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What happens when I am exposed to EMFs?
AC fields create weak electric currents in the bodies of people and animals. This is one reason why there is a potential for EMFs to cause biological effects. Currents from electric and magnetic fields are distributed differently within the body. The amount of this current, even if you are directly beneath a large transmission line, is extremely small (millionths of an ampere). The current is too weak to penetrate cell membranes; it is present mostly between the cells, not through them.
Currents from 60-Hz EMFs are weaker than natural currents in the body, such as those from the electrical activity of the brain and heart. Some scientists argue that it is therefore impossible for EMFs to have any important effects. Other scientists argue that, just as a trained ear can pick up a familiar voice or cry in a crowd, so a cell may respond to induced current as a signal, lower in intensity yet detectable even through the background 'noise' of the body’s natural currents. Numerous laboratory studies have shown that biological effects can be caused by exposure to EMFs. In most cases, however, it is not clear how EMFs actually produce these demonstrated effects.
Strong electric fields, such as those found beneath large transmission lines, can cause hair on your exposed head or arms to vibrate slightly at 60 Hz. This is felt by some people as a tingling sensation. EMFs from transmission lines can also in some circumstances cause nuisance shocks from voltages created by EMFs on objects like ungrounded metal fences.
What effects of EMFs have been reported in laboratory studies?
Several kinds of biological effects have been reported in studies of electric and/or magnetic fields. A biological effect is a measurable change in some biological factor. It may or may not have any bearing on health. Overall, effects attributed to EMFs have been small and difficult to reproduce. Very specific laboratory conditions are usually needed for effects of EMFs to be detected. It is not known how EMFs actually cause these effects.
Laboratory studies to date have not answered questions about possible human health effects. These studies are, however, providing clues about how EMFs interact with basic biological processes. The cell membrane may be an important site of interaction with induced currents from EMFs.
Keep in mind that some of these effects are within the 'normal' range of variation. A biological response to a particular stimulus does not necessarily result in a negative health effect.
EFFECTS OF 60-Hz EMFs REPORTED IN SOME LABORATORY STUDIES
What are some typical EMF exposures?
First, 'exposure' must be defined. Scientists are still uncertain about the best way to do this, because experiments have shown that several aspects of the fields may be relevant to biological effects. Should exposure be an average of changing magnetic field levels over some time period, or should it focus only on time spent in high fields above some threshold value? Are rapid field changes important? Does the frequency content play a role? Even though the average field level has been used widely to represent EMF exposure, it is possible that other definitions may relate more closely to any possible effects.
Second, EMF in the environment is very complicated. We are usually exposed to EMF from a large number of sources every day. Fields change both in time and space. A person's EMF exposure depends to a large degree on what he or she is doing in the field at the time.
Several kinds of small meters are now available that can be carried or worn by a person to record magnetic field exposures automatically. The figure below is an example of data collected with one of these meters. The magnetic field was measured every 24 seconds over a 24-hour period. For this person, field exposure at home was very low. The occasional spikes (short exposures to high fields) occurred when the person drove or walked under or over power lines or was close to appliances in the home or office.
Some studies have used these automatic gauss meters to measure human exposure to magnetic fields (see picture above). These studies tend to show that appliances and building wiring contribute to the low-level background magnetic field exposure that most people receive. People living close to large power lines tend to have higher overall field exposures. However, there is much individual variation among homes.
Release from the University of Toronto and The Hospital for Sick Children ...
Field Exposure Associated with Childhood Leukemia
at the University of Toronto and the Hospital for Sick Children have found an
association between magnetic field exposures in residences and the risk of
developing childhood leukemia.
study, reported in two separate papers in the current issues of the
International Journal of Cancer (IJC) and the journal Cancer Causes and Control
(CCC), shows that children with higher exposures to magnetic fields in
residences are two to four times more likely to develop leukemia compared to
children who are less exposed. The study comprehensively measured magnetic field
exposures inside and outside the children's homes. Wire code was also assigned
to each residence. For some children, EMF exposures were measured by a personal
authors report in IJC that overall, wire code -- a surrogate indicator of
magnetic field exposure based on the physical characteristics of the line and
proximity of the residence to power lines -- was not associated with an
increased risk of developing leukemia. However, measured magnetic field exposure
was associated with an elevated incidence of leukemia in children under age six
who were exposed in residences occupied during the first two years of life.
subset of the total study population wore a personal monitoring device which
measured EMF during usual activities in the home over 48 hours. As reported in
CCC, exposure to magnetic fields was associated with a two-fold increase in risk
of developing leukemia.
other factors such as residential mobility, power consumption, child's medical
history and other environmental exposures were taken into account, children
exposed to higher levels of magnetic fields were 4.5 times more likely to
develop leukemia compared to less exposed children. Risks were higher for
children diagnosed at less than six years of age and for those with acute
lymphoblastic leukemia -- the most common type of leukemia in children.
the methods of assessing exposure were refined, we found that the association
between magnetic fields and the risk of developing childhood leukemia became
stronger, particularly in children diagnosed at a younger age," says lead
author Dr. Lois Green, epidemiologist in the department of public health
sciences at U of T and at Ontario Power Generation. "But this study does
not establish that magnetic fields cause cancer. To date, laboratory research
has not shown a plausible biologic mechanism supporting a cause and effect
compared 201 children living in the Greater Toronto Area (GTA) who were
diagnosed with leukemia at 0 to 14 years of age between1985 and 1993 at the
Hospital for Sick Children with 406 control children.
possible, magnetic field exposures were measured at all the GTA residences
occupied by the child during the period of inquiry -- the only study to date to
consider several different exposure time periods. A detailed questionnaire was
also administered to gather information about other factors such as family
history, which might be related to leukemia risk.
lymphoblastic leukemia is most commonly seen in children two to six years of
age. The association we saw with this age group is interesting because the range
of exposures to possible risk factors is smaller and shorter," says Dr.
Mark Greenberg, professor of pediatrics at U of T and pediatric oncologist at
the Hospital for Sick Children. "We don't know what it means however,
because there is no good biologic explanation for how such exposure might
consideration, Green adds, is the possibility that lifetime exposures to
magnetic fields may have been measured more accurately for younger children
because the interval between diagnosis and measurement was short, thus reducing
possible misclassification of exposure.
was provided in part by the Ontario Hydro Services Company (formerly part of
Ontario Hydro) and the Canadian Electrical Association.
For more on the study or to reach Dr. Green, contact:
and answers about electric and magnetic fields and U of T/HSC leukemia research
June 15, 1999
are electric and magnetic fields?
Electric and magnetic fields, referred to as EMFs, are invisible lines of force
that surround all electrical appliances, house wiring, power lines in the street
and high voltage transmission lines. These invisible lines of force are created
by electric charges, which produce two kinds of fields: electric and magnetic.
Electric fields are produced by voltage and magnetic fields are produced by
current. A lamp that is plugged in but turned off produces an electric field. If
the lamp is both plugged in and turned on it produces a magnetic field as well.
Electric fields are easily shielded by conducting objects like trees and
buildings. Magnetic fields are not easily shielded by most materials. EMFs exist
almost everywhere. However, the intensity of magnetic and electric fields
diminishes greatly as you move away from the source.
are you studying EMFs?
In 1979 a study in Denver, Colorado first raised the possibility that
residential proximity to power lines might be associated with childhood
leukemia. Since that time there have been many studies which have attempted to
clarify the relationship between electric and magnetic fields and the risk of
cancer. However, the results of these studies have been inconsistent and
This current study by the University of Toronto and The Hospital for Sick
Children was designed to improve the measure of EMF exposures and thereby better
characterize the relationship with the risk of childhood leukemia.
did your study investigate?
The study involved a comprehensive assessment of the residential magnetic field
exposures of 201 children with leukemia and 406 controls. In addition to
measuring magnetic fields in the homes currently occupied by the study
participants, we measured, where allowed, the magnetic fields in previously
occupied homes. We used the following types of measurements: personal
monitoring, interior and exterior point-in-time measures, and wire code.
A subset of cases and controls whose current residence was relevant to our
inquiry period wore a personal monitor that measured both electric and magnetic
fields. For this residence we also assigned wire code and took measurements
inside and outside the home.
are the key findings of this study?
For the study population overall, wire code, which is a surrogate measure of
magnetic field exposure, was not associated with an increased risk of childhood
There was no consistent trend of increasing risk of leukemia with increasing
exposure to magnetic fields.
There was a suggestion that leukemia risk was more pronounced for
children diagnosed at less than six years of age in relation to magnetic field
exposures measured in residences occupied earliest in the period of inquiry.
Analysis of magnetic field exposures in the child's current residence showed a
statistically significant increase in risk in relation to increasing levels of
magnetic fields. However, electric field exposures in the home were associated
with a decreased risk of leukemia.
It's important to note that this study did not determine that EMF exposure
causes childhood leukemia. Rather, there seems to be a correlation, which needs
still further investigation.
While I understand that there is no causal relationship between EMF exposure and
the risk of childhood leukemia, are there steps I can take to reduce exposure to
In this study, the external electrical system is a major source of magnetic
fields in homes. There is little which residents can do to alter this.
Within the home, electrical equipment and elements of the household electrical
system may contribute to magnetic field exposures. In our study, appliances
contributed only a small amount to the children's measured exposure.
there certain electric appliances that are more likely to have high EMF
It is important to remember that EMFs diminish rapidly as you move away from the
source. Therefore, appliances that are used close to the body and for prolonged
periods of time are more likely to contribute significantly to a person's
exposure. Examples of these are hair dryers, can openers and other electrical
equipment with a motor.
are wire codes? Do they differ between cities / provinces?
Wire codes are a surrogate indicator of magnetic (not electric) field exposures
first developed by researchers, Wertherimer and Leeper. Wire code is a way of
categorizing homes according to magnetic field levels. It does not involve
measuring the fields, but rather considers the size, number and type of
electrical conductors outside the home and their distances from the home.
Because electrical distribution systems vary by jurisdiction, so can wire codes.
The magnetic field levels in a given wire code category can vary from one
jurisdiction to another.
is the difference between your study and other studies that show there are no
associations between EMFs and childhood leukemia?
The most common differences between studies relate to how the exposures were
measured, how the subjects were selected, how other factors which might also be
related to leukemia were taken into account and how the data are analyzed.
factors did you take into consideration when you were assessing EMF levels?
We considered a large number of different factors that might also be related to
childhood leukemia and/or exposure to magnetic fields. These included the
child's medical history, reproductive history of the mother, exposures to
pesticides, chemicals, x-rays, and medications taken by the mother during
How did you gather data on the subjects' EMF exposure?
study was very comprehensive in its assessment of exposure. Several different
methods of measurement were used.
We took measurements of magnetic fields around the outside perimeter of the
home, inside the home (child's bedroom and two other rooms that were frequently
used) and for a subset of children, we asked them to wear a personal monitor for
up to two days. This monitor measured both electric and magnetic field exposures
associated with the child's usual activities in the home. We also looked at wire
you differentiate between electric and magnetic field exposures?
Measurements of the electric and magnetic fields were available for only a subset of the study population, 88 cases and 133 controls whose residence at the time of interview was relevant to the inquiry period. Although electric and magnetic fields usually appear together at power frequencies (60 Hertz), the relationship between the two is not constant and therefore, they must be measured separately.
What are the risks to my unborn baby if I am exposed to EMFs?
At this time, the risks, if any, are unknown.
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