Experts in epidemiology have described as weak a study that
found high levels of exposure to magnetic fields during
pregnancy may triple the risk the child will develop asthma.
The California study, published in the journal Archives of
Pediatric and Adolescent Medicine, required 801 pregnant
women to wear meters for 24 hours that measured their
exposure to magnetic fields, such as those emitted by power
lines, flourescent lights, hairdryers and microwave ovens.
EMF emitted by wi-fi networks and mobile phones were not
included in the study.
Checks on the resulting offspring 13 years later revealed
that children whose mothers had a high level of magnetic
field exposure during pregnancy were 3.5 times more likely
to develop asthma than those whose mothers experienced low
levels of exposure.
“Our findings provide new epidemiological evidence that high
maternal [magnetic field] levels in pregnancy may increase
the risk of asthma in offspring,” the authors wrote in their
paper.
However, experts in the design of health studies have
described the research as having “major deficiencies” and
ignoring previous studies that found there was limited
evidence of a link.
“There are major deficiencies in the epidemiological methods
which lack detail and, for example, specifically fail to
identify the proportions of mothers who did not take part
(refusals, non-English speakers) and children who were
excluded,“ said Professor Patricia McKinney, Professor of
Paediatric Epidemiology at the University of Leeds.
“The characteristics of the non-participants need to be
described as they may influence interpretation of the
results.”
It was impossible to know how representative the study
sample was and whether selection bias may have influenced
the outcome, she said.
Professor McKinney pointed out that the meters measured
exposure for just one day during the entire nine months of
pregnancy.
“Furthermore, the vulnerability of the fetus varies
throughout pregnancy and most exposures which cause harm do
so during a ‘critical window’ and not across the nine months
of pregnancy."
Rodney Croft, Professor of Health Psychology at the
University of Wollongong and an expert on the health risks
of
electromagnetic field emitting devices,
also warned against drawing hasty conclusions from the
California study.
“Unfortunately, there are too many issues with this paper to
make it more than a hypothesis-generating exercise, and
certainly it does not justify any concern at present. There
are problems with both the study itself, as well as how the
authors interpret their findings,“ he said.
“For example, it does not provide a good measure of exposure
(it merely assumes that a personal dosimeter from one day
provides a good estimate of gestational exposure); it does
not describe the extent to which multiple comparisons may
have invalidated the conclusions (this is particularly
important given that it is a very speculative study which,
contrary to the assertions of the authors, is not based on
reliable research suggesting an interaction between magnetic
fields and disease).”
The researchers were unable to demonstrate that exposure to
magnetic fields caused asthma in the resulting offspring he
said.
“It is also worth clarifying some mixed messages from the
paper: It claims to be assessing relations with magnetic
fields in general, but it only looks at a subset of time
varying magnetic fields (40-800 Hz), ignoring more prevalent
magnetic fields such as the Earth’s (which falls below this
frequency range but is about 100 times stronger); and it
links these ‘extremely low frequency’ fields to
‘radiofrequency fields’ in the introduction, yet they are
not related and affect the body in completely different
ways.”
Professor David Spiegelhalter, Winton Professor of the
Public Understanding of Risk at the University of Cambridge,
cautioned that the findings would need to be replicated by
other researchers before any conclusions could be drawn.
“This study has the advantage of having measured exposure to
magnetic fields during pregnancy rather than relying on
recall, but the disadvantage that the possible association
with asthma seems to be an afterthought in a study of
miscarriage,“ he said.
Prof William Stewart, Visiting Professor at University of
Southampton and an expert in electromagnetic fields, said
the study had “a number of weaknesses”, including the use of
meters that measure only AC fields and not the Earth’s DC
field.
“This is important because the biological interactions of
pure magnetic fields are very small compared with those of
electric fields – so the authors should not describe it as a
magnetic field effect at all, or mums-to-be will start to
hide from the earth’s 500mG field in magnetically-shielded
rooms!”
He also said the authors, who used the same cohort sample of
pregnant women for an earlier study on miscarriage, did not
explicitly say how many health problems they were looking
for in the offspring.
“If many other things were checked [such as autism, heart
conditions or miscarriage] then the statistical significance
threshold should have been raised accordingly.”
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