I have heard numerous times when getting x-rays, MRIs, CAT Scans, etc. that each one is equivalent to a cross country airplane trip. Disregarding the different types of radiation as asked in this question, I was wondering if they are mixing total dosage over the entire body with the same dosage in a concentrated area.

To illustrate by example, if the total radiation received on a plane trip is 100 'rad' units and the body is 100 area units each body part would only receive 1 rad unit of exposure. Whereas if an x-ray is the same 100 rad units but your arm (for example) is only 10 area units then your arm is actually receiving 10 rad units of exposure - or 10 times the amount received on a plane trip.

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    $\begingroup$ Randall Munroe did us all a huge favor on this matter: xkcd.com/radiation. Learn it. Love it. Live it. $\endgroup$ – dmckee Jul 2 '14 at 22:07
  • $\begingroup$ I have seen that chart before but if it answers my question I'm just too dense to see it. Take the chest x-ray for example, he says it's 20uSv. Let's say the tissue exposed to that 20uSv is 20% of your whole body. Next use the 40uSv for the plane trip. Since we defined the chest area to be 20% of your body does that mean the plane trip gives your chest area 8uSv of exposure or does every cell in the body get a whole 40uSv? $\endgroup$ – CramerTV Jul 2 '14 at 23:04
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    $\begingroup$ A Sievert is a measure of equivalent dose. So it does not represent energy deposition or some other quantity that you can identify as intensive or extensive, but some approximation to health risk. Rad is energy per exposed mass: an energy density. You could scale it by mass if you want, but it doesn't make mush sense to do so: better to know how much you took where so that you can compute the health risk (i.e. get to Sieverts). $\endgroup$ – dmckee Jul 2 '14 at 23:17
  • $\begingroup$ Doses... 1diagnostic x-ray=20 mrem. 1trans-atlantic flight=2mrem. rem=roentgen equivalent man=0.01 Sv $\endgroup$ – user28737 Jul 9 '14 at 13:54
  • $\begingroup$ You say x-rays, MRIs, CAT Scans, etc. But please know that the electromagnetic radiation used in MRI in in a completely different range than X-rays. In MRI radiofrequency pulses are used, which have a wavelength of about 1-100m, whereas X-rays have a wavelength of about 0.01-10 nm. So no ionizing radiation is involved in MRI. $\endgroup$ – fhdrsdg Aug 28 '14 at 12:43

When you give the dose of an exposure in Sv, you have already taken into account weighting factors related to the organ type being exposed (some are more sensitive than others) as well as the radiation type.

In other words, the number reported represents "equivalent cancer risk if your entire body had been exposed with ..."

See for example http://en.m.wikipedia.org/wiki/Sievert#Calculating_protection_dose_quantities

So yes - a trans continental plane flight (New York to LA) on average increases your lifetime cancer risk by more than a chest X-ray.

Both are absolutely tiny. It is typically considered that 1 Sievert carries with it a lifetime risk of cancer of 5% (this depends of course on the age of exposure... All these things are "population risk" not "individual risk".

That means that one flight per week = 2 mSv / year for fifty years (2500 flights) will cause one additional cancer in one person in 2000.

And that is assuming linear scaling of radiation risk - there is some evidence that the body has some repair mechanisms that may make the curve nonlinear. So don't get too hung up on the detailed calculations. In radiation protection one uses the principle of ALARA - As Low As Reasonably Achievable. The person who gets the highest radiation dose in the hospital due to a diagnostic imaging exam is probably the interventional radiologist - it is definitely not the patient getting the chest Xray.

  • $\begingroup$ The problem is, equivalent dose ("actual" dose weighted by type of radiation, with particles worse than photons) and effective dose (equivalent dose weighted by how much of the body is exposed) are both measured in sieverts (or rems), as is committed dose (which takes more nuances into account). So just knowing the unit doesn't tell you how many dimensionless weighting factors were included. $\endgroup$ – user10851 Jul 2 '14 at 23:23
  • $\begingroup$ Thanks for the link Floris. @Chris White, so if we accept the supposition I've been told numerous times, the following statement would be true - an effective dose of 40uSv for an airplane trip spread throughout the body is an equivalent health/cancer risk as an effective dose of 20uSv applied by x-rays to the chest cavity. Or is it as Floris says, that an airplane trip is more likely to produce cancer (because of its higher total exposure) - which is contrary to my doctor's offering? $\endgroup$ – CramerTV Jul 2 '14 at 23:49
  • $\begingroup$ @TracyCramer Yes, barring the possibility that the effective dose -> committed dose conversion introduces a factor greater than 2 (e.g. if your chest tissue were for some reason really susceptible to cancer per unit DNA damage, or if all the damage being done in 1 second is worse than being done over 6 hours). $\endgroup$ – user10851 Jul 3 '14 at 0:05
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    $\begingroup$ @TracyCramer You probably shouldn't take "equal" so literally; allow it to mean "of the same order of magnitude" or perhaps "both too small to be cause for much concern". After all "airplane trip" is not a precise measure in the first place. Randall chose a trans-continental flight that take a whole work day, but they come both shorter-n-lower and longer-n-higher. $\endgroup$ – dmckee Jul 3 '14 at 1:24
  • $\begingroup$ @dmckee, I wasn't meaning to take it literally. As noted in the original question the example was an order of magnitude. I was just acknowledging the information offered as I try to concrete my knowledge. Thanks for all your insight and information. $\endgroup$ – CramerTV Jul 3 '14 at 16:32

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