Power lines increase risk of child asthma, warn scientists
High levels of exposure have effects in the womb that more than triple asthma risk, a study suggests.
However, the controversial US research was criticised by British experts who questioned the way it was carried out.
Around 800 pregnant women from the San Francisco region of California were recruited for the study, originally designed to look at the effect of magnetic fields on miscarriage.
Each participant was asked to wear a small magnetic field recording device for 24 hours during the first or second three-month period, or trimester, of their pregnancy.
The meters measured exposure to magnetic fields from electrical sources such as power lines, microwave ovens, vacuum cleaners, hair dryers and fluorescent lights. They did not measure radio frequency magnetic fields from wireless networks and mobile phones.
After the children were born, data on the numbers developing asthma were collected over 13 years. The findings were published online today in the journal Archives of Pediatrics & Adolescent Medicine.
Each one milligaus (mG) increase in magnetic field level during pregnancy was associated with a 15 per cent increased rate of asthma in offspring.
Children whose mothers were exposed to a high magnetic field level of more than two mG had a more than 3.5-fold increased risk of asthma compared with those whose mothers had low levels of exposure.
"While the replication of the finding is needed, the message here is exposure to electromagnetic fields is not good, and we need to pay attention to its adverse effects on health," said study leader Dr De-Kun Li, from the Kaiser Permanente Division of Research in Oakland.
Kaiser Permanente is a large private healthcare provider in California. The researchers said they took account of a wide range of factors that might have influenced the results, including maternal age, social background, smoking habits, family history, breastfeeding and birth weight.
Two known risk factors for childhood asthma, a genetic history of asthma in a mother's family and being the first-born child appeared to enhance the impact of magnetic fields.
Dr Li said: "The best way to reduce your magnetic field exposure is distance. Magnetic field strength drops dramatically with increasing distance from the source. So pregnant women should try to limit their exposure to known MF sources and keep their distance from them when they are in use."
But Professor Patricia McKinney, an expert on childhood epidemiology at the University of Leeds, said the research lacked a "biological basis or hypothesis to test" and pointed to "major deficiencies" in its methods. She criticised the fact that exposure was restricted to a single 24-hour period, since vulnerability of the foetus varied throughout pregnancy.
"The strong conclusions drawn from this paper that magnetic field exposure in pregnancy increases the risk of asthma in offspring cannot be justified based on the evidence provided," said Prof McKinney.
David Spiegelhalter, Winton Professor of the Public Understanding of Risk at Cambridge University, said it was "essential" that the findings were replicated by other researchers.