Comments on Draper et al BMJ 1290 4June 2005
These comments are primarily from the viewpoint of a mathematician.
While the paper reports a major study in scale, some of the statistics are
weak, particularly in the findings at greater distances from powerlines.
On the other hand some responses have seized upon those results at
greater distances (up to 600 metres), where associated fields may be
negligible, to dismiss any hypothesis for a magnetic field effect. Such
dismissive claims do not stand up to mathematical scrutiny. Such claims
also overlook the possibility of associated exposure in time spent closer
to the line, for example at nursery or school. Further, while hastily
relying on weak statistical results, such claims dismiss the stronger
statistical association with childhood leukaemia established for flux
densities above 0.4 microTesla.
Possible understatement of potential effects
The exposure metric in this study is proximity of birth address to
National Grid power lines. That would seem an uncertain proxy for any
particular field effects, compounded by uncertainty in the timing of
exposure. Genetic evidence suggests, in many cases, a two-stage causal
process of in-utero genetic damage followed by conversion in childhood to
the disease. Address at birth may be better correlated with the first
stage than the second. Uncertainties in relevant exposures would tend to
dilute statistical evidence indicative of causation.
General population studies which ignore susceptible subsets can
greatly mask possible causal associations . It would be helpful to
study the relation of exposure in utero to incidence of genetic damage
(identifiable by blood tests) and, separately, the relation of exposure
prior to diagnosis in children with genetic damage to incidence of the
disease. The risk in these two subset-related stages could be in the
region of 1 in 200, in contrast to the whole population risk of 1 in
20,000 per year, with potentially much greater statistical resolving power
for small relative risks.
Table 1 shows some odd features in the data. Firstly, the relative
risk (RR) is more like a step function with distance (suggesting a
possible exposure threshold) than an inverse power relation. Secondly,
within the 200 - 600 metre range there is a strange counter-trend; as
results in this range are barely statistically significant (CI 1.02 -
1.49) this suggests chance variations or chance events rather than
something more systematic.
The authors estimate, with qualifications, that about 1% of childhood
leukaemia would be attributable to National Grid lines. That leads to
about 5 attributable cases per year, some ten times higher than suggested
by previous studies. The 1% of cases would reflect an average relative
risk of about 1.25 on the 4% of children living within 600 metres.
Discounting the contribution from the most uncertain range of 200 - 600
metres would leave a population of about 0.7 % of children (population is
lower closer to powerlines) within 200 metres with a relative risk of
about 1.7 of which 0.7 is attributable. That amounts to about 0.5 % of
cases, which would only reduce the estimated attributable outcome to 2.5
cases per year. So the stronger statistical findings in the range 0 - 200
metres alone support about half the increased attribution.
Table 1 shows large variations in the distributions of the three sets
of controls. The controls need not be similarly distributed, as they match
different case sets, but such differences are not explained.
Maldistribution of controls would not wholly explain the finding, as the
authors observe, but the differences remain disconcerting.
1. The study is important in that it is on a large scale and deals
with proximity of birth address to powerlines. In contrast, other key
studies, which lie behind the statistical association of childhood
leukaemia with magnetic flux density, refer mainly to pre-diagnosis
exposures. The extent that this study might represent exposure to EMF in
utero or pre-diagnosis is unclear.
2. The study finds statistically significant results of two kinds.
First there are stronger results for birth addresses within 200 metres of
a power line. Second there are weaker results in the range 200 - 600
metres with statistical quirks.
3. The results within 200 metres broadly reinforce the known doubling
of risk of childhood leukaemia for pre-diagnosis exposure above 0.4
microTesla. However, they suggest the number of attributable cases from
National Grid lines would be about 5 per year, some ten times more than
previous estimates; this reduces to five times more using only the
stronger results. This may be a reflection of a greater effect of pre-
natal exposure compared with pre-diagnosis exposure, but this is not
clear. There are uncertainties in both old and new estimates.
4. The results in the range 200 - 600 metres are likely to be
spurious. They should not be relied on to support or deny an effect up to
600 metres. The argument that these results are incompatible with magnetic
field levels should not be relied on to dismiss magnetic field hypotheses
nor to counter the established statistical association with magnetic flux
5. The uncertainty in exposure metric, and between pre-natal and pre-
diagnosis exposure, would tend to understate any potential underlying
causation. Better focused studies on the two stages would be helpful and
could be much more robust statistically.
 M J O’Carroll, Searching for causes: focusing epidemiology,
Paper P1-15, Children with Leukaemia: Scientific Conference 6-10 September
Competing interests: No competing interests