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Medical Magnetic Fields Versus Health

  1. Oct 24, 2007 #1
    I know high magnet fields can be dangerous to health. My questions are 1) at what point does a magnetic field become powerful enough to be actually cause problems with human health and 2) why exactly is it hazardous at high levels? Thank you for your time.
  2. jcsd
  3. Oct 24, 2007 #2
    How do you know they can be dangerous?
  4. Oct 24, 2007 #3


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    More to the point: there is no evidence that magnetic fields are dangerous to health. That's a hoax.
  5. Oct 24, 2007 #4


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    Depends on what you mean by "magnetic field".
    DC fields are not dangerous unless you have a pacemaker.
    AC fields are a bit trickier. Wheteher or not they are dangerous depends on the strength and the frequency so there is no general answer.
    Some studies have conculded that there is an increased risk of cancer of you spend a lot of time on strong AC field, e.g. the fields around power lines or in certain factories.
    The most well known example is that there is a small, but according to some studies statistically significant, increase in leukemia among children that live very close (e.g. in a house directly below) to power lines that carry high currents (e.g. 400V lines). Note that this only concerns AC fields stronger than about 0.5 uT; i.e. fields that are quite strong (the earths magnetic field is 50 uT, but that is a DC field).
    We don't know the reason for this increased risk. Some research suggests that strong AC fields can affect the transport through the cell membranes (ion channels) but there are many other theories as well.
    However, note that we are talking about strong fields here. There is no evidence that weak field are dangerous and DC fields are complettely harmless (I occasionally work near DC fields that are of the order of 1T, the worst thing that can happen is that my credit cards stop working). Also, there is no evidence that short time exposure is dangerous.
  6. Oct 24, 2007 #5

    jim mcnamara

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    Well, FWIW - high gauss magnets embedded in a "sock" are standard procedure in speeding healing following podiatric surgery. Especially for patients with secondary vascular problems - like diabetics.

    They may also induce analgesia - (reduce pain):
  7. Oct 24, 2007 #6
    I hate the smell of the placebo effect in the morning. Smells like...dirty socks.


    You familiar with Derren Brown?
  8. Oct 24, 2007 #7
    Can you say high field MRI? They are ac and dc and health care workers are not dropping like flies. mmm?
  9. Oct 25, 2007 #8

    jim mcnamara

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    There was a thread (cannot find it) that discussed what people 'know'. A lot of the time people are exposed to the right information in debunking some odd idea. However it turns out that they remember the odd idea as correct about 40% of the time even though they learned it was wrong to start with.

    I do not know if that thread had a citation and psychology is not my area. Anyone got a reference to real research? :looks for answer from Zooby:
    Last edited: Oct 25, 2007
  10. Oct 25, 2007 #9
    You may mean this thread:

  11. Oct 27, 2007 #10


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    Although, those workers are not spending a lot of time in the machine itself. The people I know who work with MRI have reported sensations of extreme vertigo (to the point of needing to sit down a few minutes to recover) while assisting patients within a machine that was on, and there was a recent paper published (I'll have to look for the citation later) citing similar findings in rodents. The most likely interpretation is an effect on the vestibular system, since we, and other animals do perceive magnetic fields in our environment to aid in direction-finding.

    Whether there are really any long-term health hazards of magnetic fields, we do not know. If I'm recalling the paper correctly, the behavioral effects in rats were observed beginning at about 4 tesla. "Dose" is likely as relevant here as with anything else. Just for comparison, getting a few x-rays in your lifetime and being exposed to the low-level radiation naturally present in the environment isn't going to adversely affect your health, but if you're getting the equivalent of an x-ray per day, or get a high dose of radiation exposure in a major spill, sure, you are likely to have health consequences. The same could potentially be true for magnetic fields, but is untested. If you're getting the equivalent of an MRI every day, maybe that would have some long-term health effects (those working with MRIs do not even get that exposure, because they are staying out of the room while the machine is on), but the likelihood of that sort of exposure is slim to none.
  12. Oct 27, 2007 #11
    Most workers that I have spoken with (including myself) experience some vertigo, but the really strange effect occurs after perhaps 15 minutes in the bore when you lose the ability to tell up from down. As you point out, that suggests the field is doing something to the vestibular system.

    I don't know what type of facility you have experience in so my point may not be valid for you, but my experience is that the dc field is rarely off and the ac fields are often on. Further, the fields extend well outside the room. I know workers who have received literally hundreds of MRIs.

    You are, of course, correct in pointing out that the long-term effects are not fully known. Yet, most of the workers I'm familiar with are engineers, scientists, technicians, and physicians, most of whom are likely to be fairly knowledgeable and who are likely to have spent many years doing this kind of work. They do not seem to be aware of any great health risk. I concede that all these professions are well populated by people who routinely ignore dangers in order to collect data.
  13. Oct 27, 2007 #12
    I often hear it proposed that humans might have this ability but, as far as I know no one has shown it to be the case. Have you read some new research about this?
  14. Oct 29, 2007 #13

    jim mcnamara

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    There is a weak association between EMF exposure from transmission lines and certain childhood leukemias.

    NIH has a wait and see approach on this subject from a public health standpoint right now. This is the only research summary I've seen regarding humans and EMF fields from power transmission:

    Main link:

    An old microbiology paper with yeast and magnetic fields:


    There are few lab studies out there. Maybe there are some really old papers, I dunno. For humans, there are a series statistical analyses - mostly on the leukemia association.
    There was research in the '80's but not much of the research was published because it did not refute or support anything.

    If you're interested, the PUC (or PRC) of most US states limits how close dwellings and high power transmission lines can be to each other. Most also mandate reviews of new papers on publications. There are currently no limits on the much lower powered distribution lines - except some cities require them to be underground, mostly for esthetics,

    If your living place has electric power, there are going to be really fields near any circuit with load. Any operating electric applicance, including your PC generates EMF. Many of those same appliances are shielded in some ways.

    Those EMF fields obey the inverse square law, so at distances of meter+ the fields may be hard to detect.

    A lot of cartilagenous fishes (sharks and rays) detect the EMF fields of nerve systems of prey animals nearby. FWIW. If you're interested, google for 'ampullae of lorenzini'.
  15. Oct 31, 2007 #14


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    I knew an emeritus professor who spent a considerable amount of effort addressing this question; specifically how time-varying magnetic fields relate to human health. Up to that point, there were the studies correlating long term exposure of ELF fields to incidence and onset of childhood leukemia. This professor passed away after an abrupt onset of adult leukemia. For as long as I'd known him, he was a quite healthy and energetic fellow. I have to wonder if his acquisition of leukemia was a coincidence, given his lifelong career with electromagnetic field studies.
    Last edited: Oct 31, 2007
  16. Nov 1, 2007 #15
    Anecdotal testimony here. I had two MRI's three years apart for degenerative knee joint. About 24 hours following each the knee pain went away for about 2 months. This was the equivalent to later injections of Hyalgan. And again later Bextra also was equivalent. Something I associated with MRI seemed to relieve the pain. I know there is no support for this. Also a guy at the mine would rub a little WD-40 into a joint that was bothering him. I tried it, afraid of liver damage, and it seemed to be associated with a lessening of pain.
  17. Nov 1, 2007 #16
    The DC field of an MRI unit is only off twice: during shipping and installation, and during removal. The rest of the time the field is on continuously. For cryogenically cooled magnets, the process of filling with cryogen and ramping the field up generally takes 3-5 days.

    The AC (gradient) fields generally range from 10-20 mT/m (depending on application and coil type) and typically change at a rate of 100 mT/m/ms, again depending on application and coil type. Gradient fields are only active when a patient is being scanned.

    For almost all new MRI units (within the past 5-7 years), magnet shielding (a combination of active and passive) is typically designed to keep the 5 gauss line well within the procedure room. Unless the magnet is installed in a really small room, the field strength at the control console is typically between 1-3 gauss.
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