Living
with risks
Leader
Monday June 6, 2005
The Guardian
It looks like an eye-grabbing statistic: a report that infants living near
overhead high-voltage power cables have a nearly 70% higher risk of developing
leukemia than other children, according to research from Oxford University
published in the latest British Medical Journal. Figures such as 70% sound
dangerously high and demand action - and yet, as is so often the case with
statistics as with electricity cables, they must both be handled with extreme
care.
At issue is the difference between two measures of risk -
absolute risk v relative risk. Consider a group of 10,000 people, of whom three
develop a disease over a period. They are then compared with another group of
the same size subject to some variable, of whom five develop the disease. In
comparing the two cases, the relative risk of developing the disease in the
second group is 66% higher (five being two-thirds more than three). Yet, in
absolute terms, the risks remain tiny: a member of the first group has a 0.03%
risk, while a member of the second group has a 0.05% risk. In the example of
the overhead power lines, it is the relative risk of developing childhood
leukemia that increases by 70%. Meanwhile, the absolute risk of developing
leukemia remains small. The BMJ's science editor, Dr Geoff Watts, estimated
that the difference here could amount to perhaps five extra cases. While this
is significant in purely statistically terms, it hardly shows a causal link.
The problem, however, is
that the existence of even such relatively small differences can be enough to
set off vigorous protest movements. This is in spite of the warnings of
attaching too much significance to the findings - as the BMJ article suggests,
it is as likely that other factors may have caused the higher incidence of
leukemia. "Before activists begin blowing up power pylons, a bit of
perspective might help," Dr Watts notes.
Several studies have now
been done on this issue, over several years, and the results remain
inconclusive. In this regard, the debate over the effects of power pylons is a
precursor of similar debates that could run on for years to come: the MMR
triple vaccine; mobile phone masts and handsets; genetically modified food. It
is unlikely that knock-down evidence will be found to show, one way or the
other, to settle all the arguments. And that will not simply be because the
amount of research is lacking. Rather, the world is a complex place and it is
exceedingly difficult to neatly divide causes and effects. More likely, society
will slowly adapt, as it has with other matters, and learn to live with the
risks involved.