electromagnetic radiation - Is the body's exposure to an x-ray equal to an airplane trip across the country?

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.

Answer

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.