There really are a couple of interesting posts in this threat, nevertheless, I do want to clarify some important issues here.
It is very well established, that radiation causes effects on biologic systems. There are two distinctively different effects: stochastic and deterministic. Deterministic effects always will appear and cause damage to the living, ranging from radiation burns to failure of the immune system with subsequent death. Radiation burns of the skin are commonly seen in radiation therapy for tumors, and one of the aspects of radiation treatment planning is to keep this at an acceptable level. In former times, radiation burns were also frequently seen under fluorescopy, especially when performed by cardiologists for coronary angiography, but even this group of physicians lately has learned certain aspects of radiation protection. Deterministic radiation effects causing deaths can be seen during accidents, e.g. one of the helicopter pilots doing fire extinction work in tchernobyl was one of the unlucky. In general, deterministic radiation effects do not play a role for the general public and also not in conjunction with flying.
More complex are stochastic radiation effects, which do occur stochastically (well, the name does imply that). It is well established, that there is a pretty much linear dose response relationship for stochastic radiation effects for single doses above 100 mSv. This is known from the close monitoring of the survivors from Hiroshima and Nagasaki as well as from exposure especially in the early days of radiology.
Based from this data, the currently used concept is a linear interpolation into the low dose range, that is the dose range below 100 mSv (annual exposure is in the 1-3 mSv range, so even much lower). To make it quite clear, there is no study showing stochastic dose effects in this low dose range and there is no study showing no dose effects in this low dose range (an adequate study would need to randomize something like earths inhabitants against a second earth, which is a kind of difficult). All calculations of potential radiation deaths in this low dose range are from pure extrapolation.
Now, the big question is, is it scientifically acceptable to do a linear interpolation in this low dose range. Everybody who is honest can not say this. Summing up all the literature I know about this topic, the question remains open. There is some literature, which in fact does suggest that there is a linear dose response relation even in the lowest range. Other literature shows that very small doses do lead to a reduction of biologic effects. The pathomechanism being discussed for that is triggering of intracellular repair mechanisms, which thus also repair other below threshold damages on the DNA, therefore lowering potentially critical mutations. In addition, the low dose group of the A-bomb survivors in fact do not react in agreement with linear dose response relationship.
Thus it clearly is uncertain whether a low dose exposure to radiation has negative effects on your health or may be even positive effects. The current concept for diagnostic imaging is the so called ALARA (as low as reasonably achievable) principle, which in fact is a very practical an acceptable approach for low dose radiation exposure for medical indications. I would also act in that way for any other aspects, including flying. But there is also not at all reason to be overly critical, and calculating potential death rates for flights is just (un)scientific spin control.