I will review this topic wearing both my hats as a former physics professor (who also did research on the effect of ionizing radiation on tissue) and a former medical professor. I am not here going to criticize the statistical methods used to draw conclusions (e.g. in "Projected Cancer Risks From CT Scans..." Arch. Int. Med. Vol 169, pp 2071-2077, 2009.) Rather I will discuss the medical evidence for the carcinogenic effects of radiation and see if there is any valid scientific evidence that enables us to extrapolate these results downward so as to predict the possibly carcinogenic effect of the Xrays we are exposed to by CT scans and cosmic rays. I understand that this is a "hot" topic (no pun intended), and as usual in such cases, the heat of an issue tends to be in inverse proportion to the light that experimental facts shed on the issue. As Voltaire famously mentioned, no one is burned at the state because people disagree with the statement that 7 x 8 = 56, although Giordano Bruno was burned at the stake for maintaining that the earth was not the center of the universe.
I am going to restrict my discussion to adults. The different units of radiation measure will also not concern us directly, because we have no way of accurately determining how much of a given wavelength of Xradiation a body organ will absorb, let alone the DNA damage it will cause. All the hard data we have really comes from the development of cancers in the survivors of the two atomic bombs dropped on Japan, and we begin by assuming that the response of Caucasian and black tissue to radiation is the same as the response of Japanese tissue, insofar as its carcinogenicity is concerned, an assumption that is, thank God, not borne out by any accidents of nature or man. (Chernobyl is a special case, since the doses there were not comparable to those of diagnostic radiation, and a lot of the radiation damage and cancer were caused by ingestion of radioactive isotopes.)
The question to which there has been no clinical answer is whether or not the risk from the radiation released by an atomic bomb can be extrapolated down in a linear fashion to medical Xrays or our daily bombardment by cosmic rays. Is there a threshold, a level of radiation below which Xrays will not induce cancer in humans? Is the only "safe" dose (i.e. non cancer-causing) of Xrays no Xrays? This question has never been answered by any clinical study, and all the assumptions and estimations of the dangers of diagnostic Xrays are based upon the unproven assumption that there is no such threshold, and that even one photon in the Xray energy range can cause cancer in a human cell.This assumption completely ignores the reparative capabilities of DNA. We also have never answered the question of whether or not the risk from successive Xrays is cumulative, so that we can add the putative carcinogenic risk of one Xray to the next one. Somehow this is counterintuitive, unless we are saying that 10 mammograms in 10 days are no more dangerous than 10 mammograms each spaced a year after the previous one. This would seem to say that there is no DNA repair process for Xray damage, even though man lives his whole life exposed to very high energy cosmic rays.
I would argue that there must be a threshold dose below which Xrays do not cause cancer, and therefore linear extrapolation downward from the cancer incidence in A-bomb survivors is invalid. We know that we are constantly bombarded by cosmic Xrays which have energies thousands of times higher than you are exposed to by an Xray machine. Note that I said energy, not intensity. (Intensity is measured in photons per square centimeter per second.) We are partially shielded from cosmic radiation by the ozone layer and by the earth's atmosphere. People living in Denver are a mile higher than denizens of New York City, and therefore have one mile less of atmospheric shielding. Nevertheless, the cancer rate in Denver is identical to that in the lowlands. Therefore the increased cosmic Xradiation that Denverians receive is not enough to measurably affect their cancer rate. It therefore follows that there is a threshold, and that we have no idea of what a safe dose of radiation is, except that "too much" can damage tissues directly by overheating and frying them.
I can also adduce evidence that the cell damage from Xrays is cumulative. It is well known that as you get older, the incidence of cancer increases. But as you get older, you have been exposed to more and more cosmic Xrays originating from deep in the universe. Is it possible that cosmic Xrays are the main cause of the increase in cancer with aging, in part by damaging the nuclei of cells in our body and in part by damaging our immune system? For that matter, we know that the coronary arteries under the left breast have in increased rate of calcification if the breast is radiated for breast cancer. Could the exposure of all our arteries to the continuous rain of cosmic Xrays also be the explanation for the increased hardening of the arteries with age? Could this also explain the loss of brain cells with aging, and possibly contribute to Alzheimer's disease in that amyloid plaques are the body's attempt to heal radiation damage to the brain by creating scar tissue?
There has been no satisfactory explanation of why our cancer incidence increases with age, except to note that our immune system also weakens with age. But correlation is not causation, and, as I suggested above, we are then left with the question of why the immune system weakens with age. Since we know that radiation can damage the human immune system, then the continuous cosmic radiation is as good an explanation as any. It may even explain why few if any humans live beyond 120 years: the shortening of telomeres with age may also be a consequence of cosmic radiation bombardment, and we can never test this hypothesis because the energy of cosmic radiation is thousands of times higher than xrays we generate here on earth.
In summary, there is no clinical evidence one way or another that our estimates of a "safe" dose of radiation are correct, and we have no way of determining the life-long effect on us of our bombardment by ultra high energy cosmic Xrays.