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

BMJ 2005; 330 doi: https://doi.org/10.1136/bmj.330.7503.1290 (Published 02 June 2005) Cite this as: BMJ 2005;330:1290

geography and Controls

Draper et al’s result 1 that: “Compared with those who lived >
600 m from a line at birth, children who lived between 200 and 600 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)”, coupled with their conclusion that: “If the association is
causal, about 1% of childhood leukaemia in England and Wales would be
attributable to these lines”, has made their paper an important document
in the political debate regarding the case for precautionary policies to
limit EMF exposure 2, 3.

The authors qualify their data with the comment: “The results are
highly significant but could nevertheless be due to chance - for example,
if the leukaemia controls are not sufficiently representative of the
relevant population. Some support for this explanation can be derived
from the different distance distributions observed for the leukaemia and
non-leukaemia controls in table 1. Comparison of the leukaemia cases with
the latter still suggests that there is an increased risk for leukaemia
but it is much lower than that found using the matched controls. We
emphasise, however, that the use of matched controls is the most
appropriate approach.

The impact of control selection was discussed in the “Rapid
Responses” from Hepworth et al, Kheifets et al, Roman et al and the
original authors. Hepworth et al concluded their letter with the comment:
“The findings of this study are of interest in that they point towards
geographical correlates of risk for childhood leukaemia but do not support
the hypothesis that electromagnetic fields have a causal role.” The
possible influence of geographical variation will be considered below.

Draper et al matched cases and controls within registration districts
and a map of England and Wales showing their boundaries can de downloaded
from the “National Statistics” web site
(http://www.statistics.gov.uk/geography/reg_districts.asp).
There are currently some 400 districts and the map shows that there are
vast differences in size. Draper et al’s study ran from 1962 – 1995 and,
because large numbers of registration districts were abolished in the
local government reorganisation of 1974, there were many more of them at
the beginning of the study than at the end.

The authors were considering the possible impact, on childhood
cancer, of the 7000 km of transmission lines which make up the National
Grid in England and Wales. Cases were considered to be “exposed” up to a
distance of 600 m either side of the lines, giving a total “exposed” area
of 8400 km2 which is comparable to that of North Yorkshire, which is the
largest English county (administrative area 8038 km2). There is clearly
scope for considerable geographic variation within such a large area.

A map of the National Grid can be downloaded from chapter 6 of the
National Grid Company’s Seven Year Statement
(http://www.nationalgrid.com/uk/ ). It is apparent that many districts
are not crossed by its transmission lines and, in Draper et al’s terms,
the populations of these districts will be unexposed. (Although a lot of
districts are not crossed by transmission lines they are still likely to
host many lower voltage distribution lines.)

The authors took one control per case, matched for sex and date of
birth within the registration district.

The total number of cases and controls = S1

= Sum over time and registration districts of (district population *
Age-standardised rate of disease incidence)

The number of exposed controls = S2

= Sum over time and districts of (population * ASR * fraction of
population which is exposed)

The fraction of the controls which is exposed is then S2 / S1 and, if
the ASR’s were uniform over the districts, then the calculated exposed
fraction would be equal to that for the total population

In its eleventh report, the “Committee on Medical Aspects of
Radiation in the Environment” (COMARE) 4 provides data on “The
distribution of childhood cancers in Great Britain 1969 – 1993.” Over
this period, which is comparable to that of the Draper et al study, there
were significant geographical differences in the regional incidence of
leukaemia and other cancers. The calculated exposed fractions are,
therefore, not likely to be equal to that for the total population and, as
Draper et al found, the distance distributions for the leukaemia and total
cancer controls will be different. To quote from section 3.6 of the
report:

“In the counties of England and Wales the rates for all cancers
combined vary between 132.2 per million (Buckinghamshire) and 94.1 (West
Glamorgan), a ratio of 1.40. For leukaemia the rates varied between 48.3
(Buckinghamshire) and 29.1 (West Glamorgan), a ratio of 1.66”

It is of interest to note that, at 1.66, the Buckinghamshire / West
Glamorgan ratio for leukaemia is almost equal to the excess risk of 1.69
reported by Draper et al.

The COMARE data show that variation in the leukaemia ASR’s, at 1.66
is greater than that for all cancers combined (1.40). A calculation of
the exposed fraction using the “all cancers” controls is thus likely to be
closer to the value for the total population than a calculation based on
the leukaemia controls. It is therefore suggested that geographic
variation may provide an explanation for the differences between the risk
ratios calculated using the leukaemia and total cancer controls and a
reason for favouring the use of total cancer controls.

References

1 Draper, G., Vincent, T., and 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 Stakeholder advisory group on ELF EMFs (SAGE). Precautionary approaches
to ELF EMF. Supporting document S4 (2007)
http://www.rkpartnership.co.uk/sage/

3 Cross Party Inquiry into childhood leukaemia and Extremely Low
frequency electric and magnetic fields (ELF EMF); (2007);
http://ePolitix.com - CPIELFEMF

4 Committee on Medical Aspects of Radiation in the Environment; The
distribution of childhood cancers in Great Britain 1969 – 1993. (2006)
http://www.comare.org.uk/comare_docs.htm

Competing interests:
None declared

Competing interests: No competing interests

19 March 2008
David E Jeffers
Retired
Meadland, Three Gates lane, Haslemere, Surrey, GU27 2LD