There are and will continue to be articles about the dangers of radiation, so I thought a few additional remarks about the basic physics of radiation and its effect on human tissue are in order. I will restrict my discussion to external radiation that occurs in the form of gamma rays, aka Xrays, and ignore alpha rays (helium nuclei), beta rays (electrons) and neutrons. I will just mention that the shorter the wavelength the higher the energy of the incident radiation, so that infrared rays have less energy than do ultraviolet rays, which is why UV rays can penetrate the skin more deeply and thereby create Vitamin D.
Xrays are a form of energy, travelling at the speed of light because they are composed of photons. We are exposed to Xrays from the earth's natural radioactivity, from medical Xrays, and from cosmic rays. Xray energy is often defined as the ability to ionize a column of air, but this definition from physics is not a useful definition for medical purposes. Instead we have the "rad" and the "rem", although "roentgen","curie", "gray" and "sievert" are in use in some places.
The rad is a unit of absorbed radiation dose, in other words it is the dose of Xradiation that is needed for a certain sample (animal, vegetable or mineral) to absorb a certain fixed amount of energy per gram of material. It is thus clear that the rad depends on the wavelength, i.e. the energy of the incident radiation as well as the absorptive properties of the material. The rem, or "roentgen equivalent man" is a measure of the biological effects of the absorbed radiation, which is clearly not only dependent on the wavelength of the incident radiation but also on the tissue (brain, bone marrow, thyroid gland, etc.) that is absorbing the radiation. I should here mention that different countries and different international organizations have different conversion factors from rad to rem. It is also obvious that it is impossible to determine the precise rem value of incident radiation on man because we have no way to examine living human tissue after exposure to radiation to determine its biological effect, so the conversion factors are "guesstimates".
The idea that the risk of cancer increases linearly with Xray dose rate is a consensus decided upon by international bodies, and has no demonstrated basis in clinical fact. The risk is really stochastic rather than deterministic. It is also stated, without clinical evidence, that the risk is higher for children and fetuses than for adults, and higher for women than for men. The international recommended "safe" exposure dose from medical Xrays and other sources over and above environmental exposure is 0.1 rem/year, but this is also a consensus number with no demonstrated basis in fact. For comparison, because of their higher altitude and therefore less shielding by the atmosphere, residents of Denver are exposed to an additional 0.3 rem/year, and airplane crews are exposed to between 0.5 rem/year to 1.0 rem/year, depending on their routes. BTW, the International Commission on Radiological Protection recommends evacuation of any area where the excess dose of radiation is greater than 0.1 rem/year, even though residents of Denver have a lower cancer incidence than do residents of the flatlands. I also have found no published clinical studies demonstrating that women who have mammograms have a higher incidence of breast cancer than women who never had a mammogram.
Addenda: All granite contains some uranium and therefore exhibits low-level radioactivity. We have no good model as to whether or not Xradiation is teratogenic. The International Committee for Weights and Measures does not accept the rad as a unit of radiation exposure, but this unit is widely used in the U.S. The lethal dose of radiation depends upon the time frame over which it is delivered, which is why radiation therapy for cancer is spread out over weeks. In general, a dose of 1,000 rads delivered over a few hours will be lethal, and doses of 100 to 1,000 rads over a few hours will cause acute radiation sickness, with the probability of dying being greater of course at the higher dose.