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Magnetic Fields during Pregnancy ... Risk of Miscarriage
The study (ref. below) is strange for a number of reason. The authors tried
time-weighted average (TWA) but it didn't work. Then they tried another
method and found a positive correlation the peak exposure (Maximum Magnetic
Field = MMF) for a 24 hour period - a way to select for the women moving
around the most during the day (selecting for unnormal pregnancies,
discussed in a critical commentary by D. Savitz in the same issue of
Epidemiology, Vol. 13, 2002:1-3). The authors then had a subsequent paper a
few months later where they say (if I recall it correctly) that no selection
bias should be expected.
The authors chose to make the analysis where they had the strongest
statistical effect (at 1.6 microtesla some asymptotic threshold - a level
inconsistent with most other EMF studies) - a hypothesis generating approach
(evil tongues would say data dredging). The total number of pregnant women
in the study (San Francisco and San Mateo Counties) was about 2729 but only
969 were used in the final analysis - a total loss of 64.5 %. Of those
refusing to participating (1185 subjects) 47.9 % were "too busy/not
interested/too stressful". A selection was made for English-speaking women
(which reminds me of my seven months in the area - that according to my
opinion - half of the population speaks poor English or no English at all).
Note if you read the study that for Fig. 2, the MF exposure scale is not
linear - the data point for 1.4-1.8 microtesla has been split (otherwise the
number of subjects for the three lowest exposure levels would have been
around 97 x 3 = 291 rather than 252 (969/10 = approx. 97)). I regraphed the
data with a linear scale - the distribution is, not surprisingly, very
skewed. I dislike the use of the word "dose" in the Results as there is no
meaningful dose definition for EMF exposure at these frequencies (household
appliances etc).
Fishing expeditions: It is strange that this way of working - first make the
observation - and then the statistical analysis which can pass without a
stir. Once the observation has been made (note that the authors state that
they picked the level 1.6 microtesla) - the probability for the observation
is 100 %. If I say that this one woman always had blooming cacti in her home
and then died from Alzheimer's most (?) people would understand that you
can't argue that way for a causal relationship. But for EMF it seems to be
OK - the first powerline study (Boulder, Colorado around 1979) was also of a
hypothesis generating kind.
I may mention that the study below caused thousands of people (mostly
worried women) in Sweden to call car companies and ask for "safe cars" etc.
This was after an "EMF person" had fed a scary magnetic fields&cars message
to our sensational media.
The same, if not worse, fallout was in France three months ago. The
following can be found in Sciences et Avenir (May 2002): "Reste qu’il est
impossible de déterminer aujourd’hui à partir de quelle durée et pour quelle
intensité l’exposition aux champs électromagnétiques peut être néfaste à
long terme. «C’est comme se demander combien de cigarettes il est possible
de fumer par jour avant de développer un cancer du poumon! », conclut le
scientifique suédois" ( http://www.sciences-et-avenir.com/archives/ key
words you can try are: champs, rayons, cem = EMF, fausses couches =
miscarriages)
(//Last part: "It is like asking how many cigarettes it is possible to smoke
per day before you get lung cancer."//). I strongly encourage any EMF
interested Radsafer who had a year or two of French to browse through the
articles in "Sciences et Avenir".
Please comment or correct if I got anything wrong.
My personal ideas only,
Bjorn Cedervall bcradsafers@hotmail.com
http://www.geocities.com/bjorn_cedervall/
----
A Population-Based Prospective Cohort Study of Personal Exposure to
Magnetic Fields during Pregnancy and the Risk of Miscarriage
De-Kun Li1; Roxana Odouli1; Soora Wi1; Teresa Janevic1; Ira Golditch2; T.
Dan Bracken3; Russell Senior3; Richard Rankin4; Richard Iriye5
>From the
1Division of Research, Kaiser Foundation Research Institute, Kaiser
Permanente, Oakland, CA; 2Department of Obstetrics and Gynecology, Kaiser
San Francisco, San Francisco, CA; 3T. Dan Bracken, Inc, Portland, OR;
4Oregon Applied Research Services, Lake Oswego, OR; and
5Enertech Consultants Inc, Campbell, CA.
EPIDEMIOLOGY 2002;13:9-20
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