
brain tumours is observed. For the 500-600 meters category we observed a
modest excess for both leukaemia and brain tumours. Of note is that the
trend reported in the original paper is not present when the combined
control group is used, thus indicating that the trend depended on the
leukaemia controls rather than on the leukaemia cases. We agree with the
authors that the results of this study do not support a possible magnetic
field association, as has been reported by the IARC monograph [2].
However, distance is known to be a very poor predictor of magnetic field
exposure, and therefore, results of this material based on calculated
magnetic fields, when completed, should be much more informative.

Further insight might be gained by details on the methods used for
the control selection and sensitivity analyses by age, sex and time
period.

1. Draper G, Vincent T, Kroll ME, Swanson J. Childhood cancer in
relation to distance from high voltage power lines in England and Wales: a
case- control study. BMJ 2005; 330:1290-2.

2. Ahlbom A, Day N, Feychting M, et al. A pooled analysis of
magnetic fields and childhood leukaemia. Br J Cancer, 83, 692-8 (2000).

3. IARC Monographs on the Evaluation of Carcinogenic Risks to
Humans. Vol 80 Non Ionizing radiation, Part 1: Static and Extremely Low ¨C
Frequency Electric and Magnetic Fields. 2002

Competing interests:
For LK work with EPRI and consulting for utilities.

Like many of similar studies (e.g. Ahlbom, Feychting), Draper,
Vincent et alt. lack a relation between the dose and effect. Not
calculated is the duration of living close to power lines. Excluded is the
11 kV (kilovolt) system, which is much more widespread then higher voltage
systems. The magnitude of the magnetic fields of 11 kV-systems is the same
like those of higher voltages. This is true also for cables lying
underground because of the proximity to the public (around 1m).

At a distance of about 50 ... 100 m from the axis of a overhead
powerline system the strength of the electric and magnetic fields fall to
the background level.

When measuring electromagnetic fields, I always found the highest
levels (out of occupational locations) in households, especially close to
devices whith magnetic components like dishwashers, transformers (battery
chargers), speakers, computers.

What about trains? Because they use 25 kV single-phase low frequency
alternating current (ac), the fields of those powerlines are a multiple of
that of “ordinary” powerlines.

Even systems not intended to carry electric current like central
heating, gas pipes, water pipes can produce magnetic fields of
considerable strength due to balancing currents.

Competing interests:
None declared