Strong magnetic fields produced by trains, electric household appliances such as vacuum cleaners and food mixers and vehicles increase the risk of miscarriage by up to three times claim researchers in California.

Their findings also suggest that most previous investigations into the health effects of electromagnetic fields (EMFs) have been measuring the wrong thing.

One study was led by De-Kun Li, a reproductive epidemiologist at the Kaiser Foundation Research Institute in Oakland, California. His team asked 1063 women around San Francisco who were in the first 10 weeks of pregnancy to spend a day wearing a meter around their waists that measured magnetic field levels every 10 seconds.

Overall, they found that women exposed to peak levels of 1.6 microteslas or greater were nearly twice as likely to miscarry as women not exposed to such strong fields.

More significantly, says Li, among the 622 women who said the measuring period had been a typical day, those who experienced high peak fields were three times as likely to have a miscarriage. "That's another confirmation that the effect is due to EMF," says Li.

Shavers and Hairdryers

Other factors can have a more dramatic effect, however. The risk of a miscarriage increases tenfold as women age, for example, from 5 per cent for women under 30 years old to 50 per cent for those in their mid-40s.

Li's team didn't look at what was producing the fields, but appliances such as shavers, hairdryers and vacuum cleaners can produce strong alternating magnetic fields, as can electric vehicles such as trams and trains. The key is proximity to the source, as fields drop off rapidly with distance.

The team did not examine which appliances were producing the strong fields, but devices with powerful motors are known to be the worst culprits. Vacuum cleaners and drills emit around 20 microteslas - more than 12 times higher than the critical level in the study. Food mixers give off around 10

Alternating magnetic fields also have associated electric fields. The few previous studies of the effect of low-frequency EMFs on miscarriages, such as one involving 727 women done in 1991 by Raymond Neutra's group at the California Department of Health Services in Oakland, have been inconclusive.

But Li thinks this is because Neutra looked at people's average exposure to electromagnetic fields over time, not peak values. "People have never looked at peak EMFs before," Li says. "My study opens a new chapter for these EMF effects. Not just for miscarriages, but for other health effects."

EMF Spikes

When Neutra reanalysed the data from his earlier study, which has only now been published, he discovered the results were similar to Li's. Women exposed to peak EMF levels greater than 1.4 microteslas were nearly twice as likely to miscarry.

Localized Heating

In the past, EMFs have been blamed for various other ill effects, especially leukemia in children. But no one can explain how relatively weak fields might cause the DNA mutations that lead to cancer, and most studies have failed to find evidence of a link.

The peak values measured by Li are way below the recommended exposure limit of 1600 microteslas. Above this level, EMFs can induce electric currents in the body, which leads to localized heating.

Li speculates that EMF spikes could cause miscarriages by subtly disrupting cell-to-cell communication. "But as epidemiologists, we should not feel weaker because we don't understand the mechanisms."

Epidemiology January 2002 13:9-20