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.”