快猫短视频

San Francisco

THE strong magnetic fields produced by some electric appliances and vehicles
increase the risk of miscarriage, 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.

鈥淭he studies really represent state-of-the-art research into the causes of
pregnancy loss,鈥� says epidemiologist David Savitz of the University of North
Carolina in Chapel Hill. Nevertheless, he says the researchers鈥� interpretation
of their findings may be wrong. California is already initiating public hearings
to discuss the findings.

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. 鈥淭hat鈥檚 another confirmation that the
effect is due to EMF,鈥� says Li.

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鈥檚 team didn鈥檛 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.

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鈥檚 group at the California
Department of Health Services in Oakland, have been inconclusive. But Li thinks
this is because Neutra looked at people鈥檚 average exposure to electromagnetic
fields over time, not peak values. 鈥淧eople have never looked at peak EMFs
before,鈥� Li says. 鈥淢y study opens a new chapter for these EMF effects. Not just
for miscarriages, but for other health effects.鈥�

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

But Savitz sees things differently. 鈥淏oth studies found a reassuring lack of
association for the most well-established measures of magnetic field exposure,
that is average magnetic fields.鈥� The correlation between exposures to EMF
spikes and an increased miscarriage rate may be due to other factors, he says.
For instance, women who have a healthy pregnancy are more likely to suffer from
nausea. This may make them more likely to stay at home and do less, and thus
also reduce their exposure to magnetic fields.

After Savitz鈥檚 comments, Li鈥檚 team looked at their data again. They also
checked for other possible risk factors such as drug use, carrying heavy loads
and previous induced abortions. 鈥淲e adjusted for 30 different kinds of risk
factors. Nothing changed,鈥� says Li.

But Michael Bracken, an epidemiologist at Yale University, is unconvinced.
鈥淭here are numerous ways of measuring these fields, and one worries that if you
do it enough times, then you are going to find positive associations,鈥� he says.
鈥淭here鈥檚 a real risk in these things getting over-interpreted and scaring the
dickens out of people.鈥�

In the past, EMFs have been blamed for various other ill effects, especially
leukaemia 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 localised heating. Li speculates that EMF spikes could
cause miscarriages by subtly disrupting cell-to-cell communication. 鈥淏ut as
epidemiologists, we should not feel weaker because we don鈥檛 understand the
尘别肠丑补苍颈蝉尘蝉.鈥�