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Radiation from dental x-rays?

  1. Mar 2, 2012 #1
    Hey guys, I wasn't exactly sure where to put this one--but figured here might be best. I went to the dentist this morning and they took two bitewing x-rays, which are a low amount of radiation (~0.038mSv). The claim is that, since we routinely receive ~3-4mSv per year due to radiation incident from space or domestic radioactive traces, it's not something to be worried about. I agree, and think it's a good argument--but one thing was bothering me.

    The problem with ionizing radiation is that it disrupts DNA replication, and in an unlucky case, the radiation can happen to knock out the wrong gene during DNA replication, leading to cells becoming cancerous and proliferating unchecked. However, if I recall my bio100 correctly, it takes several genes of your DNA to get knocked out to actually lead to cancer. So I'm wondering a couple of things.

    1) This is biology related: if only one gene gets knocked out, will that cell line eventually die out, so say, in 10 years, that gene is no longer missing in any cells in your body (or at least is in a negligible number of cells).

    2) This is the physics question I'm actually concerned about: 0.038mSv in a fraction of a second seems like it could be more dangerous than receiving 3-4mSv over a year. Further, these dental x-rays are concentrated on a specific area, and seem like they could do more damage than you are likely to get from other sources throughout the year.

    Can anyone offer any wisdom on this? I've been very curious about it.

    Here is my very rough source on the numbers: http://www.huffingtonpost.com/thomas-p-connelly-dds/dental-xray-radiation_b_960573.html . Feel free to contribute better numbers if mine are way too off.

    Thanks so much !
  2. jcsd
  3. Mar 2, 2012 #2
    1) The body has repair and safety mechanisms that take care of most broken DNA. If it cannot repaired, most likely the cell will die or be killed very quickly. Cancer is caused by the cases where such a cell is (a) not killed and (b) divides at a much higher rate than usual.

    2) It depends very much on what part of the body receives the radiation dose. Bone marrow is very sensitive, but the extremities (hands for example) are not. "Dead" material (tooth enamel) is not sensitive at all. Dental x-ray machines are set up to irradiate only where it is necessary so the "dose density" will be higher than what you get in an airplane (because that is spread out over the whole body), but the most sensitive part of the body will receive nothing.

    I would not worry about a dental x-ray. But if another dentist wanted to take another x-ray for no apparent reason, I'd ask him to look at the ones you already have first and explain why new one is needed. There can be good reasons...
  4. Mar 2, 2012 #3

    jim hardy

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    the chromosome is a very small target.

    and dental xrays are a small dose.

    Take comfort in the astronomical odds.

    old jim
  5. Mar 2, 2012 #4
    The majority of damage from ionizing radiation is from ionizing water in the body (since the body is the most abundant possible target. This causes free radicals to form, especially hydrogen peroxide, which causes a cascade forming more free radicals. At some point the process stops. It is this cascade of free radicals that are very localized which disrupts the DNA. As stated, the cellular repair/apoptosis (programmed cell death) mechanisms usually can take care of the problem (there are oodles of cells that die every second due to normal metabolic functioning). So, one additional cell killed isn't much of anything. It's when a bunch of cells are killed (because the dose rate is so high) that causes a problem.
  6. Mar 2, 2012 #5
    hah, I didn't expect to receive so many 'biology' answers on this forum, much less this particular sub-forum. These are all very comforting, and it's nice to think about the actual mechanism (ie: water radicals) to have an idea of what's actually happening.

    I'm still slightly uneasy about the contrast between how much you get on average per day, and how much you get in a brief amount of time at the dentist, but I don't know if there's anything I can do about that.
  7. Mar 2, 2012 #6


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    Think about it this way. The dose you get the few times you get an x-ray from the dentist is low compared to the daily average does added up over your lifespan.

    Your average dose per year from natural sources is about 300 mrem/year. A dental x-ray is about 10 mrem. Assuming that you get 3-5 x-rays in one go, as I think I get, thats 30-50 mrem per visit. I only get new x-rays about every 2-4 years, so my average yearly dose is about 15-25 mrem. In my case the dental x-rays increases my average yearly exposure by less than 9%. I think that's a very acceptable risk considering that the risk I might expose myself to if I didn't get an x-ray and ended up having bad teeth, which many times requires surgery and such to remove, exposing me to the possibility of infection and reducing my quality of life.

    Those numbers are refrenced from here: http://www.physics.isu.edu/radinf/risk.htm
    Any additional sources of radiation will increase your exposure, but even in the worst case scenarios you are almost guaranteed to benefit more from receving them as opposed to not, as you will usually receive much more radiation when you have a problem such as already existing cancer, diseases that require radioactive dyes, or other disorders.

    Also, if I remember correctly, it is better to receive a larger dose in a very short span, than a lower dose over a long span. For some reason our bodies have an easier time dealing with the damage. Perhaps because the affected cells have a better chance of dying straight away instead of passing on mutated genes?
  8. Mar 3, 2012 #7
    Well, this is a problem when one gets *big* dose.

    Cancer danger arises from a different mechanism:
    DNA damage is either
    * fixed (there are enzymes which exist in the body for this exact purpose, and they working in our bodies as we speak);
    * or cell becomes unable to function properly and dies;
    * or, the unrepaired damage happened to be in inactive part of DNA (IIRC ~90% of our DNA is inactive, so-called "junk DNA");
    * or the damage does not impair cell's function too much (or at all) and cell lives on.

    This last case is a step which eventually leads to cancer. Cells with slightly altered DNA may work slightly differently. For one, they may _divide_ at a higher rate, and/or stop responding to the normal mechanisms which control cell division. If the division rate is only slightly higher than normal, then the forming tumor is not yet a cancer, but if one of these new cells will speed up its division rate even more, then it's child cells will swamp out slower dividing ones. It's a natural selection of a horrifying kind: the "worst" cell wins. (And that's why doctors pay so much attention to tumors even if they are non-cancerous).

    Theoretically, any single DNA damage event can lead to this - with very small probability for each of it. Therefore, anything which increases DNA damage increases cancer rate - but not necessarily by a measurable amount! (Well, unless there are mechanisms in our body which significantly increase amounts of DNA-repairing enzymes in response to increased rate of DNA damage. Are there such?)

    Logic says that the chances of DNA damage leading to cancer should be proportional to the dose. Taking into account that there is non-radiation induced DNA damage happening all the time, low dose (such as dental X-ray) cancer risk increase is very tiny. You'd get much better improvement in your chances not by refusing X-rays, but by paying attention to your lifestyle: what do you eat, do you exercise, etc.
  9. Mar 3, 2012 #8

    jim hardy

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    Asimov's essay "At Closest Range" explains just what a small target a DNA molecule is for a gamma ray.

    But DNA being largely carbon, it will contain an occasional C14 atom. When that one fizzles into nitrogen a chromosome gets changed.
    Carbon 14 is the only naturally occuring isotope that has significant presence in DNA, per that essay.

    So Mother Nature has built in a natural mechanism for chromosomes to get altered. I guess she too likes a little change every now and then.

    old jim
  10. Mar 4, 2012 #9
    There are many ways not related to nuclear decay/radiation for DNA to get damaged. Some are chemical (free radicals attacking the molecule), some are stochastic (errors during cell division) and some are even built-in into our genes. Read this: http://en.wikipedia.org/wiki/Transposon

    Nature indeed did not see cancer as a problem. Far bigger problem for it would be a 100% safe and stable copying of DNA. Why do you think sexual reproduction exists and is so widespread? Because in the long run, trying random new combinations is better for evolution than stagnant copying of the same old record.
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