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Do pylons cause asthma in babies?

Tuesday 2 August 2011

“Pregnant women who use hairdryers, microwaves, vacuum cleaners or who live near pylons could be putting their babies at risk of asthma,” reported the Daily Mail. “Exposing unborn children to potentially harmful magnetic energy produced by household appliances and power lines could treble their child’s chances of suffering from the condition,” it added.

This prospective cohort study measured the amount of magnetic field (MF) women were exposed to on one day during their pregnancy and looked at whether there was an increased risk of their child being diagnosed with asthma during their first 13 years. The researchers concluded that a higher exposure to MF during pregnancy was associated with an increased risk of asthma in the child.

This study has some strengths but also several limitations. In particular, the women’s exposure to MF was measured only once and then used to estimate their MF exposure throughout their pregnancy. In addition, the women were not asked which appliances they used or whether they lived near electrical pylons, therefore it is not possible to say which electrical appliances could be associated with a high MF exposure.

On balance, the weaknesses of this study mean that it is not robust evidence that magnetic fields can cause asthma in unborn children. Further research is needed to answer this question.

Where did the story come from?

The study was carried out by researchers from the Kaiser Foundation Research Institute, California, USA. Funding was provided by the California Public Health Foundation. The study was published in the (peer-reviewed) journal: Archives of Pediatric and Adolescent Medicine .

This study was covered by the Daily Mail , which covered it accurately, but it could have given more emphasis to its weaknesses. The Daily Mirror gave a very brief report of this story and did not report any data from the study.

What kind of research was this?

This was a prospective cohort study, which looked at whether maternal exposure to high levels of magnetic fields during pregnancy was associated with an increased risk of the child being diagnosed with asthma before the age of 13.

The researchers said that the prevalence of asthma has been steadily growing over the past several decades and the rate of increase suggests that there may be environmental risk factors. They suggest that environmental exposures during pregnancy could affect the development of the baby’s immune system and lungs while in the womb.

The researchers suggest that people are being progressively exposed to more electromagnetic fields (EMFs) than before, because of the increase in use of mobile phones and other wireless devices in the workplace and home.

In this study, the researchers used a meter to objectively measure the magnetic fields (MFs) that the women were exposed to during their pregnancy and followed up their children for up to 13 years to see if there was any association between exposure to MF and the risk of developing asthma.

What did the research involve?

The researchers recruited women who were registered with Kaiser Permanente, a hospital group in the San Francisco area, from 1996 to 1998. The women were 5-13 weeks pregnant. The participants were interviewed to assess their risk factors for adverse pregnancy outcomes, and potential confounders that are known to be associated with a risk of asthma (such as sociodemographic characteristics, family history of asthma and maternal smoking).

Electromagnetic field refers to both electric fields and magnetic fields. In this study, the researchers only measured magnetic fields. They did this using a device that the women wore for a 24-hour period during their first or second trimester (around 13 to 26 weeks). The device enabled the researchers to record the MF the women were exposed to during their daily activities. At the end of the 24-hour period the women were asked whether that day had been a typical day in terms of the activities they performed. The women’s average (median) MF exposure over that 24-hour period was then used to estimate their MF exposure throughout their pregnancy. For some of the analyses, women were divided into three groups based on their MF exposures: low exposure were women with the bottom 10% of MF measurement; medium exposure were women with MF exposures between 10% to 90% of the range of MF values measured; and high exposure were women with MF values in the top 10%.

Children of the 734 women who had complete 24-hour measurements during pregnancy were followed up until one of the following occurred:

  • They were diagnosed with asthma.
  • They left the Kaiser Permanente Healthcare system.
  • They reached the end of the study period (August 2010).

To be classified as having asthma the child had to have received a clinical diagnosis of asthma on at least two occasions during one year of the follow-up period. The researchers excluded 67 children who had only one diagnosis, 17 children who had asthma diagnoses that were more than one year apart and 24 children who used anti-asthmatic medications without a clinical diagnosis of asthma. In total, 626 mother-child pairs were analysed.

What were the basic results?

Overall, 130 children (20.8%) developed asthma during the 13 years of follow-up. Over 80% of these were diagnosed by the time they were five years old. Around 250 of the 626 children left the Kaiser Permanente scheme before the end of the follow-up.

The researchers looked at whether there was an association between increasing MF exposure measurements and the risk of asthma in the child. They adjusted the results for maternal age, race, education, smoking during pregnancy and a history of asthma in the family. They found that every additional unit of magnetic field was associated with a 15% increased risk of asthma in offspring (adjusted Hazard Ratio [aHR] 1.15; 95% confidence interval [CI], 1.04 to 1.27).

Women with low MF exposure were compared to those with medium or high MF exposure. The researchers found that compared to women with a low MF exposure, children of women with a high exposure had a 3.5-fold increased risk of developing asthma (aHR, 3.52 95% CI, 1.68 to 7.35). There was no significant increase in asthma risk in children of women with medium exposure compared to low exposure.

How did the researchers interpret the results?

The researchers said that a high maternal MF exposure level in pregnancy is associated with a significantly increased risk of asthma in the child.


There are several limitations to this study and these weaken its conclusion that magnetic field increases the risk of asthma in unborn children:

  • Exposure to MF was measured on only one occasion. Although the women were asked whether the measurement day was a typical day for them this may not have given an accurate estimate of the actual MF they were exposed to over the course of their pregnancy.
  • The study did not ask women about what electrical appliances they had used or whether they lived near electrical pylons. It is not possible to say from this study what types of appliance could be responsible for a higher MF exposure in these women.
  • A large number of participants (around 40%) were not followed for the full 13-year period because they had left the Kaiser Permanente healthcare scheme. This is a high loss to follow-up, and it is not possible to say whether or not the inclusions of these individuals would have altered this association between MF and asthma.

The study does have some strengths in that it is a prospective study, following the children from before they had asthma to after their diagnosis with the condition. It also used objective measures of MF and asthma rather than relying on self-recall, which can be open to bias.

On balance, the weaknesses of this study mean that it is not robust evidence that magnetic fields can cause asthma in unborn children. To answer this question would require further research in different and larger populations.

Analysis by Bazian
Edited by NHS Website

Links to the headlines

Pylons linked to babies' asthma: Hairdryers, vacuum cleaners and microwave also a risk, say scientists

Daily Mail, 2 August 2011

Asthma linked to exposure to magnetic fields during pregnancy

Daily Mirror, 2 August 2011

Links to the science

Li D-K, Chen H, Odouli R.

Maternal Exposure to Magnetic Fields During Pregnancy in Relation to the Risk of Asthma in Offspring

Archives of Pediatrics & Adolescent Medicine 2011; Published online August 1, 2011